Population and Community Ecology
Lecture Notes
Dr. James A. Danoff-Burg
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Module 2: | 9
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Module 3: | 17
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Introduction
Diversity of Life & Natural
Selection
James A. Danoff-Burg
Population & Community Ecology
Todays
Agenda
Evolutionary
ecology
Power of
evolutionary explanations
Variation
originates naturally
Hardy-Weinberg
equilibrium
Selection
Evolutionary Ecology
Definition
using historical ecological, evolutionary, or systematic
data to explain current ecological trends
Example:
Penguins are only in the Southern Hemisphere - Why?
Functional explanation:
food is not present elsewhere, constrains their location
Evolutionary explanation:
phylogeny indicates that all related
lineages have only existed in the Southern Hemisphere
This is a more elegant explanation,
explains more data
also if it is supported, it obviates
the need for the Functional explanation
Example II:
Desert Pupfishes in
(also at this
link and at this link)
Functional explanation:
fish are specialists in the ponds in which they live
Evolutionary explanation:
found there because of historical
lake that became subsequently dried up,
leaving isolated ponds that were all
spring fed
Again,
Evolutionary is a simpler and more elegant explanation
Functional
vs. Evolutionary explanations
Many argue
that if an evolutionary explanation is not disproven
(speaking in double negatives, as is the norm in the
scientific method),
then it
should be the first explanation adopted
Before even exploring functional answers
Power of
Evolutionary Explanations
These
explanations best explain the greatest amount of data in the most elegant
manner
How variation originates in nature
Point
mutations
Substitutions
leading to transcription errors
Deletions
leading to frameshift errors
Additions
leading to frameshift errors
Chromosomal
rearrangements
normal crossing over
during Prophase I of meiosis
inversions
Translocations
Most
mutations are deleterious
only 1 in
1,000 are beneficial
All of us have mutations in our body
are continually created,
something on the order of 1 in every 100,000 sex cells have
some type of point mutation in them
Hardy-Weinberg Equilibrium
Explains why variation continues in a
population
acts as a null hypothesis when testing for changes in the population
Explanation
two
alternative alleles of a single trait
P and Q
p and q
will substitute for the frequencies of each allele in the population
p = 1 - q
and q = 1 - p
Equation
for diploid organisms
1 = (p +
q)2 = (p + q)(p + q) = p2 + 2pq + q2 = 1
p2
determines the frequency of the PP genotype
2pq
determines the frequency of the PQ genotype
q2 determines
the frequency of the QQ genotype
Assumptions of H-W equilibrium
huge populations
random mating
no immigration or emigration
no selection
no mutation
these are violated when change is occurring in a
population
An Exercise
Determine
the frequencies of each handedness allele for our in-class population
assuming that individuals with
either a PP or PQ will be right handed
Lefties are only obtained by QQ
Selection
and Speciation
Violations of the Hardy-Weinberg
Equilibrium can be produced by
selection, drift, and/or other mechanisms
These
violations can produce longer-term evolutionary changes
Violations include:
Breeding
restrictions
assortative mating, inbreeding)
Population
size
(small populations, genetic bottlenecks [more on this later
in term])
Population
demographics
(what proportion of the population
that remains can breed in the future?)
Migration
(could introduce or remove novel alleles into a population)
Natural Selection
First
published by Charles Darwin and Alfred Russel Wallace
in 1859
Interesting
historical story
Darwin, the landed gentry
Wallace, the lower-class striver
incidentally,
do not
confuse evolution with natural selection
Evolution: as observed a scientific "fact", as we
have
Natural Selection: one of the mechanisms through which
evolution can occur
was an early proponent of evolution
Lamarck: Not the loser we paint him as
Proposed
based on five observations
variation
exists in nature
food and
resources are limited
organisms
tend to out-reproduce their resources
competition
exists
differential
reproduction results
leads to some lineages contributing more to the next
generation than others
Differential reproduction =
natural selection
Nifty
Darwinian Fact
never in
the Origin of Species did
he was only interested in documenting the above five
observations
Never talked about the production of new species
#3 - The Beginning and the End of Species:
Selection, Speciation and Extinction
James A. Danoff-Burg
Population & Community Ecology
Types of Selection
Directional
selection against one extreme or the
other
Stabilizing
or Normative
selection against both extremes
Disruptive
selection against the mean
Selection
Types
Speciation
Definition:
the production of new species
Two
General Types of Speciation
Allopatric
speciation that arises as a
consequence of separation of a population
Sympatric
speciation that arises within the
normal cruising or home range of a species,
usually through some sort of behavioral
or ecological change,
often as a consequence of symbiotic
relationships
Speciation
Types
Extinction
and Speciation:
Intimately Related
Produced
from same events
The same processes that could lead
to a speciation event could lead to an extinction event
Isolation, reduction in population
sizes, strong selection pressures, etc.
The latter
is more common
More populations go extinct than
produce new species
Similar to the number of favorable
vs. disfavorable mutations
Thought experiment:
How would
each of the 5 H-W equilibrium assumptions be violated if a population were to
go to extinction?
Think this
through
Extinctions
We are in
the throes of an extinction event that may be every bit as large as the very
largest extinctions in the geological past,
including the K-T extinction and the
one at the Permian-
Triassic boundaries
Individual Selection
The
individual is who lives and dies
Not the species they only
disappear or explode locally
Natural
selection acts solely on the level of the organism
Most commonly accepted explanation
of how evolution works
Not at the
level of
Group
Species
Lineage
Phylum
Other higher-level taxa
Individual
Fitness
Individual
selection is the most commonly accepted mechanism of natural selection
currency: individual
fitness
Definition
proportional contribution of that
individual to the next generation
only relative to other individuals
in the population, not an absolute value
Individualism
Essence of
our current understanding of natural selection
Hasnt
always been such
Historically, altruism and group
selection were thought to also work differently from individual selection
Altruism: reduce your fitness for another
individual (usually relation)
Group
Selection: reduce your
fitness for the good of the group
Both were popular until the 70s
Can they be explained using
individual selection?
Individual explanations for Altruism
Simple
individual fitness would not predict altruism
At least as how it was originally
constructed
Focused exclusively on the
individual
Individual selection modifications that WOULD explain
altruism
Inclusive
fitness
J.B.S. Haldane,
the monks, and your drowning brothers
Reciprocal
altruism
Most commonly found in long-lived
groups
Individual
Explanation for Group Selection
Group
Selection Definition:
selection above the level of
individuals
Popular in the recent past as a way
to explain why altruism occurs
Currently
thought of as an untenable idea
At least as how it has been created
and used
Individualized Explanations:
Inclusive
fitness
Reciprocal
altruism
Resuscitating
the Superorganism
David
Sloan Wilson, Eliot Sober, and others are trying to resuscitate the idea of
group selection
Using
structured populations
Definition of Structured populations
A large group that is divided into
smaller cohesive groups
Each of which have a different
selective values within and between the levels of group
The groups compete against other
groups at the same level of hierarchy
Selection above the level of the
individual
Examples of
Superorganisms
social insects
cellular
slime molds
Common
feature:
relies on emergent properties that come about as a consequence of these
groups
Emergent properties are those that
only exist above the individual usually at the group level
Example: Foraging patterns of army ants,
fruiting body in cellular slime molds
Both are produced only because of a
collaboration between many individuals
Favored Ecological Settings
Common
environmental features that encourage species to act cooperatively
Environment is divided into patches
Patches are ephemeral (relative to
the life cycle of the species undergoing group selection)
Low migration
Patches are colonized by a single or
few individuals
Examples
Decaying logs
Dung pats
Favored Autoecological Properties
The
Higher-level group should have the following traits
they have the ability to replicate
themselves
survival depends on the emergent property of
the group (foraging, etc.)
a mechanism exists for that
attribute to be transmitted to the next generation
Important provides means for the
trait to be heritable
A
Theoretical Example
John
Maynard Smith's model of group selection
Trait groups
Individual fitness = fn(own
genotype)(group genotypes)
Assumptions:
Wo = fitness of non-interacting
individual
c = cost of altruistic act
b = benefit of altruistic act to
others
s = synergistic benefit accrued to
both individuals if both are altruistic
p = frequency of altruists in the
population
Trait Group
Evolution Equations
Wa = W0 c + pb
Equation for cooperator / altruistic
individual
Wn = W0 + pb
Equation for non-cooperator /
selfish individual
Initial
Conclusions
If altruism costs anything (and no
synergisms exist), altruism will not evolve
If only random mating / association
exist, altruism will not evolve
When Will
Trait Groups Evolve Altruism?
When:
Fitness combines additively θ
synergistic benefits
Altruists congregate into trait
groups (particularly if they are related common occurrence)
Then:
Group
Selection
Some Take-Home Messages
May occur
in some taxa living in appropriate ecosystems
Requires
many favorable confluences to occur
Fitness combines additively θ
synergistic benefits
Altruists congregate into trait
groups
Particularly common if cooperators
are related
However ΰ Not a
universal feature of life
In contrast to how it had been
historically discussed
James A.
Danoff-Burg
Population
& Community Ecology
Our
Emphasis Today
The abiotic factors that influence the distribution and
abundance of organisms
This is what many researchers consider to be the main focus
of ecology
Again, the difference between our
class and a traditional introductory ecology course
Most of what an organism tries to do is to maintain internal
homeostasis
Today, well be talking about how
this is accomplished
Relation to
Course
How does this relate the rest of the course?
My rationale
Selection acts on the individual
Individuals are the units that
respond to the environment
The environment is a key selective
agent
Physiological ecology is mostly coping
with this aspect of natural selection
Regulating
Homeostasis
Organisms can try to do this by
Regulating their internal body
chemistry
Changing behavior
Example: butterflies in the Nearctic
using sunning of their dark wing patches
Homeostasis
Most common in which types of organisms?
Most organisms do this = Regulators
Terrestrial
Free-moving aquatic
Conformers
Which ones would not want to regulate their internal biochemistry to
this level? = Conformers
Why not?
Common features of conformers
those that are sessile
Anchored
In relatively stable environments
Consequently
They are unable to cope with rapidly
changing environments
Ignoring
Homeostasis
Others dont even try to differentiate between themselves
and the environment
These instead conform to their
environment
Example: tube worms at the bottom of the ocean in the
hydrothermal vents
dont regulate the concentration of
salts in their blood
if they are taken to another site
that is more brackish
they take on water until their cells
burst and they die
Constant environment, little
variation
Clearly this is not an option for
those organisms that migrate great distances or are highly mobile
Important
Abiotic Factors
Those that would influence organismal
distribution which are they?
Key components:
Temperature
Humidity
Wind
Sunlight abundance (for
photosynthetic organisms)
-->these first four determine the climate<--
Climate definition: prevailing weather conditions at a
locality
Secondary
Important
Abiotic Factors
What are some other important abiotic factors?
Including the following
Soil type
Water and/or soil pH
Fire prevalence
Salinity
Space (plants and other sessile
species mostly)
Pollution
Solubility of oxygen in water
Pressure - atmospheric and water
pressure
Mineral abundance
Which
Abiotic Factors are Important?
How can we know which factors are key?
Depends on the biology of the organism
Differs between organisms
Even neighboring organisms in a
given locale
Often common between closely related
species
ΰ the
predictive power of phylogeny
Abiotic
Features and Biotic Limits
How could abiotic factors indirectly influence the presence
and abundance of a species - or do so less obviously?
Indirectly via
Any of these factors influencing the
organisms host, prey, preferred food source, etc.
Less directly via
Multiple interacting factors
Many of these are at work and
simultaneously take effect
Independently, they are not limiting
only collaboratively
Abiotic
Factors and
Typically, it is not that a species is absolutely excluded
from an area
They are usually less fit, more stressed,
or less apt to mate and reproduce
Abiotics,
Ranges, and Life Stages
The resiliency or ability to live in marginal
habitats varies based on the life stage of the organism
Resiliency ability to respond to
change
Immature stages of plants and animals are usually less hardy
than are the adults
Why?
Adaptability
Ranges of adaptation
How much can the environment vary?
in frequency and magnitude
While still retaining the species
Breadth of adaptation
Can refer to any or all of the above
abiotic factors simultaneously
Are relative values
Stenotopic - Able to live only at a very
narrow range
Eurytopic - Able to live across a wide environmental range
Adaptability
and Multiple Abiotic Factors
Not much relationship between how the organism copes with
each variable
Vary independently
as to how much of any of the above
conditions that are present in their preferred habitat
Remember that the impact on the range will vary through the
life span of the individual
And also across the geographic range
of the species
Diurnal
Patterns of Adaptability
Variation exists through the day
Particularly true for ectothermic
(or cold-blooded) organisms
Their body temperature and thus
sensitivity to a variety of stimuli vary in response to the environment
Not exclusively
Via sunning and a few other
techniques
Ectotherms can usually raise their body
temperature quicker than does the environment and to a greater level
E.g., lizards or butterflies and basking
Abiotic
Extremes and Species Limits
Not just the average, average high, or average low value
that limits species
Usually it is the extremes of the
range that limit species distribution
Even true if these are rare events
Only occasionally lethal conditions
will
be
catastrophic
most extreme cold of the year
the most saline part of the tidal
cycle
For example: the Saguaro cactus
Temperature extreme lethal
Freezing temp.
longer than 40 hrs.
