In: Biology
Landscape fragmentation is the process by which habitat is
altered or converted to other habitat types; for example, patches
of forest may be converted to agriculture. As a result, habitat
patches (forest in our example) usually become smaller and spaced
farther apart as fragmentation progresses. Fragmentation has many
effects on local communities.
This could be due to several reasons, some of which we will discuss
in class on Thursday. How would MacArthur and Wilson’s Equilibrium
Theory of Island Biogeography (ETIB) predict habitat fragmentation
should affect biodiversity within remaining habitat patches?
According to the ETIB, why would this result occur?
Two eminent ecologists, the late Robert MacArthur of Princeton University and E. 0. Wilson of Harvard, developed a theory of "island biogeography" to explain such uneven distributions. 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. 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).
That is the essence of the MacArthur-Wilson equilibrium theory of island biogeography. How well does it explain what we actually observe in nature? One famous "test" of the theory was provided in 1883 by a catastrophic volcanic explosion that devastated the island of Krakatoa, located between the islands of Sumatra and Java. The flora and fauna of its remnant and of two adjacent islands were completely exterminated, yet within 25 years (1908) thirteen species of birds had recolonized what was left of the island. By 1919-21 twenty-eight bird species were present, and by 1932-34, twenty-nine. Between the explosion and 1934, thirty-four species actually became established, but five of them went extinct. By 1951-52 thirty-three species were present, and by 1984-85, thirty-five species. During this half century (1934-1985), a further fourteen species had become established, and eight had become extinct. As the theory predicted, the rate of increase declined as more and more species colonized the island. In addition, as equilibrium was approached there was some turnover. The number in the cast remained roughly the same while the actors gradually changed.