In the next section, we consider another important ecological theory, namely island bio-geography. Why do many more species of birds occur on the island of New Guinea than on the island of Bali? One answer is that New Guinea has more than 50 times the area of Bali, and numbers of species ordinarily increase with available space. This does not, however, explain why the Society Islands (Tahiti, Moorea, Bora Bora, etc.), which collectively have about the same area as the islands of the Louisiade Archipelago off New Guinea, play host to much fewer species, or why the Hawaiian Islands, ten times the area of the Louisiades, also have fewer native birds.
Two eminent ecologists, the late Robert MacArthur of Princeton University and E.O. Wilson of Harvard, developed a theory of "island biogeography" to explain such uneven distributions (MacArthur and Wilson, 1967). They proposed that the number of species on any island reflects a balance between the rate at which new species colonize it and the rate at which populations of established species become extinct (Figure 8.5). If a new volcanic island were to rise out of the ocean off the coast of a mainland inhabited by 100 species of birds, some birds would begin to immigrate across the gap and establish populations on the empty, but habitable, island. The rate at which these immigrant species could become established, however, would inevitably decline, for each species that successfully invaded the island would diminish by one the pool of possible future invaders (the same 100 species continue to live on the mainland, but those which have already become residents of the island can no longer be classed as potential invaders).
Equally, the rate at which species might become extinct on the island would be related to the number that had become residents. When an island is nearly empty, the extinction rate is necessarily low because few species are available to become extinct. And since the resources of an island are limited, as the number of resident species increases, the smaller
Figure 8.5 Extinction and immigration curves.
and more extinction prone their individual populations are likely to become. The rate at which additional species will establish populations will be high when the island is relatively empty, and the rate at which resident populations go extinct will be high when the island is relatively full. Thus, there must be a point between 0 and 100 species (the number on the mainland) where the two rates are equal, and therefore the input from immigration balances output from extinction. That equilibrium in the number of species (Figure 8.6) would be expected to remain constant as long as the factors determining the two rates did not change. But the exact species present should change continuously as some species go extinct and others invade (including some that have previously gone extinct), so that there is a steady turnover in the composition of the fauna.
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