Interactions
between factors
Extremes may also weaken the individual / population
Then other abiotic factors may have
greater impact
Also, biotic factors could be
unleashed
parasites, competitors, or predators
can better have access or kill the organism
Environmental extremes may slow or
stop sexual maturity, or interest in propagation
E.g., Cole Porter tune
Beneficial
Abiotic Extremes
Often needed for existence
Not all abiotic extremes are bad
If a desired extreme is not present, species propagation can
be halted
E.g., many insects use a heavy rainstorm as a cue to mate
Rainstorm may have obliterated most
of the other insect species populations
if they are transported to areas
that get the same amount of humidity, but not in heavy rainstorms, they will
not mate
Common among desert organisms
Extremes often both determine the extent of and make
possible species presence
Biomes
Clearly all of the above factors are important for the
distribution of organisms.
These factors do not occur independently of each other
There are characteristic Biomes within which
organisms / populations / species / communities tend to be associated.
Define Biome?
One definition:
A major regional or global biotic
community, such as a grassland or desert, characterized chiefly by the dominant
forms of plant life and the prevailing climate. (Dictionary.com)
Types of
biomes
Tundra
Tiaga
Coniferous forest
Deciduous forest
Desert
Grassland
High-altitude cloud forest
Rainforest
Biomes,
Abiotic Factors, and
Abiotic factors determine biomes
Abiotic factors determine species presence
Both limits and makes possible
Species presence identifies the biome
Circular relationship exists between these three variables
James A.
Danoff-Burg
Population
& Community Ecology
Actuarial
tables
Actuarial tables = Life Tables
developed by life insurance companies
introduced to ecologists by Raymond
Pearl in 1921
Life insurance companies have a very vested interest in
knowing how long people are going to live
Basis of your car insurance and life
insurance policies
Humans
vs. Other Species
Lots of information on human life expectancy
Little for other animals
Less in plants or other organisms
Consequently
Little is known about basic biology
of most species
Even economically important species
Two
types of life tables
Static
Stationary in time
a snapshot of what is going on in
the population at one time
a population cross-section at a
single time
a.k.a. time specific life table
Cohort
reports the data observed from
following a single generation from birth to death
a.k.a. age specific life table
Differences
in Life Tables
If life expectation improves through time, the static would
undershoot the cohort curve
Improved life expectation can occur
via:
improved health care in humans
improving environment for other
organisms
fewer predators
whatever would improve the quality
of life for the population
Cohorts also tend to be more accurate than statics
if a pops life expectancy is
decreasing, then so will the cohort curve
inverse of above
Deriving
Life Table Data
Most derived from
Survivorship directly observed
follows cohorts through time
Observe age of death
Age structure directly observed
Most useful for static life table
One type of data can be used to derive the others
Types of
data
All of these columns can be derived
from each other
Usually based on the information in
x and nx columns
These are usually the observed data
Factors measured / recorded
x = age interval
often implicitly an interval
this can be determined to be whatever (month, yr)
Nx = survivors beginning at age
interval x
lx = proportion of
orgs surviving to start of x
Dx = number of orgs dying between x
and x+1
qx = mortality rate between x and
x+1 (dx)
ex = mean life
expectancy of orgs alive at x
this is the main interest of Life
insurance Co.
How to
Derive These Factors?
Derive subsequent columns from the data of x and Nx
Dx = Nx - Nx+1
lx = Nx / No
qx = Dx
/ Nx
ex = the sum of all of capital Lx from age x to the last age /
N of age x
Lx
= (Nx + Nx+1) / 2
used only
for calculations of ex
Survivorship
curves
Derived from graphing Nx
against x
usually with a log normal converted Nx
Three main types of survivorship curves
Type I
A low death rate through most of life, until right at the
end of life
Typical of humans and many other vertebrates
Type II
Constant death rate throughout all age classes
Typical of many bird populations & some extremely less
developed human countries
Many other species will have a curve
that is intermediate between Types I and II
Type III
Most Common
Staggeringly high initial death rate, followed by a leveling
off and a constant death rate
Typical of those with mass spawning
sea urchins, many marine fish, most
trees, parasites
Population
Changes
Due to both intrinsic factors of growth as well as extrinsic
factors
Intrinsic
only looking at factors at work
within the species, and not those that operate on the population
e.g. reproductive capability,
physical growth
Models that include this as well are
usually more realistic than those models that do not
Extrinsic
Resource abundance, climate,
competition, etc.
this can be an idealized way of
looking at population growth
If only this factor is considered
Causes
for Population Change
What is a general term for the internal mechanisms that can
be used to cope with changes in some of the extrinsic factors?
Hint: we studied them during last lecture
Answer: Physiological Ecology
Mechanisms for maintaining internal
homeostasis
Lecture 6
Exponential Population Growth
James A.
Danoff-Burg
Population
& Community Ecology
Populations
Change
Derivation of the growth rate of a population
Growth rate is equal to r
A term well get to shortly
What are the general factors that change a populations
size?
They are:
birth (B)
death (D)
immigration (I)
emigration (E)
Relation Between Change Variables
How could we put this into a simple equation?
B and I increase the population
D and E decrease the population
Beginning equation:
N(t+1) = N(t) + B D + I - E
N(t+1) = size of the
population at t+1 in the future
N(t) = size of population
now
Simplification
in Equation
Close our population
No E or I
This is assumption 1 in our model
Well change this later when we discuss metapopulations
New equation: N(t+1) = N(t) + B
D
Rates of
Change
Thus far
Absolute number of individuals born in the time unit
B = # born
D = # died
If we desire a per capita RATE of change
b = B / N
d = D / N
b & d are often called the instantaneous
birth and death rate
Happening continually and
instantaneously
Growth rate
Rate for a closed population is
equal to r = b - d
r has many equivalent names
the intrinsic rate of increase
the instantaneous rate of increase
Exponential
Assumptions
Closed population
No Immigration or emigration
Instantaneosity
individuals in the populations are
assumed to be capable of reproduction the moment they are born
Constant b & d
having more individuals in a
population does not matter
There is no interindividual
difference in reproductive or death potentiality
Can get a partial individual increase or decrease
No time lag
r = The
Malthusian parameter
Thomas Malthus = predicted human
overpopulation
exponential growth = growth of populations when there are
no external limiting forces
E.g., abiotic or biotic like predators, parasites,
competition for resources, etc.
This is an idealized situation
Necessary to provide a null hypothesis
Violations allow us to better understand population
Natural
Occurrences of Exponential Growth
Does occur in nature
Usually only briefly
Examples:
introductions of species into unnatural habitats
ecological release
Could also occur because of human
alterations of the environment
Elimination of natural predators
Differential
Growth Equation
The differential exponential
growth equation
dN / dt = rN
Can also represent this as DN
/ Dt = rN
This is the base of all subsequent growth equations that
well explore
Integrating
Differential Equation
If this is integrated, we get the Nt =No(e)rt
Symbols mean
e is the base of the natural logarithm (approx 2.717)
r and t are multiplied together to get a power to which to
raise e
You can also derive r from the slope of the straight line
obtained by plotting the ln of the abundance data
Doubling
time
A special case of the exponential growth equation
when Nt = 2
(No)
Derivation
t double = ln(2) / r
divide through by No to eliminate it
then take natural log of equation to
get the bolded equation
r tends to be related to body
size
smaller body size θ
larger r (generally)
Smaller body size θ
shorter doubling time
Continuous
Versus Discrete Population Growth
Species do not reproduce continuously
Thus far weve assuming that has been the case
Assumptions # 2 & 5
Continuous vs Discrete Graphs look
different
continuous is smooth,
discrete has serrations like the teeth on a saw
Reproduction often occurs later in
life (assumption 2)
Jumps up when an individual is born
and decreases when an individual dies (assumption 5)
Discrete
Growth Curve
Difference in Equations?
Continuous & Discrete equations are identical
Use a different growth rate term in Discrete growth
lambda (l) is used rather than r
Clear difference in equations
You will always know it is a discrete growth
Has a finite rate of increase
Limited by discrete individuals and
temporal clumping
Properties
of Lambda
l always positive
It is a ratio
May be small, but never negative
l is the ratio of population size
during the next time period to the current time period
Similar
to N(t+1) / N(t)
A Key
Difference
Continuous and discrete models are very similar when
resources are unlimited
Differ tremendously when they are limited
Discrete growth will be highly erratic
Continuous growth will scale itself to the resources
Stochasticity
Stochasticity definition:
A force that produces non-deterministic
models
models that will not necessarily
produce the same result when run repeatedly
All weve done thus far have been
deterministic
Two types of stochasticity
Demographic sequence of births and abundance of different groups in the
population
Environmental external changes
Deterministic
vs. Non-Deterministic
Deterministic models
all that weve discussed thus far
will produce the same curve irrespective of how many times
we run the model
Non-deterministic models
Incorporates the environmental
stochasticity
variability associated with good and
bad years due to resource availability
Incorporates demographic
stochasticity
that there is not an even
distribution of ages in a population
Variability
in Non-Deterministic Models
Incorporated in a population in non-deterministic models
using variance
Use mean population values for r (r with line over it)
Use mean population sizes (Nt
with line over the N)
Substitute these into Nt
= No (e) r t
Mean Values are Key in Non-Deterministic Models
Non-Deterministic
Curves
we get an increasingly erratic curve
Similar to FIGURE 1.3 in Gotelli
From this we determine that
population variance increases with time
the variance of N at time t (Nt)
depends on the mean size and variance of r
high r size and variance --> high variance
if no variance in r, we have the deterministic model
Variance
High variance θ instability
variance in r is greater than 2r (average r) θ the
pop will likely crash to zero & go extinct
E.g., Demographic stochasticity
probability of a birth or death occuring
next in the sequence depends on the relative magnitudes of b and d
variance increases with time, particularly important at
small pop sizes
High Rate
Magnitudes θ Instability
High b and d θ greater variance and elevated
probability of extinction
particularly if both are high
Reason
A population will have much greater turnover of individuals
Increase the probability of chance runs of one sex
Logistic Population Growth
James A.
Danoff-Burg
Population
& Community Ecology
Logistic
growth
Also incorporates density dependent factors in
population growth
Definition: factors that increase in intensity with
increasing population sizes
Density independent growth = main change in assumptions from
exponential growth equation
How would density dependent factors affect growth?
Logistic
Growth Assumptions
Inconstant b and d
Exponential growth assumes constant b and d
Logistic: slowing b and increasing d with increasing N
K is constant
Does not change with increased crowding
Linear and incremental relationship
Increasing N θ decreasing b and increasing d
Logistic
Model
Exponential growth is a special case scenario of the
logistic growth model
When there are no crowding effects
No density dependence
Most density dependence impacts are negative
Logistic
Growth Curve
sigmoidal if starting below K
exponential decline if above K
both settling on K, given enough
time
What would be the effect of increasing or decreasing each of
the values in the logistic equation?
N, K, r (more later)
Logistic
from Exponential Model
Exponential model: DN
/ Dt = rN
DN / Dt
= (b-d)N
Need to modify b and d
Using density dependence constants to account for the
differential effect of crowding
How to do so?
Modify b
b = bo - aN
a = a constant that reflects the populations response to
crowding in terms of the birth rate
The impact of crowding increases
with N
aN will increase with increasing N
If a = 0 then it becomes the same as the density independent
growth birth rate
b = bo
- aN
b = bo
(0)N
b = bo
Modify d
d = do + cN
c = a constant that reflects the populations response to
crowding in terms of the death rate
If c = 0 then it becomes the same as the density independent
growth death rate
d = do + cN
d = do + (0)N
d = do
Defining K
Incorporate these modified b and d into the exponential
model
the Karrying Kapacity
is derived from this substitution
K = N ((bo - do) /
(a + c))
Facts on K
K = the summary term for all the forces at work on the population
a and c reflect the effects of density dependent growth
K = the relationship between unfettered growth and the
effects of crowding
Relating intrinsic b and d to a and c
Deriving
the logistic equation
Logistic growth equation derived from this substitution
DN / Dt
= rN
After some simplification:
DN / Dt
= rN(1-(N/K))
Which part of this is novel relative to the exponential
growth equation?
Novelty?
