Lecture 6 –
Arrival and Change

Islands: Ecology, Evolution, & Conservation

Dr. James A. Danoff-Burg

Department of Ecology, Evolution, & Environmental Biology

Columbia University

Today

• Special impacts of small population size

• Sex on islands

• Niche shifts

Content Flow

• Last week

– Macroevolution

• Production of new species from older ones

– Speciation patterns

– All were the consequence of microevolution

• This week

– Microevolution

• Changes within a species

• Not always leading to new species (speciation)

Arrival and Colonization:
The Rule of Tens

Empirical observation – not well understood

Rule of Tens –
Interpretation & Consequences

• Great reduction in number of individuals that make it through each stage

• Consequences

1000 Introduced

100 Established

10 Integrated

1 Invasive

Rule of Tens – Why?

• Possibilities

– Difficulties in biotic factors

– Difficulties in abiotic factors

– Difficulties in dispersal

– Small population sizes

– Stochastic impacts

– Density dependent or independent factors

• In short: Any kind of factor that limits population growth

Relevancy to Populations

• Relevant to

– Numbers of species

– Number of individuals within a species

• Abundance

– Same factors that are limiting the arrival and success of a species

– Also limit population size of colonists

• Thus initial founding population

Population Size and Microevolution

• Selection is most intense on small populations

– More likely to be impacted by chance environmental occurrences

– More likely to have revelation of recessive traits

– More likely to be founded by oddities

• Drift also fastest in small populations

– More likely to have chance fixation of new traits

Small Population Features

• Founder effects

• Genetic drift

• Bottlenecks

Founder Effects

• Islands usually founded by small population size

– Due to either a dispersal or vicariant event

– One or a few events typical

• Genetic composition of founders

– Reduced genetic diversity in founders

• Relative to source pool diversity

– Reduced heterozygosity in island population

– May lead to other changes

Genetic Drift

• Chance changes / fixations of novel traits

– Due to small population sizes

– Revelation of otherwise hidden & recessive traits

• Drift speed

– In large populations (infinite) è miniscule change

– In small populations è greatly accelerated

Bottlenecks

• Founding events: due to small population sizes

– Great reduction in population size

• If bottleneck is long-lived or repeated

• Intuitively obvious

• Bottlenecks don’t always reduce genetic diversity

– If bottleneck is short (one generation) & population rebounds

ècould actually increase genetic diversity

• May serve to break up genetic dominance

• May allow additive genetic variation to arise

– “shaking up the order”

• E.g., sexually selective females need to be less selective

• Could lead to novel interspecific hybrids è introgression

Impact of Small Populations – Summary

• Restrictions to a small population

– Due to small number of colonizers

– Greater influence in less vagile species

• Rely on chance happenstance for dispersal (human-mediated)

• Leads to several genetic changes

– Loss of genetic diversity and heterozygosity

• Founder effect / Genetic drift

– Novel genetic combinations

• Genetic drift / Founder effect / Bottlenecks

– Introgression between species

• Bottlenecks

Colonizers and Sexuality

• Which do better on islands – sexual or asexually reproducing species?

• Which is more likely to colonize islands?

• Are there island-specific trends among sexually reproducing species?

Island-Specific Trends

• Hawaii & New Zealand

– Have high concentrations of dioecious plants

• Hawaii (21% vs. 9% on BCI)

• Relative to mainland

• Dioecious = plants with either only male or female plants

• Is there a reason for this?

– Colonization?

– Establishment?

– Ecological need?

– Expectation: monoecious plants would do better

• More likely to have a single colonizer create an established colony

Dioecy Superabundance

• Arrival Explanations

1. Dispersal syndrome hypothesis

• Dioecious plants are better seed dispersers

– More likely to have fruits

• Establishment Explanations

2. Pollination syndrome hypothesis

• Dioecious plants: pollinated by small generalist insects

• Monoecious plants: pollinated by larger specialists

3. Growth form hypothesis

• Woody, shrubby forms more likely on islands than mainlands

• These lineages are more likely to be dioecious

• Post-Establishment à speciation

4. Enhanced survival hypothesis

• Dioecious more likely to speciate

5. Autochthonous development hypothesis

• Hermaphrodites initially, then selection for outcrossing, anagenesis

More Sexual Trends on Islands

• Parthenogenetic lizards

– More common on islands than mainland

– May overcome founder effects & bottlenecks

• Through polyploidy

• Through greater likelihood of colonization

– a.k.a. rescue effect

• Through introgression and further hybridization

– Introduces genetic variation into otherwise reduced genomes

Niche Shifts

• Many trends common on island lineages relative to mainlands

– Reduction in dispersal ability

– Gigantism & nanism

– Character displacement

– Ecological release

– Other patterns

• Before these: Define niche

Niches

• Definition?

1. The ecological role played by a species in a community

2. An n-dimensional hypervolume

• Huh?

Niche =
N-dimensional Hypervolume

• Dimensions are the limiting factors

– pH, soil type, humidity, temperature, nesting space, mates, parasites, etc.

• Fundamental niche

• Along a series of resource gradients

– Stressed at extremes or extreme combinations

• Empty niches on islands?

