Island Biogeography

MacArthur and Wilson (1967) investigated the species-area relationship for islands in an archipelago (number 2, above) when they developed their theory of island biogeogra-phy. There is a consistent relationship between the area of an island and the number of species living on it. MacArthur and Wilson noted that the number of species present on an island represents a dynamic equilibrium between the rate of extinction and immigration of species. At equilibrium, the number of new species arriving equals the number of species going extinct. The taxonomic diversity of the island fauna may be changing (with different species arriving and disappearing), but the species richness stays constant.

The rate at which species immigrate to an island is most closely correlated with the distance of the island from the nearest land. The immigration rate (in number of species per year) is approximately the same for islands of different size but equal distance from the nearest land mass. That is because the colonizing organisms have the same distance to travel to reach any of the islands. However, the extinction rate is negatively correlated with the size of the island; a larger island can support a larger viable population, with less risk of extinction. Therefore, on large islands, the extinction rate reaches equilibrium with the immigration rate only after many species have colonized the island. Large islands will have a greater number of species than smaller islands at the same distance from the nearest landmass.

If we now consider islands of the same size but at different distances from the nearest land mass, we can see that the extinction rate is approximately the same for all islands. This is because the islands have the same available space for supporting viable populations. However, the immigration rate is negatively correlated with the distance of the island from the nearest land; the organisms have farther to travel to reach the more isolated islands. Thus, on distant islands, the extinction rate reaches equilibrium after only a few species have reached the island. The distant islands will have a smaller number of species than a less isolated island of the same size.

This theory of island biogeography has been applied to fragmented habitats and ecosystems. Using this theory, we can estimate the number of species a fragmented landscape can support, and predict whether that number will be enough to prevent the ecosystems from collapsing or prevent the extinction of a species.

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