Island Biogeography

The relationship between land area and number of species

Wilson and MacArthur - 1963

• E.O. Wilson and Robert MacArthur developed the Theory of Biogeography in 1963. – They noticed that faunas and floras of islands around

the world showed a consistent relation between the area of the islands and the number of species living on them.

– Also noted a distance effect.– For example, Cuba has more species than does

Jamaica or Antigua. – The relationship shows up all around the world.

Equation

• S=CAZ

• S is the number of species• A is the area of the island• C is a constant• Z is also a constant that is dependent on the group

of organisms (birds, reptiles, grasses, etc.) that are being considered. – Z also depends on whether the island is close to other

source areas (for example, a continent or other islands).

Equation

• More area means more space, more space means larger populations for each species, and larger populations mean a longer life for each species (i.e. a lower extinction rate).

• Small islands are precarious; they have fewer species, fewer individuals per species, and much higher rates of species extinctions.

Their original experiment

• They took small islets in Florida Bay with the goal to test their equation.

• The work was inspired by Krakatau – they cleared all of the arthropods from the islets via fumigation.

Their original experiment

• Results: species began to return immediately and they very quickly recovered their pre-fumigation species diversity (number of species, not necessarily the exact same species as before the experiment).

• Larger islands did indeed support more species and, over time, had a lower extinction rate (species that showed up were less likely to go extinct).

Their original experiment

• Also, they saw the distant effect that they had hypothesized.– Two islands of the same size would have different

numbers of species based on how closely situated they were to other land masses (in this case Florida and the islands of the Florida Keys).

• Conclusion: species diversity was directly related to island size and inversely related to distance from other land masses.

Barro Colorado – Panama Canal

The Galapagos

This is a figure that demonstrates the relationship between the size of the island and the species diversity. Note, as the island’s area increases, the number of species that it can support increases (assuming the same type of ecosystems). Looking at it from another perspective, the larger the land area, the lower the extinction rate and the greater the biodiversity.

Hawaiian Archipelago

Hawaiian Archipelago