Everything on the right side in the parentheses is new
DN / Dt
= rN(1-(N/K))
(1-(N/K)) can be referred to as the unused
portion of K
When N < K there is still unoccupied portions of K
Population growth will therefore occur
Equilibrium
and stability
Equilibrium is achieved when:
r = 0
N = 0
N = K
at the intersection of a declining bo
and increasing do is K and stability
Also some trivial situations
K = 0
t=0 (or no time for population
change to occur)
How to
Attain Equilibrium
The population will return to equilibrium if it is disturbed
from it
by decreasing bo
(when?)
By increasing do (when?)
Also can be achieved in an unstable system
where N continuously vacillates
incorporates a different definition
of equilibrium
not constant, but rather consistent ΰ dynamic
equilibrium
If the time lag between an action and its effects on the
population are greatly delayed (more later)
Allee
Effect
Not all density dependent effects are negative
Benefits of Crowding = Allee
Effect
b continues to increase with increased density and/or d
continues to decrease
generally are very species and ecologically-dependent
Usually only true up to a certain density
above a critical value, the regular effects of density
dependence will take hold
Examples of
Allee Effect
Can you think of some?
the evolution of altruism
social facilitation of feeding
predator defense
nest preparation
formation of zebra herds or musk oxen, etc against predators
Time
lags
Originate because there are delays in births deaths, etc in
response to environmental change
Closer to reality: cannot immediately compensate for
environmental changes
This is usually represented by tau
(T)
Most relevant population size is that of the population at a
time in the past (T) - the time lag
Time Lags θ
Oscillations
Effects of
T
The response time (1/r) will be inversely related to r
Larger r, smaller T θ
logically
With large rT
values, get dynamic equilibrium always vacillating
With intermediate rT, get convergence to K
With small rT,
get RAPID convergence to K (essentially get base logistic growth curve)
Oscillations reduce with time (dampened oscillations) until
K is reached or approximated
Then population vacillates about the
mean K
Variation in carrying capacity
With decreasingly stable K, we get
instability in the population
Read the
three Model variations in Gotelli
Discrete population growth
Curves that are closer to reality
Recognize that individuals do not reproduce partial
individuals
Births and deaths often occur clumped in time
Differences between discrete and continuous growth in
Logistic relative to the more minor differences in exponential growth
Stochastic logistic population growth
Lecture 8
Age-Structured Population Growth
James A.
Danoff-Burg
Population
& Community Ecology
Temporal
Changes in Age Structuring in Populations
Cultural
Changes in Age Structuring in Populations
Contextualizing
Today
Most of this section we have already done
when we covered how to construct life tables and the
different types of curves that could be derived from different types of
survival curves
Our goal today is to be able to at least approximate r
weve been assuming this is provided
Will approximate r using the data that we are normally given
Deviations
from previous models
Using exponential growth curve
Changes in assumptions of exponential growth
No instantaneousity
Uneven age distribution in the
population
Another way to say this is that the
birth and death rates depend on the age of the individual
Young do not reproduce
Elderly are more prone to death
Also are continuing to assume that there are equal sex ratios
Changes
from the earlier life table discussion
A few minor terms are different between Life tables and
those in Gotelli
Age classes are defined as the
uppermost age that an individual has in a class range (e.g., a 1-4 class would
be referred to as the 4 year age class
Sx = Nx
as weve been using
New term
gx = probability of surviving to
age class x+1, given that we made it to age class x this may increase or
decrease through time
in contrast to lx, which
can only decrease
A few new
terms that well need today
Bx = #births (raw data) in that age
class
the per class birth rate = bx
a value that is usually given and
differs from the birth rate (b) that weve been using
Generally only use the number of
females and assume that the proportion of males to females is equal
if it is unequal, then we have
skewed estimates of the POPULATION birth rate
but accurate estimates of the per
female rate irrespectively
If bx
= 0 then that age class is incapable of reproducing
Repetitiveness
of Birthing
Parity - births per female through their lifetime
Big bang reproduction or one-time
Semelparous in animals (-parous = Latin
for giving birth from parere)
Monocarpic in plants
Continual reproduction or ability to continually reproduce
through life
Iteroparous in animals
Polycarpic in plants
Relating
this to Life Tables
How would each of these reproductive trends be mirrored in
the tables of b values?
An answer:
Semel- only a b value at one age class
Itero - b values in more than one
Net
reproductive rate (Ro)
Definition
the mean number of female offspring
produced per female over her lifetime, in units of number of offspring
Assumes that the Ro is the mean number of female
offspring per female over lifetime
or by assuming that Ro is
the mean number of offspring per pair of monogamously reproducing species
however the latter is only a special
case
normally we refer only to the number
of female offspring per female
Calculating
Ro
Ro = sum lx times bx
for all life stages
if Ro is > 1, then r is likely to be positive
if Ro is = 1, then r is likely to be zero
if Ro is < 1, then r is likely to be negative
Ro
and r
Ro does not differ from r if there is instantaneousity
with generation time and thus a lack
of instanteousity, they do differ
there is a gross relationship
between the two
Ro, lambda, and r are related
Ro measures population increase as a
function of generation time
If no age structuring of population,
Ro ~ lambda (the finite rate of increase)
however, lambda is a function of
absolute time, whereas Ro is a function of generation time
r = ln(lambda)
Calculating
Generation Time
Need to calculate the generation time to get a more accurate
estimate of r
Generation time (G) = sum of lx times bx times age divided by sum of lx
times bx
r (estimated) = ln Ro / G
accurate to within 10% of r
the more specific r is determined by
the Euler (pronounced oiler) equation
We wont get into here
Discussed in Gotelli
Lecture 9: Mutualism and
Commensalism
James A.
Danoff-Burg
Population
& Community Ecology
Introduction
to communities
What is a community and how do they
interact?
Definition of communities:
A group of organisms belonging to a number of different
species that co-occur in the same habitat or area and interact with each other
Same time, same place, multiple species
Examples:
along the shore of the
those living in one of our sites at
the BRF
those organisms that are living on
each of our faces
How can two
species interact?
Mutualism
Commensalism
Herbivory
Predation
Parasitism
Allelopathy
Competition
Summary:
+ positive impact, - negative impact, 0 no impact
the
active participant or initiator is the first consideration
Types of
Interspecific Interactions
More Later
on Other Relationships
Herbivory
Predation
Parasitism
Competition
Today: Mutualism & Commensalism
Definitions
Allelopathy
When one organism (secretes a chemical that) prevents
another from living near it
Trees, Barnacles, other sessile organisms
Commensalism as a special case of
mutualism
the relative benefits to one party are so negligible as to
be zero
Symbioses
Definition
Two (or more) species live in close association on, in, or
with each other
e.g.: mutualists, commensalists, parasites, and parasitoids
Mutualism
Mutualism - tremendously
understudied field within ecology
Much of the work that has been done
has lapsed into frequent story-telling
all is as it should be
the world is a warm fuzzy place
Equally plausible:
most mutualisms originate by the
capture, slavery, and exploitation of one species by another
Both parties benefit by the current
relationship
one may just barely be benefited by
the relationship
the other party may benefit
colossally by the relationship
Dominance
of Mutualisms
The majority of the worlds biomass
is composed of mutualistic species
trees and their nitrogen fixing Rhizobium fungi in the roots
most corals and unicellular algae in
them
flowering plants and their
pollinators
the huge number of microorganisms in
the digestive systems of many animals
Commensalism as a special case of
mutualism
the relative benefits to one party
are so negligible as to be zero
Extreme Mutualism?
Could say that all Eukaryotes are
the result of a mutualistic relationship
Endosymbiotic origin hypothesis for Eukaryotes
Pressures of parasitism,
parasitoids, and predators have produced more biodiversity
more on this during the next few lectures
Huge diversity of environments and types of interactions
Mutualistic
Generalities
Generalities of mutualists
More applicable as we go along a
gradient from occasional
mutualists to facultative to obligate mutualists
Form an evolutionary gradient
Less frequent θ More frequent θ
Necessary relationship
Eight
Features of Mutualists
Simple life histories, relative to
parasites
Sexuality is suppressed
in favor of asexual budding - e.g. corals
No conspicuous dispersal phase
e.g. mycorrhyzal fungi
Evolution of simultaneous dispersal
events
e.g. Sceptobiini beetles and their
host ants
Eight
Features of Mutualists
Stable population cycles
fewer outbreaks, fewer crashes
Relative to parasites or
predators/prey
Stable proportions of each
individual in the relationship
e.g., ants and aphids
diminishing benefits of ant tending
to the aphids at high aphid densities
Eight
Features of Mutualists
Niche breadth is greater
Best viewed in facultatively mutualistic relationships, when they are and are not living
together
e.g. mycorrhyzal fungi and their
host legumes
legumes can exist w/o them but they
can only live in very few types of habitats
Multiple host relationships are
common
not many are species specific
not as true for obligately host
specific relationships
e.g. lichens and algae are very host
specific
Modeling
Mutualism
Use the logistic growth model
What are three ways in which the
mutualism could be summarized?
ANS:
increase in b
decrease in d
increase in K
Modeling
Mutualism
How could we summarize this in the
logistic growth equation?
Use the original equation
Add a term at the end that summarizes the beneficial effects
of the presence of the second species on the first
Modeling
Mutualism
Logistic Equation as basis
DN / Dt
= rN(1-(N/K)) + AM M)
Explanation of terms
A(alpha) = the beneficial
conversion of the presence of M individuals into N individuals
(X) = time with
which mutualistic relationships are formed between
the species
M = density of the second mutualist in the population
Extending
the Model
For the second species
Put equation in terms of benefits to species 2 with
increasing population size of species 1
Use Alpha () for impact of species 1 on species 2
Increasing any of the values above θ mutualism
Well get to another way to model
this growth at the end of the competition discussion next time
Lecture 10: Models IV - Competition
James A.
Danoff-Burg
Population
& Community Ecology
Competition
Two types of competition:
Resource (or exploitative as Gotelli calls it)
directly takes away some of the
limited resource
also includes pre-emptive competition
Interference
an individual or population directly
reduces the competitive ability of a competitor
Competing
Competitions
Another way to split up competition:
Indirect
resource or exploitative and
pre-emptive
Direct
interference
competitive exclusion
when one species outcompetes
and excludes another from their habitat
Lotka and
Volterra
The creators of the Lotka-Volterra Competition Model
Independently derived the competition model in 1925 and 1926
respectively
The
Model
Similar to the brief model that we composed for the
mutualism relationships
Based on the logistic model
Competition Equation
DN
/ Dt = rNN
(1 - (N + αM)
/ KN)
DM / Dt = rMM
(1 - (M + βN) / KM)
Symbol
Explanations
α (alpha)
per capita effect of species 2 on the population growth of
species 1
relative to the impact of intraspecific competition within
species 1
β (beta)
the per capita effect of species 1 on the population growth
of species 2
relative to the impact of intraspecific competition within
species 2
Model
Novelties
Intraspecific vs. Interspecific Competition
Low α (or β) value (< 1)
θ
Greater Intraspecific competition than interspecific competition
Large α (or β) (>1)
θ
Greater Interspecific competition than intraspecific competition
α (or β) = 1
each individual added of the same or
different species would have equal effects on the population growth
Intraspecific
Competition
Usually relationship is asymmetrical
Reflected by unequal β and α
Example
If α = 4 and β = 1, how else could you
state this?
Equilibrium
in the competition model
Derivation
set DN / Dt = 0, then simplify.
Result: Nt = Kn αM
Interpretation
Kn is diminished by presence of M
individuals
αM represents the amount of the
environment that is consumed by M individs, in terms
of N individs
If α = 0, then it reduces to N = K - 0, as in
the logistic equation
Graphical
representations of
the model
These equations allow us to determine the equilibrium for a
given community
they dont allow us to tell how stable is that equilibrium
nor do they tell us much about the dynamics of the
relationship between the species
State-Space
Graph
Y axis is sp. N
X axis is sp. M
the combination of abundances of sp. N and M are graphed out
This graph shows all the possible joint abundances between
the species
Still doesnt show any stable points
Time and
Stability and the Graph
Now let's incorporate time and stabilty
to get a linear isocline for an individual
species, which shows the stable point for all communities
This shows the set of abundances for which the growth rate (DN / Dt) of
one species is zero
Isoclines
The isocline ONLY shows the
stability for species N
The point at which N has a zero value (directly on the X
axis) is Kn/ α
roughly equivalent to the maximum number of M individuals
that Kn could contain
Isoclines
If we move off the isocline, we
must return to it
only by moving vertically, as we are only noting change in
numbers of sp. N
If we move the species off the isocline
and then move back to the isocline
single species movement vector
If we plot another line that shows the isocline
for sp. M, we have two lines plotted on the graph
Joint
Species Competition Outcomes
We get four joint species competition outcomes
after plotting onto the state-space graphs the individual
movement vectors
the resultant summed community vector for all unique
quadrants in the graph
Four
Outcomes
Competitive exclusion of N
Competitive exclusion of M
Stable coexistence
Unstable coexistence
Competitive
Exclusion
Competitive exclusion of sp. N by M (M is a higher and
parallel line to N)
Competitive exclusion of sp. M by N (N is a higher and
parallel line to M)
Stable
Coexistence
Coexistence and stable equilibrium (crossing lines,
with Kn / > Km
and Km / > Kn)
Results in lower equilibrium values for both species
Unstable
Equilibrium
Outcome depends on initial
conditions
Produces an unstable equil
Crossing lines with the Km
value above the Kn/ value
Unstable Equilibrium cont.
