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Figure 8.6 The equilibrium number of species. Any particular island has a point where the extinction (EXT curve) and immigration curves (IMMIG curve) 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.

Figure 8.6 The equilibrium number of species. Any particular island has a point where the extinction (EXT curve) and immigration curves (IMMIG curve) 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.

will occur with fewer species on distant islands (Figure 8.7). Close islands will have high immigration rates and support more species. By similar reasoning, large islands, with their lower extinction rates, will have more species than small ones—again everything else being equal (which frequently is not, for larger islands often have a greater variety of habitats and more species for that reason).

Island biogeography theory has been applied to many problems, including forecasting faunal changes caused by fragmenting previously continuous habitat. For instance, in most of the eastern United States only patches of the once-great deciduous forest remain, and many species of songbirds are disappearing from those patches. One reason for the decline in birds, according to the theory, is that fragmentation leads to both lower immigration rates (gaps between fragments are not crossed easily) and higher extinction rates (less area supports fewer species).

Example 2: Connecticut forest re-establishing

Indications of such changes in species composition during habitat fragmentation were found in studies conducted between 1953 and 1976 in a 16-acre nature preserve in Connecticut in which a forest was re-establishing itself. During that period development was increasing the distance between the preserve and other woodlands. As the forest grew back, species such as American Redstarts that live in young forest colonized the area, and birds such as the Field Sparrow, which prefer open shrub lands, became scarce or disappeared. In spite of the successional trend toward large trees, however, two bird species normally found in mature forest suffered population declines, and five such species went extinct on the reserve. The extinctions are thought to have resulted from lowering immigration rates caused by the preserve's increasing isolation and by competition from six invading species characteristic of suburban habitats.

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