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An Extinction Theory   

The Island Biogeography Theory

About two decades ago Robert MacArthur and Edward Wilson began analysing the extensive data on the numbers and distributions of plant and animal species on islands around the world. Biologists had long accepted a view that may seem obvious: that larger islands contain more species than smaller ones. They also found that the ratio of number of species to island size was impressively consistent. They further found that the number of species doubled for every tenfold increase in island size.

MacArthur and Wilson proposed that the number of species on an island represent a dynamic equilibrium between the immigration rate of new species to the island and the extinction among those species already present on the island. They also found that no matter what its size, an island almost invariably contained fewer species than a continental piece of land of the same size within a larger stretch of continuous habitat.

The island biogeography theory stimulated new insights and research into the distribution, diversity, and conservation of species and has drawn attention to the dynamics inherent in fragmented habitats. What has increasingly been found in protected areas throughout the world is that as the unprotected habitat around them is developed and destroyed, parks and reserves become islands - habitat islands, surrounded by seas of non-habitat.

If the number of species on islands is inevitably lower than the number of species on same-sized, non-isolated mainland areas, then the creation and isolation (by human development of surrounding habitat) of a park or reserve should be followed by a regular series of extinction, an inexorable decline in species diversity within that reserve itself.

Further, as a large area of habitat is fragmented, the total habitat area is reduced and what is left is distributed in disjointed fragments of varying size. Habitat islands thus contain samples of populations of a much larger area. These samples contain fewer species, fewer individuals within a species, and more species represented by only a few individuals.

Up to a point no species are lost, but as fragmentation continues the remaining area is reduced to a critical size below which the habitat will not provide the requirements of many of the original species, and a number of them disappear.

The first to go are the susceptible species - those on the higher trophic levels, the habitat and food specialists, and ones that require large areas of habitat to maintain viable populations.

With regard to small islands the potential for experiencing all the problems of small populations exists - random sampling of gene frequency, genetic drift, erosion of genetic variation, increased inbreeding and accompanying loss of fitness - all increase the probability of species extinction. A case study on the fate of some of the antelopes in the region illustrates this theory here.

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