If the combination of the two species are in either the
right top most or left bottom most quadrants then the two spp. would both go
toward the equilibrium point
If not, then the community goes to
exclusion of the other sp.
Perturbation θ Movement off Isocline
θ exclusion of one or either of the species
Stability is uncommon
environmental or demographic
perturbations
Predation I
James A.
Danoff-Burg
Population
& Community Ecology
Overview on
Predation
Broadly interpretation:
Herbivory - predation on plants
Carnivory - predation on animals
also normally thought of as predation in a less refined
sense
Parasitoids - predation on animals
whereby one animal kills the other and lives on or inside of it
Parasitism - similar to parasitoids,
but do not kill the host
Cannibalism intraspecific
predation on animals
Predation
and Selection
Predation (in a broad sense)
pressure
How to detect?
Exclude the predator
Observe the growth rate of the prey
population
e.g., rabbits into
e.g., Herbivore exclosure fences
e.g., Parasitism -the sea Lampreys in the
Predation (in large sense) forces θ
huge effect on prey population sizes
Prey
Responses to Predation
How could prey respond to predation?
Some possibilities
Chemical defenses
Fighting back
Camouflage
Escape (better fleeing)
Mimicry (Batesian and Mullerian)
a. Batesian-
mimic is not dangerous, but model is
b. Mullerian
- model and mimic are dangerous and often are aposematic
Aposematism (warning color)
Posturing (pretending to be something you are not)
Dilution effect (schooling or masting)
a. Predator satiation effect
Mutualism with another org for protection
Physical Defenses
Acoustic startle response
Feigning death
Prey
Responses to Predation
Categories not mutually exclusive
Multiple responses can and do
simultaneously occur in the same species
Majority of predation is focused on
non-random sample of prey population
Usually the weakest, most ill,
youngest, and oldest
Coincidentally
(or maybe not, from the perspective of natural selection)
Least able to contribute to the future reproductive ability
Most defenseless of the prey population
θ
benefit of predation for species
Prey
Responses to Predation
Strong predatory influence
Effect the population structure of
prey
e.g., Dingoes in
When present θ
many fewer prey juveniles than adults
Juveniles are 5% of pop
in comparison to when dingoes are
absent
When dingoes are absent
Juveniles are 55% of pop
How would this influence prey
population?
Life-dinner
principle
Why are
prey usually one step ahead of their predators?
Coevolutionary arms race
Between predators and prey
Repeated compensatory change between
the two species
Changes occur in response to changes
in the other species
Example:
Canadian snow hare and
Co-Evolutionary
Arms Race
Three assumptions of the
relationship between the two species
Species must be closely tied
together
Prey driven
Prey changes precede the predatory
response
Predator-Prey
Cycles
Strength of
Predation Pressure
Impact on prey population cycles
Oscillations
Occur when there is a lag time
between the activities and course of the prey and those of the predators
High predation pressure θ
permanently oscillating or at least dampening oscillations
Consequently, predators may
occasionally peak above sustainable prey levels
intermediate predation levels lead
to cycles that damp out quicker
predators always are below the
levels of the prey
Low levels of predation pressure
Possibly due to broad predation
preferences
Lead to no oscilations
and steady Ks
Predators are always well below the
levels of prey
James A.
Danoff-Burg
Population
& Community Ecology
Predation
Model Applicability
Easily generalizes to all the remaining two-species
community interactions
Parasitoids
closest to predation
need to allow for multiple
predators consuming a single prey and having a long-term association
Herbivory
non-lethal, multiple predators on
the prey
Parasitism
would be most similar to herbivory,
given these modifications
Predation
Population Growth Assumptions
Prey (V) is limited largely by predation
Prey and Predators (P) encounter each other randomly
Predators are prey specialists and depend on them for their
food
Predators can continue to expand their prey consumption rate
without limit
Predation
Population Growth Assumption 1
The prey is limited only by predation
In the absence of predation
the equation for prey population
growth would be the exponential growth equation (dV /
dt = rV)
If predators were present
need to incorporate a term to
summarize the detrimental effects of the predator species being present
dV / dt = rV - αVP
α * V * P
Losses due to predation are proportional to the product of
predator and victim numbers
α is the encounter rate of prey by predators
α is not the same alpha as in the competition
model
Solve for equilibrium (dV / dt = 0)
simplified, we find that the
equation for the Prey would simplify to P = r / α
Only in terms of the PREDATORs
Paradoxical?
the equilibrium solution is set by
the number of predator species nearby
A logical consequence of assumption
1
Predation
Population Growth -
Assumption 2
Predators and prey encounter each other randomly in a
uniform environment
There are no hiding places for prey
Prey are dispersed randomly
Prey are not clumped together into refugia
Predation
Population Growth Assumption 3
The predators are extreme specialists
Feed on nothing else other than the
prey
Predator growth equation
The inverse of the above prey
population growth equation (at equilibrium) would be dP
/ dt = -qP
q = per
capita death rate of predators
What would happen if we eliminated the prey population?
dP / dt = 0
Therefore: the predator pop declines exponentially as the
prey population declines
Expanding
the Predator Equation
Include both the positive and negative effects of the
influence of prey abundance
dP / dt = βVP - qP
β is the conversion efficiency with which
predators can convert prey into more predators
Predator
Growth Equation
dP / dt = βVP
- qP
Positive growth only occurs when the prey is present
as the first term on the right side
of the equation would be zero if not, and the predator population would decline
to extinct
Equilibrium situation (dP/dt = 0)
Simplified, we would find that the
equation for the Predators would simplify to V = q / β
Paradoxical?
the equilibrium solution is set by
the number of prey species nearby
Logical consequence of assumption 3
Responses
of the predator population to the prey pop
The functional response of
the predators to the prey pop can be summarized by αV
More encounters θ more predators
The numerical or growth response of the predator population
to prey abundance is βV
More efficient conversion of V θ P,
more predators
Prey
Population Growth Assumption 4
Predators can continue to expand their prey consumption rate
without limit
Unrealistic, but necessary
It is a consequence of the mathematics and graphs
More on this in a moment
Prey
Isoclines
Prey isocline - flat horizontal line with Y
intercept = r / α
Prey abundance is determined only by predator abundance
Prey Isocline
At the Y intercept, there are no prey
if we look at this value on the Y
axis, how could we explain this situation?
The growth rate of the prey is equal to or less than the
handling time of the prey by the predators
r / α = the number of predators that control the
growth rate of the prey population
K for the Prey?
There is no K value
Assuming exponential growth
Isocline equation: dV
/ dt = 0
Draw the movement vectors only horizontally
If we were to disturb the prey pop away from the isocline
Isocline = negative sloped line that has
been elevated to unlimited K
K =
infinity
Prey
Movement Vectors
Above isocline
prey moves to the left
more predator individuals than the
prey population can sustain
so it decreases toward zero
Consequence of Assumption 4
Below isocline
prey moves to right
fewer predators present than can
control the prey population
the prey population increases
without bound
Consequence of Assumption 1
Predator
Isocline
Vertical line with X intercept equal to q / β
Predator
Isocline
A logical consequence of equilibrium
the predator population: determined
by the number of prey
Equilibrium = critical number of
prey necessary for the predator population to exist
A suggestion
Think of this line as a sloped line
a positive line with K at the end of
it at infinity
predator population increases with
the prey population
Predator
Movement Vectors
Draw only vertical arrows for the predator population
Displace the predator population off
the predator isocline
Possibly by augmenting or decreasing
the number of predators present locally
Movement is up to the right of the line
Movement is down to the left of the isocline
The equation of the isocline
dP / dt = 0
same as all other isoclines
Graphing
the Two-Species State-Space Graph
Sum community movement vectors differ in each quadrant
Result: a smooth continual ellipse that flows around counter
clockwise
Summary
Community Movements
Summary ellipse
adheres most closely to the prey isocline
prey drive the system
if we have no prey, we have no
predators
The predator population peaks when
the prey is at its midpoint and vice versa
Peaks of the predator and prey pops
are off by a quarter cycle
victim numbers are usually well
above predator numbers
because of the reduction in biomass
as you go up the food chain
Extinction
Probability
High initial population sizes θ Increased probability of extinction
Use a high initial value of any of the populations
Closer to the axes
Increase the probability that one or the other of the
species would go extinct
As long as the isoclines do not change
Paradox of enrichment
Amplitude
of the cycle
Amplitude is determined by the starting points of the ellipse
Consequence of initial population sizes of V and P
Constant ellipse shape
merely expanded outward accordingly
so that each point would be on a similar shaped ellipse
Period
of the Cycle
Period is the speed with which the populations cycle from peak to
peak
Increase r and q
these determine the speed of
population cycling
Equation of the period is roughly = C ~ 2π / (Φ(rq))
Approximating
reality with the model
Incorporating a victim carrying capacity
Prey do not generally grow
exponentially
Usually the isocline
has a negative slope and a K
Rather than using the (1-(N/K))
Substitute a constant (c)
density of the prey population = cV2
New equation:
dV/dt = rV - αVP cV2
intersection point of the X axis =
r/c
r/c ~ carrying capacity of the prey
population
c (or unused portion of K) = Stability
c stabilizes the interaction to static equilibrium
Missing now:
Dramatic oscillations
Cycling
Making
the predators satiatable
One of our 4 initial assumptions
Modification: predators have also an upper limit
A novel plot: n / p / t on Y, V on the x axis
Lotka-Volterra
predation = Type I response
Unrealistic: predators do not eat
without end
predators are limited by handling
time
Predator
Satiation
Modify the response to reflect these realities
Type II response
Need handling time (h) incorporated
Need satiation point = maximum
consumption ability
Satiation point = 1/h
A.k.a., the maximum predation rate
How predators respond
Functional response
Differing prey abundance consumption
Search
Image and Handling Efficiency
When a predator first begins to feed on a new prey item
They are slow in recognizing the new prey
They are clumsy in handling them
Eventually improve with experience
Type III response
Prey switching is common
Impacts
of Functional Responses
Reduced predation selective force on the prey at higher prey
densities
Implications of masting or prey
aggregations
Implications for controlling prey population cycles
important for
biological control
Predator
Functional Response Curves and Biological Control
Species with Type I predator responses are best
Low to intermediate densities, the Type II would be useful
To a lesser degree Type III also at intermediate levels
Additional
Focus on the last few sections in Chapter 6
The Paradox of Enrichment
Incorporating Other Factors in the Victim Isocline
Modifying the Predator Isocline
Wont discuss these in class
Predation III - Parasitism and
Herbivory
James A.
Danoff-Burg
Population
& Community Ecology
Changing
Various Factors
What factors can change in the predator-prey cycling?
dP / dt = βVP qP
dV / dt = rV - αVP
Any!
r, α, β, q, V, P, etc.
We briefly talked about this earlier
Review and expand upon this now
Increasing
r
Example: annual grass is suddenly able to reproduce at twice
the rate it was previously
increase r θ
increase of prey replacement rate
number of herbivore species that the
patch can produce in any given time span is increased
Effect of this on the K of V?
Handling time and ability of P to
consume limitlessly has not changed
K of V would not increase
predators can consume the same
proportion of prey
Type I predator response to prey
Increasing α
What would be the effect of predator search efficiency
You would decrease the number of V
throughout cycling
Prey would be found and killed
quicker
Leads to a fewer P throughout cycling
Increasing β
Make the predators better at converting prey resources into
other predators
Increasing predator efficiency
Making the predators better
predators
More efficient θ greater β
more instability and greater
vacillations in the populations and lower K of V
Increasing
q
What would be the effect of increasing the q of the
predators?
Predators die quicker
Maintain same overall consumptive rate
Predators get more food per individual
Situations
unique to Herbivory or Parasitism (Parasitoidism)
Beneficial herbivory
Can herbivory actually be good for the plant?
Supporting data: plant growth tends to increase following
herbivory
Houseplants
Often get overcompensation for
the damage
The plant grows more vigorously or
reproduces with greater fervor than it would without the damage
Other
Support for Beneficial Herbivory
Increased seed production from additional terminal branches
being added to the tree
Usually only the terminal branches
of a plant flower & produce seed
Mangrove trees
more aerial roots form when the main
branches were infested with herbivores
θ
greater stability in the shifting soils of the estuary in which they live
Modeling beneficial herbivory
increase r
Leads to above changes with
increasing r
Evidence
Against Beneficial Herbivory
Lack of statistical power
Not really been demonstrated that it
is because of herbivory
Could be because of mere removal of
tissue (by wind, etc.)