– Disharmonic biota?

Niche Shifts

• Many trends common on island lineages relative to mainlands

– Reduction in dispersal ability

– Gigantism & nanism

– Character displacement

– Ecological release

– Other patterns

Reduction in Dispersal Ability

• Conflicting trends

– Excellent dispersal ability needed to arrive

– Poor dispersal abilities common on islands

• Particularly on smaller islands

• Characteristics:

– Flightlessness

– Reduction in wing size

– Poor flight ability

– Reluctant fliers

Reduction in Dispersal Ability: Examples

• Tristan de Cuhna

– 18/20 endemic beetles reduced flight abilities

– Descriptive data only

• British Columbia coastal islands

– 200 in Barkley Sound

– Trend in Asters over 8-10 years

• Initially large dispersal ability

• With increasing age on island è decreased dispersal ability

• Dispersal ability: ratio of seed (achene) to ball of fluff (pappus)

• How does this come about? Why?

Explanations for Dispersal Reduction

• Stronger fliers è blown off island

• Selecting against dispersal ability

• Reduction in predation pressure

• Reducing selection for flight ability as escape mech.

• Energetically costly to maintain those muscles

• Particularly among ground-feeding birds

Both will be more likely on smaller, windier islands

Gigantism & Nanism

• Trends in the average organismal size

– Gigantism: larger body size than mainland

– Nanism: smaller body size than mainland

• Example (Lomolino 1985):

– Smaller mammals tend to be larger

– Larger mammals tend to be smaller

Mechanisms for Gigantism

1. Predation hypothesis

– Absence of large predators on islands

• On mainlands, vertebrate predators remove larger individuals

– Human influence has historically been longer on mainlands

– Ecological release

2. Social-sexual hypothesis

– Greater competition among insular community

– Selects for larger and more showy individuals w/ecological release

3. Competition (Food Availability) Hypothesis

– More competition è larger body size

– In accordance with ecological release

Mechanism for Nanism

• Population and food availability hypothesis

– Reduced resources on islands

• Larger bodied individuals use more resources

– To maintain viable population sizes

• Smaller bodied individuals

– Enabling larger population sizes

– Given a constant amount of resources

Character Displacement

• Examples

– Niche shifts (shorter term ecological displacement)

– Size polymorphisms (longer term evolutionary displacement)

• Results from competition

• Definition:

– When competition causes two initially allopatric species to diverge from each other in some character upon attaining sympatry [or syntopy] (Diamond et al. 1989)

– Components:

• Separate evolutionary trajectories, divergence, dispersal event

Examples of Character Displacement

• Anolis lizards in the Caribbean

– Shifts in perch height & diameter

– Due to competition

– Greatest among closest related species

– Greatest among those of similar size classes

• Hawaiian crickets

– Shifts in song type & diversity when species syntopic

• Sympatric in the same habitats

Character Displacement and Speciation

• Does not necessarily lead to speciation

– May be a mechanism

– Particularly for reinforcing incipient species limits

• Usually only in effect where two species are in contact

– Often short lived through time and through the year

• Remove syntopy è return to original char. values

Ecological Release

• The antithesis of character displacement

– Already discussed

• E.g., as a mechanism for gigantism

– Essentially is a removal of a negative selective pressure

• Predation, competition, parasitism

Ecological Release Outcomes

• Two possible outcomes of release

– Decrease in intraspecific diversity

• Loss of non-essential features

– Flight, defensive traits, expensive camouflage patterning, diverse song types

• Often due to predatory or parasitic release

– Increase in intraspecific diversity

• Removal of the selective forces canalizing the species

• Often due to competitive release

Other Island Syndromes

• Trend to weedy woodiness

• Rodents and the “Island Form”

Trend to Weedy Woodiness

• Common on Hawaii & remote islands

• Lack of true trees

• Probably due to typical colonizing species

• Weedy woody plants: r-selected & better dispersers

– Trees tend to be K-selected & poorer dispersers

Rodents and the “Island Form”

• Characteristics of the rodent Island Form

– Reduced dispersal ability (discussed earlier)

– Greater body size (discussed earlier)

– Reduced aggression

• Possibly due to bottleneck of related individuals

– Positive response to crowding

• Isolation, reduced predation è larger population size

• Few large population cycles = Density Stasis

– Lower reproductive output / individual

• Response to crowding

– Greater life expectancy

• Reduced number of mortality agents

• May be common to other small mammals on islands

Isolation and Population Density

• Population density may increase with increasing isolation

– Example: rodent Island Form

• Common on

– Strongly isolated islands

– Small, but not too small, islands

• Need to be able to maintain persistent populations

• Need to not have many predators

– To reduce the selective pressure on the population

For next week

• Read

– Ch 9 in Brown and Gibson Biogeography – “Reconstructing Biogeographic Histories”

• Write

– Compare 2 species in 2 dissimilar (but somewhat related) taxa with different responses to the same island

– One taxon: increased intraspecific diversity on island

– One taxon: decreased intraspecific diversity on island

– Hypothesize as to why this was the case

• Use the mechanisms discussed above