Chapter 19

Island

Biogeography

Harned Hall 301 - (615) 963 - 5782

Sections

Island Biogeography

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)

MacArthur & Wilson's rules

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

  • There is a relationship between island area and the total number of species in a community
  • Rates of turnover can be predicted from the time it takes to reach 90% of equilibrium
  • Number of species (in a community) remains constant on islands
  • Turnover occurs

Species area relationship

Not expected to be linear, but a power function

S = number of species, A = area of the island

  • z and c are both constants needed to fit area data (in m2, km2, etc.) to number of species
  • This can be linearized by taking the log of both sides:

  • In this form, you can estimate c and z from a plot of S versus A

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

  • Fumigated mangrove islets
  • Observed the recovery of the islet
  • Saw rapid recovery to equilibrium number of species
  • Only low rate of turnover, although the rate was not 0

    First interpreted this as confirming island biogeography

    Later revised conclusion to say that the turnover rate was too low

    • Felt that biotic interactions (competition and predation, mostly) were more important than the colonization/distance and extinction/area relationships

Impact of island biogeography

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

Terms

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