Sampling bias?
Possibly plants that are chosen by
herbivores do this independently of herbivory
Comparing Azaleas and Grasses
Reduced production of plant
secondary compounds
Comparing two different groups of
plants
1- slower growth because of
increased secondary compounds
2 - faster growth, higher herbivory,
and decreased secondary compounds
Parasite-Induced
Behavioral Changes
How do parasites differ from the other examples weve been
discussing thus far?
many many individuals jointly can
feed on the victim (similar to herbivory)
host doesnt (usually) die (similar to herbivory)
parasite lives inside its food source (unique from all
earlier interactions)
Parasite
alteration of host behavior
Example
snails infected with a worm, climb aquatic vegetation,
apparent to avian predators, snail cant retract body because eye stalks are
swollen
A question for thought: What impact would this change have
if the parasitic worm had no detrimental impact on the avian predators?
Lecture 14
Causes of Population Change
James A.
Danoff-Burg
Population
& Community Ecology
Key
factor analysis
How would you determine the most important factor regulating
a population?
What would regulate the population?
The biotic and abiotic factors that we have discussed thus
far
Predation
Parasitism
density dependent effects
fungal infection
thermal gradients
etc.
A Possible
Way to Answer the Question
Before and after a force
We can measure the impact of a force
by measuring the population before the presence of the force and also then
after it has been present
This is usually summarized as lower case k for each force
The difference is usually expressed in terms of log Nt - log N(t+1)
Nt = number of individuals in the
population before the force was introduced
N(t+1) is the number
after the force
once you have factored out all the
other forces
An Overall
K Value
Summarize all the effects into the overall ktotal value
ktotal = k1 + k2 + k3...
The key factor
The force that most strongly affects
the survival
In reality
there will usually be multiple
factors that together determine the population size
ecologists usually refer to the most important
one.
Key factor calculation
k = Log10
Nt - Log10
Nt+1
Example:
Colorado potato beetle
Example:
Colorado potato beetle
Strongest factor is by far emigration during the female x 2
stage
sex ratio was female biased
females left the population?
Larger k values = greater impact
k ~ 0 θ chance?
k > 1 θ
important factor
k Values
Limitations
Do not tell us the relative
importance of each factor as determinants of the year-to-year fluctuations in
mortality
only that a factor is important in
determining population size in the current year
not in determining CHANGES in
population size from year to year
Longer-term calculations
Use average k values
Example using 10 years data in the
case of the Colorado Potato Beetle in BHT
In sum:
K values θ
quantifying relative strength of influences on population growth &
mortality
Populations
Control
Definition of terms
Top-down control
limitations on population growth
imposed by those higher up in the food chain
e.g. predators on prey, herbivores on
plants, parasites on host, etc.
Bottom-up control
controlled by biotic resources
e.g., prey on predators, etc.
Just the inverse of the above
Population
Control and Models
Previous models
Both competition and predation models have included both
top-down and bottom up controls
We were simultaneously looking at the reciprocal effects of
both on the other
Example
Progressively increase numbers of trophic levels in a food
chain
Example
Conclusions
Top levels of the system
controlled only by their supporting resources and
intraspecific competition
Lower levels of the food web
change depending on the number of trophic levels in the food
web
Cascade downwards from top consumer
Alternating impacts
Simple
Trophic Systems
Few trophic levels are rare
Found mostly in island ecosystems
e.g., tropical
plants (many species)
giant tortises
are found
no predators on the tortise
More
Complex Trophic Systems
More common
Food web
many trophic levels
more relationships among the species in the web than a food
chain
A Quick
Summary
Stiling has an excellent review of the
conceptual models testing these ideas for natural environments that are very
complex
most of them try to explain how ALL
communities function across the entire planet
Probably unrealistic
More likely: have multiple models
that are applicable to different communities
Determine the conditions under each
model will be most applicable
Lecture 15
Metapopulations I
James A.
Danoff-Burg
Population
& Community Ecology
Metapopulations
Definition
a group of populations that are
linked by migration
A species = sum of all its metapopulations
Limited migration between the populations
thus far weve been ignoring the
influences of neighboring populations by assuming that migration was
non-existent)
Metapopulations
in Context
Individual population changes = the
best indicator for the entire species?
the assumption of no migration &
closed populations is not that far off
Determinants of migration rate
Vagility
Size of home range
Prevailing environmental conditions
do they have to move
Density of the local population
do they have to move?
Migration
Rates
Migration is often on the scale of very few individuals per
generation moving between populations
Would this be sufficient to affect the genetic composition
of a population?
Depends in part on the population
size
Small pops θ
very important
Uniformity
in Population Growth Models
Uniformity in distribution in models
Assume that all individuals are
equally distributed across space
Assume equal probability of survival
in a diversity of
How can we now introduce heterogeneity into the environment?
Patchiness
of Metapopulations
Introducing Patchiness
The world is hardly a salad bar for
herbivores
most species have only a few host
species they can feed on
A parasite would look at each of the
hosts as they wander around the landscape as mobile patches
Other animals (non-hosts) are
inhospitable territory
Scaling
Definition
smaller organisms view the world
more fine grained
larger organisms view the world as being
more coarse grained
Example: the leaves on a tree
Each is a novel unexplored territory
for aphids
when one individual invades a new territory, it could grow
exponentially
For an herbivorous mammal
each tree would only be a small portion of the resources
that it would need to exist
Organisms interact with the world
differently
And differently in different
habitats
Novelties
in the Metapopulation Model
Migration
In past, no migration was a viable
assumption
True for huge population sizes and
isolated populations
In reality
Most animal species migrate, and
nearly all plant species disperse either their seeds and/or their pollen
thus you do have to account for
metapopulation dynamics
Seque
We do not consider the population
size, only its persistence
Dont distinguish between size of
the component populations OR cycling OR constant populations
Building
the Model
We have only two possible states of the pop
local extinction - 0
local persistence - 1
Seque also up in scale
No longer concerned about the fate
of individuals
Also geographically, we are looking
at a larger area
Building
the model
Scale of Regional vs. Local extinction
Local extinction much more likely
than extinction of the metapopulation
An analogy: a single individual
dying and the entire population going extinct
Probability of extinction of a
population
pn = 1 - pe
pn = probability of local persistence
pe = probability of local extinction
Example: 1 - 0.8 = 0.2 = pn
Multiple
years
Mathematically
(1-pe)n
could also be (1- pe)(1-
pe)(1- pe)(1-
pe)...
repeated n times
where n is the number of years
Probability of regional extinction increases tremendously
across years, with high persistent local extinction probability
Example: if pe
= 0.8 for 4 years
pn = 0.04%, almost no chance of
survival
Probability
of Extinction of a Metapopulation
If we have two pops, we have two independent probabilities
of extinction
We multiply the probabilities together and then subtract
that resultant value from 1
1 - (pe1)(pe2) = Px
(1 - 0.8*0.8 = 36% Px)
Px = probability of regional
persistence
This equation could be expanded
indefinitely for whatever number of pop probs you
have
Expanding
the Concept
Special case
when all the pe
are equivalent
you could simplify the above
equation as 1 pe2 = Px
Figure 4.1 in Gotelli
calculate all possible Px values for all component populations
assume that all have the same
extinction probabilities
Px increases logistically as the
number of n increases
With multiple populations
e.g., pe = 0.8, Px goes from 20% (1 pop) to 36% (2 pops) to 41%
(4 pops) to 83% (8 pops) to 97% (16 pops)
Compare with pn = 0.04%
for 4 years for a single population
Metapopulations spread the risk of extinction around
An Aside
on Empty Niches
Can there be unoccupied niches?
Avered by G. Evelyn Hutchinson
niches do exist independently of
whether the animals exist to occupy them
Others would say that in fact the
niches do not exist until there are animals to occupy them.
A lack of herbivory at the tops of
trees Ή empty giraffe niche
Only means that large leaf herbivory does not occur.
As an extreme example
Think of the niche as existing in extreme environment
Define that same niche in the
the giraffe niche is still unoccupied, and will likely stay
that way
Building
the Model Parameters
f = fraction of pop sites that are actually occupied
f = 1 when
all sites are occupied
I = immigration rate or colonization rate of novel patches by
the organism
analogous to B (number of births)
E = extinction (or emigration) rate
I and E are not exactly the same as
true immigration and emigration, but comparable
pi = probability of local colonization
This probability can be empirically derived in part from Key
Factor analysis
we would also need information at each life stage as to the
organisms vagility
pi also depends on f
high f values θ
increase the probability of colonization
Simple
Metapopulation Model
Equation: df / dt = I - E
Comparable to the initial
exponential growth rate model (dN / dt = B - D)
Assumptions
there is no influence of regional
occupation of other patches on pe or pi
changing f does not change pe or pi
Metapopulation
Equilibrium
When would the metapopulation be at equilibrium according to
this model?
ANS: when I = E
Complicating
the Model
Let I = pi(1-f)
same thing as saying that the
colonization rate = the probability of local colonization multiplied by the
unoccupied portion of the available populations.
Let E = pe(f)
extinction rate is a product of the
probability of local extinction multiplied by the fraction of the patches that
are occupied
A More
Complicated Model
Incorporating more realistic
assumptions
df / dt = pi(1-f) - pe(f)
Assumptions:
homogenous patches
no spatial structure
no time lags
constant pe
and pi
regional occurrence (f) affects pe and pi
when the more complicated portions are incorporated
large number of patches
we can get a small f and still have some patches occupied
Complicated
vs. Simple Models
Differences in heterogeneity
Simple model assumes constancy
across range
Complicated allows for differences
in pe and pi across range
Island-Mainland
model
This is essentially what we have described above
assume huge and constant propagule
rain from either a separate mainland or by residual local propagules
as in the seed bank for plants
Propagules = new dispersers that
could form new colonies
An example of Source-Sink Dynamics
Lecture 16
Metapopulations II
James A.
Danoff-Burg
Population
& Community Ecology
Island-Mainland
model
This is essentially what we have described above
Basis of the Metapopulation model
assume huge and constant propagule
rain from either a separate mainland or by residual local propagules
as in the seed bank for plants
Propagules = new dispersers that
could form new colonies
Sophisticating
the Metapopulation Model
Internal colonization
this takes into account that most of
the recolonization events occur
because of colonization by other
patches
not included in the base
island-mainland model
Internal
Colonization
Mathematics
need to modify pi = (i)(f)
i is a scalable constant
increases as f increases
reflecting the enhanced probability
that a patch will become colonized when more of the surrounding patches are
occupied
Sophisticating
the Metapopulation Model
Rescue Effect
when we have more patches occupied θ
usually have more individuals in the metapopulation
Consequently θ an
increased rate of migrating individuals arriving at patches
Therefore: "rescued" from
extinction
Rescue Effect
Particularly important with populations that are declining
in size
Consequently θ
extinction is less likely
Mathematics
Modify pe
= e(1-f)
e is another scalable constant that
decreases as f increases
summarizes the strength of the
rescue effect
More
Sophistication of Metapopulation Model
Incorporating even more reality
Using the island-mainland model
Incorporate Internal Colonization
AND Rescue Effects
A More
Sophisticated Equilibrium
Substitute the above two modified equations for pi
and pe
slightly more complicated model
Equation
df/dt = (i)(f)(1-f)
- (e)(f)(1-f)
df/dt = 0 = i
e
Observations
At equilibrium (meaning?)
the equation simplifies back to an
analog of the first metapopulation model
df/dt = i e
Observations
on a Sophisticated Equilibrium
Therefore
when i = e
θ df/dt = 0
no change in f
when i
< e θ negative and declining df/dt
f dwindles
until extinction, if these values do
not change
when i
> e θ positive and increasing df/dt
f will continue to increase in
occupancy until f = 1
When f = 1, all patches are occupied
df/dt goes towards zero
Reading in
Gotelli Chapter 4
Be certain to have read through
the checkerspot
butterfly
heathland carabid
beetle
Lecture 17
James A.
Danoff-Burg
Population
& Community Ecology
Island
Biogeography
Which situation would you expect to have the greatest
community species diversity?
A. Island close to mainland OR B. Island far from mainland
Island
Biogeography
Which situation would you expect to have the greatest
community species diversity?
A. Larger island OR B. Smaller island
Island
Biogeography
Which situation would you expect to have the greatest
community species diversity?
