Lecture 2
Island Environments
Islands: Ecology, Evolution, & Conservation
Dr. James A. Danoff-Burg
Department of Ecology, Evolution, & Environmental
Biology
Columbia University
Goals of Understanding Islands
Understand
ecological processes
Explaining the distribution and abundance of
organisms
Definition
of ecology from J. Krebs (1972)
Predict
future
Assess the possible future evolutionary progress
Due
to both human and natural processes
Plan for conservation actions in the future
Important Processes
Collectively
θ current distribution and abundance of organisms on
islands
Island formation
Long term environmental changes
Normal abiotic processes
Disturbances
All
influence
How (often, manner) colonists arrive
Whether they succeed in establishing
A Thought Experiment
How
would each process affect:
Colonization
Establishment
Community structure
Endemism
Diversity
Today
Types
of Islands
Modes
of Island Origin
Long-Term
Environmental Changes
Abiotic
Island Features
Disturbances
Discussion
of Whittaker 1995
Types of Islands
4
general categories (from last week)
Oceanic Islands
Over
an oceanic plate, never a part of continent
Continental Shelf Islands
On
shelf, historically may have been connected to mainland
Habitat Islands
Terrestrial
habitat islands, surrounded by inhospitable habitat
Non-Marine Islands
Islands
in the stream
A Caveat
Ecological
/ Evolutionary / Conservation conclusions
Relevancy hierarchy:
Most
directly relevant to oceanic islands
Strongly
relevant to continental shelf islands
Least
directly relevant to non-marine and habitat islands
May need some theoretical modifications to be
relevant
Not
universally applicable
Same features dont necessarily work in
similar ways in different island types
Today
Types
of Islands
Modes
of Island Origin
Long-Term
Environmental Changes
Abiotic
Island Features
Disturbances
Discussion
of Whittaker 1995
Origins of Sea Islands
Plate
Boundary
Seafloor spreading
Volcanism
Plate Tectonics
Within
Plate
Volcanism & Hot spots
Siltation, deposition
Divergent Plate Boundary Origins
Seafloor
spreading
Plate boundaries drive apart what was previously
a part of a continent
Example: New Zealand
Volcanism
Magma wells up between plates
Builds mountains θ islands
Example: Iceland
Convergent Plate Boundary Origins
Volcanism
Convergent plate boundaries
Often
creates volcanoes underwater
Magma
wells up and creates islands
Example:
islands of Indonesia
Convergent Plate Boundary Origins
Plate
Tectonics
Convergent plates
Drive
up crust in one plate
Other
plate subducts
Land
is pushed above water level
Volcanism has diminished (older)
Plates may move transversely now
Example:
Antilles
Within Plate Origins
Hotspot
Origin
Movement of a plate over a stationary hotspot in
the mantle
Hotspot melts holes in the plate as it moves
over it
A linear origin of progressively younger islands
away from the hotspot are produced
Islands
subside due to their own weight with age (seamounts) and erosion (guyots)
Example: Hawaiian Islands
Loihi
seamount is forming now
Hawaii
is 1 myo, Kaui is 8 myo, oldest guyot to NW is 70 myo
Within Plate Origins
Tectonic-control
Origin
Areas of crustal weakness
en
echelon lines
More
or less parallel to each other
Magma from the mantle can penetrate weakness
points
No age progression along island chain
Examples: Canary Islands, Cape Verde Islands,
and Galapagos Islands
Today
Types
of Islands
Modes
of Island Origin
Long-Term
Environmental Changes
Abiotic
Island Features
Disturbances
Discussion
of Whittaker 1995
Long-Term Environmental Changes
Isostatic
changes in relative sea-level
Due to processes intrinsic to the island itself
Eustatic
changes in sea-level
Due to changing volume of water in the sea
Island
climate change
All essentially result in sea-level
changes
Relative Sea-level Changes
Change
the relative elevation of the land surface
In relation to a constant sea-level
Methods
of change
Removal of icecap from land ΰ elevation of plate
