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Chapter 19
Island Biogeography |
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| Harned Hall 301
- (615) 963 - 5782 |
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Sections
Theory that island community compositions are due to a balance of colonization and extinction
Much early work done on bare islands either new (volcanic activity) or where a catastrophe has removed most or all of the flora and fauna
Many saw succession in the plants that were able to colonize these areas and so islands were viewed as good examples of succession
MacAuthur and Wilson viewed islands as dynamic communities, where the species present were the product of constant invasion and local extinction
Leads to species turnover, where it is difficult to predict which species will persist and which will go extinct when comparing different islands, although the total number of species may remain steady
They developed some rules for predicting the number of species on an island (either total or some subset &emdash say the bird community on an island)
Species number should increase through time, but rate of increase will slow until it becomes zero and the number of species no longer increases (some sort of curve with an asymptote)
Number of species at asymptote is an equilibrium between rates of colonization and extinction
Notice that speciation is not considered - not because it doesn't happen but because the time scale is too short for speciation to have much of an effect
Extinction rates depend on island size
Smaller islands have lower population sizes, fewer refugia, and a higher rate of extinction
Colonization rates depend on the island's distance from the continental source
- More remote islands have fewer colonists arrive than closer islands
- When there are islands between the source (the continent) and the island, these may act as "stepping stones" and may increase the rate of colonization for remote islands
Other rules have been added, as refinements of the theory due to new ideas and new data
When two islands are the same distance from a source the larger island will have a higher colonization rate (the interception arc is larger)
this is the Target Effect (bigger targets are easier to hit)
When an island is close to its source of species, both the rate of colonization and the rate of extinction are affected
extinction rate appears to be lower (or is lower) because a species can be replenished or re-colonized from the source
- This effect is called the Rescue effect
- What happens is that new members of a species already on the island arrive, and are counted as natives (one can't tell which organisms are migrants and which are not)
The new arrivals increase population size and make extinction less likely
When a community's habitat is patchy and each patch is surrounded by unsuitable habitat, then the rules of island biogeography may apply
Habitat islands can be large geographic objects (mountain tops, lakes) or organisms (individual trees)
Mountain top habitats often can be seen as islands
Evidence for island biogeography
Testable hypotheses generated by the theory
A theory is only testable if it leads to a prediction that can be falsified by evidence
Not expected to be linear, but a power function
S = number of species, A = area of the island
If the theory of island biogeography is true, this relationship must be true
This relationship has been shown to be true for many island communities, but not all
- Book gives evidence from reptiles on Caribbean islands, insects on small mangrove islets
However, one expects to find more species with a greater sample size without there being and island effect
- Some continental communities exist in habitat islands, some do not
- We expect the relationship to be stronger (larger positive slope) for continental communities that are confined to habitat patches with poor dispersal between patches
- birds in North America, plants in England are continental situations
- birds and mammals on mountain tops are in habitat patches
- birds disperse long distances
- animals are confined to patch
- animals should have stronger relationship (and they do)
Rates of colonization and extinction, Species Turnover
Difficult to do, as the rates are dependent on observing hard-to-see events
Often when one rate can be measured, the other can not (and both need to be established for the same community)
Simberloff and Wilson
First interpreted this as confirming island biogeography
Later revised conclusion to say that the turnover rate was too low
important way of thinking for conservation biologists
how big to make wildlife preserves
how to arrange smaller preserves around a larger preserve
steppingstone preserves
problem is that this approach is too simplistic
all islands are same type and quality habitat
clearly not true
dispersal is constant over time and among different members of the community
know that dispersal tends to be episodic, often tied to catastrophic events
know that species differ in dispersal abilities
Island Biogeography, dynamic communities, species turnover, Extinction rate, Colonization rate, stepping stones, rescue effect, patches, Habitat islands, Testable hypotheses, falsification, Species area relationship, power function, colonization/distance and extinction/area relationships
Last updated October 26, 2006