A. Larger island
A. Island close to mainland
Most of the basis of Island Biogeography
Simple model
Generally accepted
Equilibrium
Model of
Builds on
island biogeography, metapopulation
models, the climax community, and succession
More on these in a few lectures
Succession
Changes in community species
composition through time
Climax community
Climax community a.k.a. = fully
mature community
Assumes:
No change in the mean number of taxa at climax through successional time
Turnover rate can be high, with many species
becoming extirpated or colonizing the area
Equilibrial
Model of
Simberloff and Wilson (early 1970s)
Original proponents
Built on the idea of Island
Biogeography with equilibrial addition
First study fumigated mangrove
islands of different sizes & distances from shore
Looked at the species & trophic
types that were present on the islands initially and at climax community
Equilibrial
Theory of
Size of
Species-Area
Relationship
Equilibrial
Theory of
Conclusions
there was a balance between extinction and colonization on
these islands θ equilibrium number of taxa
Upon arrival at the climax
community, There was a constant number of species in the plots from one
observed time unit to the next
The same species were not found in the fumigated
islands, but similar guilds were present
Island
Biogeography Conclusions
Supported main assertions
Assertions re: # species:
Near > Far
Bigger > Smaller
There is an equilibrial number of
species
Spawned a great deal of additional research by many others
One of two main proponents (Simberloff) no longer agrees with equilibrial assertion
An
Historical Note
The ideas as we have presented them were produced in reverse
order
Island biogeography was first suggested in 1963 by E.O.
Wilson and Robert MacArthur
Equilibrium between extinction and colonization
was not formally tested until 1967 by Simberloff and
Wilson
Metapopulations did not come until much later in the
1980s
Formal
Statement of the EIB model
Principles of EIB
The number of species on an island
tended towards an equilibrium value
Ŝ
Steep curve of colonizers initially
gradually levelling
out at Ŝ
Ŝ is determined by the
equilibrium values of E and I
balance between E and I
Ŝ is
Determined by the Equilibrium Values of E and I
I is determined by the proximity to the dispersal source AND
by size of the island
higher for closer to the site
larger islands provide bigger
targets for dispersing organisms = The target effect
E is determined by island size AND by competition AND by
distance to the source population
smaller islands will have larger E
values
those species that are already
present will influence the extinction of the successive ones
smaller distance θ
lower E and increasing persistence
Island Size
and Extinction Rate
Why would smaller island size increase E values?
Smaller islands may restrict the
population sizes of the species present
Smaller pops are much more strongly
affected
by
population fluctuations
think back to the population growth models
Those species that require larger
home ranges or patches for existence would be unable to continue to live
Those species requiring a certain
type of habitat may go extinct because the habitat is not on the island
islands tend to have fewer types of
habitats and niches than do mainlands
Species
Presence and Extinction Rate
Why would those species that are already present influence E
& I?
If no species of a comparable guild
or niche is present, even a weak competitor will be able to thrive
Not so if there is already a better
competitor for the resources already present
Colonization rate decreases with
increasing number of species present on the island
the island approaches maximum number
of species that could colonize it
decreasing probability of any single
species invading
Species extinction rate increases
with increasing number of species present
better competitors win out
Distance
from Source and Extinction Rate
Why would decreasing distance to the source area decrease E?
Rescue effect
those populations that are too small
could still continue to live if there is a constant influx of novel colonizers
or immigrants
I and E
& Ŝ and Ť
Equilibrial Values
Ŝ = Equilibrial number of
species on the island
Ť = Equilibrial turnover rate
Area Effect
& I and E & Ŝ and Ť
Area effect comes about as a consequence of the Target
Effect
Larger islands are better targets
for dispersal propagules
Larger islands have lower extinction
rates & higher Ŝ
Distance
Effect & I and E & Ŝ and Ť
Distance effect
Far islands are rarely encountered
by dispersal propagules
Near islands have higher immigration
rates & Ŝ
Making the
Model More Realistic
Linear I & E is unlikely
Usually nonlinear I and E
Differential dispersal ability θ
Nonlinear I
Most species have a short dispersal
distance
Increasing species interactions θ
Nonlinear E
Species
Area Relationships
Thomas Lovejoy's experiment in Brazil
Background
in late 1970s Brazils government
required owners of tracts of rainforest to set aside at least 50% of their land
as undisturbed areas
Brazil also allowed Lovejoy to ask
farmers to set aside the tracts in regular sized patches (islands)
From 1 hectare to 1000 hectares in
size
Recorded the numbers of animals and
plants that were found after the patches had time to stabilize
Species-Area
Relationships of Lovejoys Experiments
Outcomes
Smaller patches had many less
species after a while
Huge edge effects
drying and light penetration
Patches that had 90% less area had
the species diversity reduced by 50%
Those species in smaller patches
differed from those deeper in the forest
mostly those found in the edges are
those that favor disturbed environments
Implications
of Lovejoys Work
Support the independently proposed idea of MacArthur and Wilson
The equation Ŝ =cAz
holds for the relationship between number of species and area,
Ŝ = # of species maintained by
an island at equilibrium
c = constant that summarizes the
number of species that are usually found in a unit area in a specific ecosystem
c is larger in tropics, and much smaller in deserts
A = area of the island
Z = constant that is determined by
the dispersal ability of the species and the proximity of the island to other
sources
it changes as you go from one set of islands to another the
same species
Implications
of the Species-Area Relationship
Implication
a 90% reduction in area size θ
have a 50% decrease in species abundance
It is in large part on these data
and models that conservationists try to estimate the minimum sizes of land that
they need to protect to preserve the local biodiversity
Other supporting data
Species counts of reptiles and
amphibians in the Caribbean islands
If the log of the number of species
is plotted on the Y axis and the log of the island area on the X axis
We find that there is a staggeringly
perfect straight regression line that can be fitted to the data points using 7
islands in the Caribbean
This is one of the most often
repeated sets of data to support the ideas of IB
Lecture 18
Communities, Species Diversity, and Community Stability
James A.
Danoff-Burg
Population
& Community Ecology
Communities
Defining Communities
How would you define communities?
from earlier in the semester
Definition:
An association of interacting
populations, usually defined by the nature of their interactions or the place
in which they live (Rickliefs)
Properties
of communities
How can we characterize communities?
Some possibilities:
Richness
number of species that are found in
an area
Abundance
Number of individuals
Diversity
An interaction between richness and
abundance
Number of trophic levels
Number of guilds
method or location of foraging
Relative abundance of different
species
Biomes
Definition
a large scale community of organisms
that are usually found together
Defined by their predominant
vegetation
e.g., the Tiaga
is defined by being all evergreens and is at a Northern latitude
what is the dominant vegetation in a
desert?
the tundra?
Biomes and
Communities
Relationship between these two?
It is possible to think of
communities as being formed because of two forces that may or may not
simultaneously be at work
An analogy
community to a body
each species in the community has a
role to play
in other words the species come
together because they need each other to exist
Biomes and
Communities
Advocates of the community as body
often have very sharp distinctions
between biomes (called ecotones)
Distinctions come about because some
of the key species were missing
the divisions between prairie and
deciduous forests are usually really distinct
Critics of community as body
communities are not at all akin to
an organism
the species are often found together
merely because they have the same physiological requirements
Open vs.
Closed Communities
Two theories about the origin and
maintenance of communities
Open communities
do not have clearcut
boundaries between biomes
there is no ecotone present
in these types of communities
advocated by those who view the
physiological requirements as the reason for species being distributed
Closed communities
have strong ecotones and have very
tightly overlapping species distributions
advocates of the communites
as a body view the world as being full of closed communities
General
Types of Communities
Closed
sharp boundaries
abrupt ecotones
distinct associations between
species
Open
boundaries are vague, gradual
little or no association between
species
Ecotones
Definition
Transition zones between biomes
Additional interpretation / theory
Sites of generative forces for
speciation and origin of community diversity
using the paper by Smith et al.
In Science magazine 1997, June 20
(276:1855-1857)
usually been thought of as merely
being the edges of most species ranges
more stressed regions with less fit
organisms occupying it.
ecotones have much lower species
diversity than the surrounding biomes
shouldnt be such important areas to
conserve
However, these regions may be key in
generating new species
Smith et
al. 1997 Ecotones as Biodiversity Generators
Summary of main points
Explored genetic diversity within a
bird species called the little greenbul (Andropadus virens)
in
Had an ecotone between Savanna/Sahel and Equatorial Rainfores
each of which should present
different selective forces on the birds
rainfall and other variables differ
tremendously
Measured and compared the rates of
migrations of birds
using allele frequencies on the
basis of 8 microsatellite loci
measured morphological divergence in
5 morphological characters that have a close correllation
with feeding ecology, flight, and fitness
winglength, weight, tarsus length, upper
mandible length, bill depth
Experimental
Sampling Design
Results
Results
all but upper mandible length
significantly differed between the poplations in the
ecotone versus the forest
generally ecotone had larger values
but not at all between
ecotone-ecotone and forest-forest comparisons
Gene flow between populations
differed and was quantified using the microsatellite
data
Main Result: despite varying Nm
values (effective migration rates), the ecotone-forest morphological divergence
still existed
when Nm got to be huge the difference
was swamped out
Results
Conclusions
Morphological divergence: due to selection for those species
that had the optimal values for each trait
The magnitude of the difference between ecotone and forest greenbul populations
similar to that observed when many
different species were compared
Divergent selection as observed here will often lead to the
production of new species
ecotones may be integral to the production and
maintenance of biodiversity in tropical rainforests
Conservation
implications
For the long-term good, ecotones should be preserved as well
Even though they may have fewer
species in them
Even though there may be lower
abundance values in them
Even though the individuals in them
are usually more stressed
Even though the individuals in them
are usually less fit
Should be retained because they are
such important generative forces for biodiversity
Species
Diversity and Community Stability
Traditionally
diversity = stability
Stability Defined
often defined as unchanging
communities
E.g., the same plant species are
found in a certain patch from year to year to year without change
Alternatively: dynamic equilibrium
the host and parasites fluctuate
continually
without either of them being exactly
at equilibrium
Stability
Terms
Resistance
Lack of change in response to
perturbations
perturb a system & measure
whether the community actually changes at all
measure any aspect of community
before and after the perturbation
Resilience
How much of a disturbance can be
absorbed and still rebound
Two features
Elasticity - how quickly the community returns to the stable
equilibrium point
Amplitude of the resilience - how large of a disturnbance
that the community can take and still bounce back
Stability
Example
Dan Simberloff and Edward O. Wilsons
work on Florida Mangroves
Surveyed all the arthropods on each
of the mangrove islands
Tented entire mangrove islands
Fogged the tree with a biodegradable
insecticide that killed ALL animal living on the tree
Noted the arthropods that returned
to the tree islands over few months
Results of
Mangrove Experiment
Results
Species richness had stabilized
after 200 days
elasticity = 200 days on average
even for total devastation
The actual species that recolonized differed from those that were there before the
fogging
Same trophic structure was present
in the islands following recolonization
suggesting that the proper way to define the stability of
these island communities was by trophic or guild structure
NOT by the actual species present
Rate of turnover of the species on
the islands continues
at a natural rate of 1.5 species going extinct and recolonizing the islands at a background rate
--> therefore the communities on the islands are
always changing
Some
Questions
Do the same species have to be present after a disturbance
as were there before for a community to be stable?
Can the same community ever exist?
Are there multiple stable states of
a community?
Can you ever dip your toe into the
same river twice?
Merely semantic?
What is a community?
Is it defined only on the basis of
the species that are present or on the trophic guilds present or general animal
lineages present?
More later
Stability
and diversity
Traditional view
diverse ecosystems are more stable
Called the equilibrium hypothesis
Bases of this conclusion?
Mostly data on pest species
outbreaks
Data for
Stability & Diversity Hypothetical Relationship
Naturally simple ecosystems =>
much more susceptible to invasions
have huge outbreaks of the pest
species when they invade
Outbreaks on an unnatural and simple
ecosystem tend to be colossal
particularly in agricultural
ecosystems
If these ecosystems are reduced even
further by the application of pesticides, outbreaks are even more common among
pest species
Likely due to extermination of the
predator species
Diverse tropical ecosystems seem to
not have the huge outbreaks of temperate ecosystems
Metaanalysis of more than 40 food webs
more stable food webs tended to have
a higher species diversity than frequently disturbed areas
Conclusion θ Diversity
= Stability
Data Against Diversity = Stability
Generally: reevaluations of the
above data
May be a dynamic
interrelationship between diversity and stability
Data:
Fluctuations of small mammals in tropical regions
tend to be as large as are those found in northern temperate
regions
Agricultural systems are not good model systems
The plants in them are totally artificial
Have never had time to evolve resistences
to the pests
Actually many of the responses were bred out so that the
plants would be more palatable
These are not at all valid comparisons
Diverse tropical ecosystems DO have huge fluctuations in
some pest species
at least in the one system under study
Tropical ecosystems are tremendously susceptible to human
disturbance
Moreso than are northern forests
Not such a strong point given that the tropical forests
soil is notoriously poor
Not a seed bank in the soil for things to regenerate from
Confound soil type with diversity effects here
Data still have yet to be collected to adequately address
this issue
Non-Equilibrium
Hypothesis
Researchers opposed to Equilibrium Hypothesis
Dynamic relationship between
Diversity & Stability
Species composition is constantly changing through time
Intermediate levels of disturbance
tend to produce the highest levels of diversity
High levels of disturbance
select strongly for those species
that have high growth rates and are r-selected
Low levels of disturbance
Competitively dominant species will
rule the roost and chase out all others
Outcome: Intermediate Disturbance Hypothesis
Intermediate
Disturbance Hypothesis
figure 14.9 in Stiling
middle values produce the greatest
diversity values
Lecture 19
Amazon Biodiversity and Community Change
James A.
