Nearctica - Ecology - Biogeography

Biogeography - Island Biogeography

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The field of Island Biogeography is the brain child of Robert McArthur and E.O. Wilson. The basis of island biogeography supposes there is a large source area of species and surrounding the source area is a series of islands or different sizes and distances from the source area. Species disperse from the source area to the islands. In the theory's simplest form species from the source area disperse to an island at a rate (the immigration curve) depending on the distance of the island from the source area. The fewer species are on the island, the higher the immigration curve. The more species on the island, the lower the curve because most potential new immigrant species will have already reached the island. Similarly there is a rate at which species go extinct on the island (the extinction curve). The more species there are on the island, the higher the extinction rate. The shapes of the extinction and immigration curves are not particularly important to the broad conclusions reached by island biogeography, but the slopes of the curves (or lines in this case) are.

The slopes of the lines depend on the size of the island and its distance from the source area. For a small island the extinction rate should increase more rapidly (the line should be more vertical) as the number of species on the island increase because smaller islands have fewer potential habitats and resources to colonize. Larger islands should have a more horizontal extinction rate (relative to the number of species already there) because there are more potential habitats and resources to use. The slope of the exintinction curve, therefore, depends on the size of the island.

The immigration curve for islands close to the source area should be fairly vertical because the potential immigrants should reach the island fairly rapidly and then fall off sharply as the number of potential new immigrant species from the source area decreases. Similarly islands far from the source area should have a more horizontal immigration curve because new immigrating species take longer to get to far away islands than to those closer into the source area.

Any particular island has a point where their extinction and immigration curves intersect. At this point the number of new immigrating species to the island is exactly matched by the rate at which species are going extinct. This equilibrium point is called the "Equilibrium Number of Species" for that island.

The influence of the distance of an island from the source area on the equilibrium number of species can be examined graphically. For example, you can compare islands of the same size but different distances from the source area. In this case the extinction curve is identical, but the immigration curves are different. An examination of the graphs above predicts that islands closer to the source area should have a higher equilibrium number of species than would be found on more distant islands.

A similar exercise can be used to compare islands of different sizes, but equal distances from the source area.

The two main predictions of Island Biogeography are:

1. Islands close to a source area should have a higher number of species than islands further from the source area for islands of equivalent areas.

2. Larger islands should have more species than smaller islands for islands located at similar distances from the source area.

These two predictions have been largely supported by observational and experimental data you will find in some of the biogeography links located in the general biogeography page of Nearctica.

Island biogeography applies not only to islands, but can also be adapted to any series of discontinuous habitats such as forest fragments. Of course the theory of island biogeography has been expanded and refined since its earliest theoretical beginnings as outlined above.

For information about Island Biogeography, visit the general biogeography links page.