Tectonic uplift
Subsidence of the lithosphere
Increased
mass
Ice, rock, or water loading
Decreased
density
Subsiding oceanic volcanoes
Isostatic Subsiding Oceanic Volcanoes
Sink
because of decreased density of collapsing volcanic rock
Coral growth keeps up with island subsidence
Coral
can grow at about 0.5-2.8 cm/year
Older islands have thicker coral beds than
younger ones
Produces
a predictable progression (Darwin, 1800s)
Island with fringing reef
Sinking island with barrier reef
Submerged island with atoll reef atop it
Eustatic Changes in Sea-Level
Isostatic
or eustatic changes often interact
Occasionally produce same biogeographic result
Eustatic
changes in sea-level result because
Glaciation
Moves
water from ocean on land ΰ lowering sea levels
Reconfiguration of the seafloor
Produced
by underlying within- and between-tectonic movements
Enough
to connect Britain with mainland Europe
Climate Change
Impact
of global climate change greatest on islands
Reduction in, or lack of, glacial refugia
Sea-level changes dramatically affect available
land surface
Species
present on islands need to be adaptable
Reduction in genetic diversity of small
populations is problematic
Today
Types
of Islands
Modes
of Island Origin
Long-Term
Environmental Changes
Abiotic
Island Features
Disturbances
Discussion
of Whittaker 1995
Abiotic Island Features
Topography
Climate
Water
Oceanic
movements
Air and water currents
Topography
Islands
of tectonic origin
Larger islands tend to have the highest
mountains
Islands
of volcanic origin
Have disproportionately high mountains for
island area
Expectation of smaller peaks on smaller islands
doesnt hold
Increasing dissection with erosional time
Dissection ΰ population isolation ΰ speciation
Climate
Oceanic
influence is disproportionately large
Result in anomalous climates for the latitude of
the island
More
polar than those at same latitude
At
equator: 1°C less than same latitude
Due
to land mass size & wind
Topography by Climate Interaction
Rainfall
Low leeward
shores ΰ desertlike
High windward
mountains ΰ rainforest
orographic rainfall
Telescoping of
altitudinal zones
Same elevational
stratification, only compressed
Average atmospheric humidity level important
More important
than proximity to sea
Droughts on peaks ΰ temperature
inversions
Immaturity of volcanic soils
Elevation of highest peak (as important as island
area)
Water
Determine human
use & natural composition
Minimum of 10 ha
for maintaining fresh water source
Volcanic &
coral based islands with much fresh water
Porous &
permeable soil
Properties
decrease with erosion (popping bubbles)
Zonation in water
tables
Vadose zone
(above sea-level) fresh water
Ghyben-Herzberg
lens (sea-level to below) fresh & brackish water
Salt water (below
G-H lens) salt water
Rain shadow on
mountains
Oceanic Movements
Air
& water currents
Change through the year & through years
Usually
have some consistency in the movement of air and water
Can
determine the source of colonizing species
Some regularity in patterns
Example:
Mona Island and butterflies origins
62
km2, 46 spp, 9 shared with Puerto Rico
None
shared with Hispaniola
9:1
Hispaniola : Puerto Rico species richness
Today
Types
of Islands
Modes
of Island Origin
Long-Term
Environmental Changes
Abiotic
Island Features
Disturbances
Discussion
of Whittaker 1995
Natural Disturbances
Important
Shorter term changes
More than previously recognized in ETIB
Categories
Climatological
hurricanes,
tornadoes, tsunamis
Volcanism
eruptions,
lava flows
Short-term
reductions in biodiversity
Possible longer-term increases
Natural Disturbances
Definition
Discrete event that removes organisms, opens
space, and enables colonization by new individuals (and possibly species)
Scaling
issues
Relevancy for different species
Structuring
influence of disturbances
Natural systems often structure island
ecosystems
Same sized disturbance will have longer-lasting
impact on island ecosystems