Danoff-Burg
Population
& Community Ecology
Why Are the
Tropics So Diverse?
Tropics are more diverse than similar temperate zones
More species are found only there than in any other
ecosystem
Approximately 50-80% of all species that are found in the
world are found ONLY or predominantly in the rainforests
Global
Distribution of Biodiversity
Greatest in areas where NPP is greatest
Terrestrial: toward Equator - Why?
Aquatic: near shore, marine
upwellings Why?
With many regional exceptions!
Neotropics
focus of what we are talking today
More than 65% of all tropical
ecosystems are in Central and
The number of species present are
much more than in other geographic locales
Amazon is home to more endemic
biodiversity than any other single location on Earth
Reasons for
Amazonian Biodiversity
Three main categories of explanations
Abiotic Proximate
Ecological Proximate
Evolutionarily Ultimate
Abiotic
Proximate
Explanations 1, 2
Lack of freezing temperatures
extreme values are most important
The area with the extreme values
usually defines the extent of most species
Abundant rainfall
even in the dry season, there is
abundant rainfall
Remove one of the main impediments
to plant growth
some plant lineages CANNOT cope with
such extreme rainfall
die if moved there
Constant rainfall θ
more plant species occupy a given area
leads to our next two mechanisms
Ecological
Proximate
Explanation 1
Increased species packing (= "spatial
heterogeneity theory")
Many plant species in the tropics θ
greater number packed into an area of identical size
Supported by data
More plants θ
more niches of a greater diversity θ greater species packing
Also θ increase in the number of canopy
layers in the forest
sometimes up to five canopy layers!
increase # niches even farther
Ecological
Proximate
Explanation 2
Increased productivity
Increased productivity θ greater species packing
each species is able to accumulate more resources in a unit
time
More important in very productive
areas than in less productive areas
the niches can be much more
fine-grained in high-productivity sites than in low productivity sites
Ecological
Proximate Explanations 3, 4
Increased interactions
this is a consequence of reasons 3
and 4
more species θ
more intereactions θ more opportunities for
specialization
Competition
Predation and herbivory
Mutualisms and coevolutionary
relationships
Larger area
θ
many potential habitats
Many niches to occupy and therefore
for many species to evolve
Ultimate
(or Evolutionary) Explanation 1
Glacial Refugia hypothesis
traditional view: the tropics have been frequently
fragmented into refugia due to glaciation
Jurgen Haffer
1969, a petrochemist and birdwatcher
during the ice ages the temperature
stayed the same, but the precipitation levels plummeted, θ
shrinking of the tropical forest and isolation of the tropics into those areas
that were able to maintain relatively high rainfall
Much work has been inspired by this
suggestion, most of which has of late contradicted it
Glacial
Refugia Tested
Cracraft and Prum
1988
looked at the geographic
distribution and phylogenies among four groups of closely related species of
parrots and toucans
found that barriers were present and
that all four of the groups had similar biogeographic history
However, spots that the barriers
isolated predated those postulated by the refugia hypothesis
were likely due to geological events
that well preceded glaciation
Ultimate
Explanations 2, 3
Stability through time
contrary to what the refugia
hypothesis suggests
intermediate levels of disturbance
produces highest diversity levels
the idea that the tropics have been
stable for a long time period is not borne out by the geological data
There have indeed been many changes
in the tropics
as was emphasized by the refugia
hypothesis
Lower extinction rates
May come about as a result of
stability
those sites that are longer lasting
(stable) will accumulate more species
will become more diverse
Community
change
Succession
Definition: the nonseasonal,
directional, and continuous pattern of colonization and extinction on a site by
populations
A species cannot occur locally without the following
It can reach the site and is within
the roaming distance
The appropriate resources exist in
the site
It is not outcompeted,
over preyed upon, over parasitized
Therefore
the appearance and disappearance of
a species requires that conditions, resources, and/or the influence of enemies
varies through time as well
if they were constantly good, the
organism would not disperse
Succession
Definition
Chronological distribution of organisms within an area
The directional sequence of species within a habitat or
community through time
Shared:
Time
Single area
Successional
Role Players
Early successional species, pioneers, or colonizers
Later successional species
Changes in diversity
Through time
Pioneer
Species
Hallmarks of a pioneer species
Hardy
Fast growing (high photosynthetic
rate for plants)
Good dispersers
r-selected species
Poor competitors
Paradoxical?
These dont put energy into
secondary chemicals
Cant fend off the herbivores as
later arrival will be able to
Early successional trees have leaves
that continue all the way into their canopy, not just at the crowns
Later
Successional Species
Hallmarks of Later Successional species
Good competitors
Shade tolerant (plants)
Larger species
K-selected species
Later successional species trees have their leaves at the
extremes of their branches
Modeling
Succession
Can we predict the future if
?
knew the species that were initially
present
the likelihood that one species
would replace another
How do you know when a community is at the climax community
for a site?
Production = respiration (input
equals output)
organic materials cease to
accumulate
Simple models assume the climax community will always be the
same
Not that viable or naturally
accurate
Succession Across Biomes
Differs depending on the native biodiversity present
Succession in more complex communities
Will take longer
Have more intermediate stages
Have different community members
throughout
Succession in simpler communities
will of course be simpler
e.g., in desert flora, the first
plants to invade will often be those that compose the climax community
Succession
Types by Process
Degradative
Consumption of a finite resource
Allogenic
Requires ongoing extrinsic
environmental changes
Autogenic
Intrinsic factors within the
community
Degradative
succession
Definition
successsion that occurs as a finite resource is
decomposed
e.g., log decomposing, garbage dumps
and discarded trash, dead animal in the forest
Usually occurs over a short time
scale of months or years
The process has a termination point
in that the resource will be completely used up
metabolized and mineralized
Usually the process entails each
species changing the environment enough so that it no longer suits the
abilities of that species and it will have to leave
this is one of the hallmarks of succesion in general and is called facilitation
Degradative
Succession - Example
Medical and forensic entomology
Able to date the time of death of an
individual by the insects that are present on the corpse at the time of
discovery
Changes from locale to locale, and
insect profile must be analyzed and determined specific to each site
Allogenic
succession
Definition
succession that occupies a new
resource that does not become degraded or disappear
the new resource forms as a result of changing external geophysicochemical forces (allo-
external, genic - originated)
e.g., siltation
Generally happens on a very long
time scale (many years)
Siltation by the
As the land becomes more firm,
plants that require a firmer soil (grasses and other herbaceous plants) can
invade and displace the pioneer species (sedges and rushes)
Autogenic
Succession
Autogenic Definition:
succession that occurs as a result
of biological processes driven by ecological forces of species within the
community
generally take place over many many years
e.g., accumulation of litter in a
forest, or peat in a bog, or increase in shading by the canopy
when a newly exposed patch of land
is colonized
Allogenic
vs. Autogenic Succession
How to distinguish between these two?
take a soil core of the site
the pollen from the resident plants
that would be present would differ as you went from the bottom up in autogenic
In allogenic,
the plant community would stay relatively consistent
The distinction between allogenic and autogenic becomes less clear when biological
processes can accellerate the process with their
roots holding the silt in place
Autogenic
Succession Types
by Habitat
Primary
New habitat from barren ground
Mechanisms
of Succession
Facilitation
Inhibition
Tolerance
Facilitation
Definition
Early occupants change the abiotic
environment in a way that makes it comparatively less suitable for themselves
but more suitable for the recruitment of others
Particularly important in sites
where primary succession is occuring
in that the ground usually has not
been able to harbor ANY species at first
e.g., glaciation, new sand dunes,
new lava flows, pumice plains from volcanic eruptions, sudden deposition of
massive amounts of sediment due to tsunamis
Usually takes the form of the early
pioneers changing the soil type
pH, drainage, decomposition of
organic carbon, nitrogen content
Most important of the three
mechanisms of succession & Oldest in the literature
Facilitation
Example
Lichens on a boulder
accumulate wind-blown soil around
themselves
the soil eventually covers them up
and buries the lichens
making the lichens unable to compete
the plants start to grow in the area
now covering the lichens since there is sufficient soil to allow root growth
Inhibition
Definition:
the early pioneers actually PREVENT
later arrivals from gaining a toehold
A mechanism for this in
plants?
Allelopathy
Later species will only gain
admission if the early succession species die off
due usually to extrinsic forces
Otherwise, the early successional
species stay on forever (if no external disturbances)
Tolerance
Definition
Pioneer species do not change the
environment to make it easier for later arrivals to take hold
Later species may still eventually
displace the pioneer species
they may eventually outcompete the
earlier species
James A.
Danoff-Burg
Population
& Community Ecology
Community
Structuring
Building on previous information
Food webs / chains
Top-Down, Bottom-Up control
Community Diversity and Stability
Food web
participants
Producers
Primary producers (Autotrophs)
the generation of plant material by
converting energy into matter
or protists or bacteria by photosynthetic protists/bacteria
chemosynthetic bacteria
light or energy released by the
cleaving of chemical bonds for chemoautotrophs
Secondary Producers
Decomposers
Consumers
Primary consumers - herbivores or planktivores
Secondary consumers - primary
carnivores
Tertiary consumers - secondary
carnivores
Types of
Interspecific Interactions
Direct interactions
Simplest types of interactions
predator eating the prey, herbivore eating
the plant, parasite parasitizing the host...
most readily apparent
Indirect interactions
Interactions that do not have the
species directly encountering each other
E.g., effects of forest
fragmentation size in
Small sized patches and trophic
cascades
All species were affected, even those that could have lived
there
Peccaries and poison dart frogs
Indirect
Interactions
Unexpected consequences
Peccaries and poison dart frogs
Both herbivores
Could be competitors
Decreasing one herbivore leads to a dramatic decrease in
another species
Other species is at least partially
a competitor
normally get an increase in one
competitor when the other competitor is excluded
Strength of
Interactions
Weak interactors
often ignored from the analysis of food webs
would greatly complicated the web
analysis
Strong interactors
tightly woven into the fabric of the food web
Strongest interactor = keystone
species
That species that, when removed,
lead to a total breakdown of the food web
Keystone
Species
Definition
The strongest interactor
That species that, when removed, leads to a total breakdown
of the food web
Originally = terminal predators
Currently
keystone species can be at nearly any trophic level
Relevancy of top-down or bottom-up
control of communities?
A question
can detritivores be Keystone species?
Determining
Keystone Species
How?
Numerical abundance?
Biomass?
Productivity?
Consumption?
A Special
Case
Biomass of the
Inverted biomass pyramid
how can you have more biomass of the
highest trophic level than of the primary consumer level?
And even more of that than of the
primary producer?
How to solve this dilemma?
Lecture 21
- Energy Pathways & Exotic Species
James A.
Danoff-Burg
Population
& Community Ecology
Flow of
Materials Through an Ecosystem
Transmissibility of materials
How is a molecule or ion or chemical passed through the
system?
Are the raw elemental materials of the molecule used up?
Stages in
Nutrient Cycles
Unassimilated
Biomass
Biomass
Biomass
Biomass
Necromass
Materials
Cycled
Nutrients
Carbon
Hydrogen
Nitrogen
Oxygen
Phosphorus
Sulfur
Energy?
Is energy cycled?
Energy
Does energy cycle?
What defines a cycle?
Is energy lost / gained in an ecosystem?
How is it lost?
How is it gained?
Energy vs.
Nutrients
Nutrients cycle
Conservation of material
A lot of new material does not generally
enter an ecosystem
Energy flows
A one-way movement of energy through
an ecosystem
Energy originates by gathering solar
energy
Energy lost through growth and
metabolism
Flow of
Energy
What are some possible fates of the Joule?
Moved through in a unidirectional fashion
Stored in one form of structure of one species and converted
into another
Dissipated during respiration as
heat
Power muscle movement
Pass through the digestive tract of
an animal unchanged
(e.g., roughage in humans)
Organic
Material in an Ecosystem
What goes where? And how do we refer to it?
3 categories in which organic energy can be locked up in an
ecosystem
Standing crop - all organic material in the
system
this is often used inappropriately
and interchangably with biomass
Biomass - mass of living organisms in the
area
Usually expressed in units of energy
(J/m squared) or dry organic material (tonnes /
hectare)
Necromass - the dead biomass
Includes decaying organic matter
humus, litter, peat, organic components
of soil, etc.
Often used to also refer to the dead
parts of living organisms
What are some examples of this?
A
Cross-Biome Comparison
Compare tropical rainforest and the temperate deciduous
forest where is more Necromass?
Using the more restrictive definition of necromass
(ignore the amount in the dead parts of living organisms)
Tropical Rainforest
Dead and decaying together
In general
Temperate forests
Soil and leaf litter layers are
notoriously depauperate in the tropics
farming usually fails in a few years
after the land has been cleared
Slash and Burn agriculture is only
temporary
Two types
of productivity
Primary productivity
rate at which biomass is produced per unit area
Performed by plants, and photosynthetic and chemosynthetic
bacteria
Secondary Productivity
Productivity from digestion of other biomass
E.g., Fungi grow from the energy locked up in the necromass
Primary
Productivity
Expressed in terms of J / m2 * day
Also expressed in terms of dry organic matter (kg/ ha yr)
Gross primary productivity (GPP)
Total amount of energy fixed by
photosynthesis
Loss of GPP
Respired away by the plants
themselves during cell growth and organellar movement
Also lost as respiratory heat (R)
Net Primary Productivity (NPP)
Portion of the energy that is
available to the higher trophic levels
NPP = GPP -
R
Secondary
Productivity
Terms
Also have an R loss
Also have a NPP for energy then available to other organisms
An interesting observation
fungi do not only feed on dead organisms - they also feed on
living ones
Therefore secondary productivity is also applicable to
other heterotrophs
including animals and bacteria
Patterns
of Productivity
Recap of earlier discussions
Which ecosystems are the most important for consumption of
CO?
NPP and CO consumption
directly comparable because
productivity has the by product consumption of CO2 and production of
oxygen during photosynthesis
Patterns of
Productivity
Terrestrial ecosystems
Equatorial tropical rainforest
(highest)
Lowest in deserts and polar regions
Aquatic ecosystems
NPP is highest nearest the shore
light penetrates nearly to the
bottom of the water
therefore benthic life is possible
as opposed to the deeper water
NPP is high in regions on the
continental shelf
rain of organic matter accumulates
on the bottom
bottom dwelling orgs are able to
feed on that
Productivity
and Species Diversity
The most diverse sites are those with the highest
productivity in general
Not a clean trend
Taiga in northern
the biodiversity abundance is
absolutely inverted between the two sites
Exotic
Species
Most of the pest species that we deal with on a regular
basis are introduced species
Examples in
gypsy moth (Lymantria
dispar)
fire ants (Solenopsis
invicta, from
kudzu (from south eastern
pigeons, starlings, house sparrows
(from
part of an effort to have all the
birds mentioned in Shakespeares sonnets in the
Asian tiger mosquito (Aedes albopictus)
yellow fever and dengue vectors in
both illnesses had been eradicated
until recently
Effects of
Introduced Species
Losses due to pests
Definition of pests: any species that is where we want do
not want it to be
Could compete with humans for
resources
Extinction of local taxa
Defoliation at extreme levels
e.g. Gypsy moth population outbreaks
Enhanced erosion due to defoliation
Hybridization with Native Species
Extinction
of Local Taxa
Local extinction b/c of biotic interactions
Subcomponent
lead to a breakdown of the entire community
the introduced species would throw
most of the interactions out of balance
Chaos theory
disrupting a single link in the food
web/chain may lead to the destruction of the entire community
At the least
strongly interacting introduced species would disrupt the
balance among the species in the community
Exotics
Causing Extinctions
House Cat
Possibly the most disruptive singled directly interacting
predatory species
more bird species have gone extinct
due to the house cat in
Exotics
Causing Extinctions
The African Rift valley lakes (
a single species of cichlid fish had
invaded when the lakes first formed and speciated
like mad
more than 300 species were formed
from this single species
In 1959 British Colonists introduced
the Nile Perch
Huge predatory fish has eliminated
nearly half of all the endemic cichlid species
will likely exterminate more than half at completion
Acts to kill the cichlids directly
(predation or competition)
Also indirectly
as the algal eating cichlids go extinct, the algae are
released from that selective force and get huge algal blooms
Leading to eutrophication and
reduction in oxygen levels and light penetration in the lake
Introduction lead to extinction of many other species
never before has a single ill-advised introduction lead to
such a devastating wave of extinction
Will likely lead to the extinction of most traditional ways
of life
Mostly fishing-based in the past
Locals wont eat Nile Perch too oily
Enhanced
erosion due to
defoliation
Example
introduction of rabbits, goats, and sheep to island habitats
often lead to huge population
outbreaks of the herbivores
Decimation of the plant population
The plants usually are lacking the
chemical defenses
These types of herbivores are rare
on island chains
Erosion
Loss of ground cover and no new
species to replace it
Bare ground θ
high erosion rates
Hybridization
with Native Species
Mating with non-conspecifics
leading to introgression
where the genes of one species
overwhelm those of the other species
may result in the extinction of the
species that is being swamped out
Particularly a problem with
subspecies
barriers to interbreeding are absent
as is the definition of subspecies
of course
Also happens between species
particularly in plants
can have hybridization between very
distantly related species
The Big
Question
Why do introduced species outcompete
many local endemic species?
Predicting
Invaders
Early detection and control is key
Limited resources for eradication
How to best focus efforts?
Early?
Late?
What would make for a good invader species?
most reflect adaptability or fecundity
Nine explanations / observations
Rule of
Tens =
Early Intervention
Early detection is key
Cheapest
Least effort
Easiest
Most efficient
Most successful
Prediction is best route
Predicting
Invaders
Abundant in original range
Polyphagous (generalist species)
They generally have the ability to
adapt to novel environments
invaders have been imported to are
usually areas that are not their native range
it is unusual that their normal
associates would be present
Short generation times
Able to make a quick toehold in the
novel environment
Few individuals of the invader
species are introduced
Predicting
Invaders
Genetically diverse
most diverse species have the raw
genetic diversity to cope with novel environmental conditions
Greater adaptability
Able to colonize from a single individual
either a single fecund female or by
a parthenogenetic individual
Important in the instances of a
single or few colonizers
Larger individual body size
if a single colonizer arrives, the
larger individuals are able to survive for a longer time
Larger are more hardy than smaller
individuals
Predicting
Invaders
Associated with humans
those species that opportunistically
adapt to living with us do best
Eurytopic species
Ability to withstand a broad range
of habitats θ ability to invade novel areas
better than stenotopic species
Ecological Release
Introduced species are usually free
from normal constraints from competitors, predators, or parasites
Called ecological release
Inverse of this has been useful for
biological control
Using parasitoid species
Invasions
on
Effects are usually most marked on islands
Island species tend to not be such good competitors
Havent had the necessity to be better competitors due to
enhanced number of competitor species as is on the mainland
Fewer types of niches are actually occupied θ there may be many more unoccupied niches
Fewer number of large keystone predator species
Heaviest
Hit Location
Comprises < 0.2% of total US land area
Has more than 25% of US endangered species
Approximately 72% of recorded extinctions are in
Hawaii has
more endangered species (per area) than anywhere else on the planet
Lecture 22
Biodiversity: Threats & Extinctions
James A.
Danoff-Burg
Population
& Community Ecology
Threats to
Biodiversity
Exotic Species
Reduction of Habitat
Degradation of Habitat
Improving Habitat for Pests
Greenhouse Effect & Ozone Hole
Acid Rain
Pesticides
Enrichment & Eutrophication
Reduction
of habitat for most
organisms
Examples of types
Clearing of land
Agricultural reasons
Long time period needed
Recolonization & Restoration
requires a habitat around that could
serve as the source
Degradation
of habitat
I.E. things are still surviving, but the habitat has become
less viable
Individuals are stressed
Habitat supports fewer numbers of
each species
have reduced K values)
Mechanisms
Pollution (air, water, chemical,
heat, etc.)
Edge effects
changes percolate a good deal of the
way into an environment
changes
includes changes in heat, light, wind,
aridity, soil moisture, exposure, sound pollution from humans, etc.
Improving
habitat for pest species
Invasives
released from inhibition
principles of succession, competitive and ecological release,
and exotic species come into play here
Greenhouse
Effect and Ozone Hole
gas emissions are primarily CO2, NO, CFCs, and
methane,
CO2 is the most abundant
of them all
Short-term effects
Retain heat by preventing the
release of reflected heat from the surface of the earth
CFCs destroy ozone in the higher levels of the atmosphere
ozone screens out dangerous UV rays
that kill most life
CFC is inert
can degrade the ozone layer for many
years
Ozone hole annually occurs over the
Threats of
Global Warming and Ozone Hole
To aquatic ecosystems
kills the phytoplankton, the basis
of the entire food web
Long-term effects
Raising the ambient global
temperature
Raising the oceans temperatures
Melting the glaciers
Inundation of coastal land
Destroying imperiled coastal habitat
Forcing most of the worlds
population to flee inland and degrade other land
Famine, degradation and imperiling
terrestrial ecosystems, etc.
Global destruction
Acid rain
Nitrogen, Sulfur, Carbon gasses
leads to Habitat destruction and degradation
Increases acidity of water
Increases mobility of ions
Decreases habitability of the area
Pesticides,
radiation, metal tailings, etc.
These lead to habitat degradation mostly
most of them are lethal to all life
as we go up the food chain, the relative concentration of
toxins increase in concentration
Biological magnification
If we were cannibals, we couldnt
eat each other
We wouldnt pass the inspections by
the USDA
Enrichment
and Eutrophication
Waste water treatment plants spew the excess water out into
the environment on big rains
also seen by nearly all industrial
ranching operations
leads to an enrichment of the local ecosystem
feces are loaded with lots of
limited resources
nitrogen, phosphorous, and potassium
(NPK - the same as in fertilizers)
usually the limiting nutrients to
plant growth
Carrying capacities of many species in a community are
enhanced
Paradox of enrichment
Increased competition
Dominance by one species
Examples of
Eutrophication
Kelp beds of
cities dumped raw sewage
expansive kelp beds were disappearing
sea urchin that fed on them was exploding
urchins were able to feed on suspended organic material
maintain a huge herbivore population
killed all the kelp
when the sewage dumping was halted, then the sea urchins
stopped growing so quickly and the kelp beds returned
Extinctions
Geologically regular events
Geologic record
work mostly by David Raup at
history of the world into two main
types of extinction events
Mass extinctions or extinction
paroxysms
1. only 5 or 6 of these in addition to the present
one,
2. most familiar one is the Permian-Triassic boundary
that killed most of the dinosaurs and nearly all marine animal life,
3. probably due to an asteroid that hit off the coast of
Background extinctions
1. More regular extinctions
2. Empirically demonstrated from looking at extinctions of families of marine
animals to be a regular event every 16 million years
3. Cause of the regularity could be anything from a statistical artifact to
being caused by a shadow sun that is just outside our view that goes through an
asteroid belt every 16 million years, collides with them and sends the
asteroids to Earth -- wacky idea, but one called the Nemesis Hypothesis that Raup himself has promulgated in book length form
4. The average extinction rate historically has only been around 0.000009% per
year
or in a world with 10 million species (about the total number known to science
at present), one species lost per year
2. Average extinction rate at present
Unknown with certainty, but estimated to be around a species a day in the
1970s, but by now, that rate is estimated to have risen to 1 per hour! (Approx
8,760 per year) Myers 1979 (but estimated without much of a basis)
Appreciably
more than 1 a year at average
We are in the largest extinction crisis ever at
present
1. Greater than any average extinction rate that has been estimated for in the
past
2. Sole cause: us
3. Consequences of past extinction events
For most past events, too little is known
Following
the Permian extinction event that killed the dinosaurs and most marine animals
-- what would you predict had happened, based on what weve been talking about
thus far in class?
a. Extrememly lowered diversity
b. Few species left would be released from competition, predation, parasitism,
etc.
c. These species would explode in abundance (seen in trilobites and other
groups)
d. The species that were dominant at the time of the event would not be so
afterwards, --> lead to a shift in dominant species, a.k.a. Keystone
predator species
1. E.g, dinosaurs were replaced by mammals
e. However, there are many cases of what he calls Lazarus taxa, or those that are greatly
diminished in diversity and abundance after the event, but subsequently
resurrect through geologic time and become diverse again
These
trends were observed because there were land on which this speciation could occur
after the extinction event (asteroid, volcano, etc.)
a. Not the case at present, since we build sterile cities or agricultural lands
or mining pits that denude the land (at least for many years) of viable habitat
- we are therefore reducing the amount of available habitat for life
b. If we continue to have no intention of changing that, there is little hope
for preserving existing biodiversity -- our best hope lies in preserving and
then restoring damaged areas as best as is possible