Island biogeography is the study of past and present animal and plant distribution patterns on islands and the processes that created those distribution patterns. Historically, island biogeographers mainly studied geographic islands—continental islands close to shore in shallow water and oceanic islands of the deep sea. In the last several decades, however, the study and principles of island biogeography have been extended to ecological islands such as forests and prairie fragments isolated by human development. Biogeographic "islands" may also include ecosystems isolated on mountain-tops and landlocked bodies of water such as Lake Malawi in the African Rift Valley. Geographic islands, however, remain the main laboratories for developing and testing the theories and methods of island biogeography.
Until the 1960s, biogeographers thought of islands as living museums—relict (persistent remnant of an otherwise extinct species of plant or animal) scraps of mainland ecosystems in which little changed—or closed systems mainly driven by evolution . That view began to radically change in 1967 when Robert H. MacArthur and Edward O. Wilson published The Theory of Island Biogeography.
In their book, MacArthur and Wilson detail the equilibrium theory of island biogeography—a theory that became the new paradigm of the field. The authors proposed that island ecosystems exist in dynamic equilibrium, with a steady turnover of species. Larger islands—as well as islands closest to a source of immigrants—accommodate the most species in the equilibrium condition, according to their theory. Mac-Arthur and Wilson also worked out mathematical models to demonstrate and predict how island area and isolation dictate the number of species that exist in equilibrium.
The driving force behind species distribution is dispersion—the means by which plants and animals actively leave or are passively transported from their source area. An island ecosystem can have more than one source of colonization, but nearer sources dominate. How readily plants or animals disperse is one of the main reasons equilibrium will vary from species to species.
Birds and bats are obvious candidates for anemochory (dispersal by air), but some species normally not associated with flight are also thought to reach islands during storms or even normal wind currents. Orchids, for example, have hollow seeds that remain airborne for hundreds of kilometers. Some small spiders, along with other insects like bark lice, aphids, and ants (collectively knows as aerial plankton ) often are among the first pioneers of newly formed islands.
Whether actively swimming or passively floating on logs or other debris, dispersal by sea is called thallasochory. Crocodiles have been found on Pacific islands 600 miles (950 km) from their source areas, but most amphibians, larger terrestrial reptiles, and, in particular, mammals, have difficulty crossing even narrow bodies of water. Thus, thallasochory is the medium of dispersal primarily for fish, plants, and insects. Only small vertebrates such as lizards and snakes are thought to arrive at islands by sea on a regular basis.
Zoochory is transport either on or inside an animal. This method is primarily a means of plant dispersal, mostly by birds. Seeds ride along either stuck to feathers or survive passage through a bird's digestive tract and are deposited in new territory.
Anthropochory is dispersal by human beings. Although humans intentionally introduce domestic animals to islands, they also bring unintended invaders, such as rats.
Getting to islands is just the first step, however. Plants and animals often arrive to find harsh and alien conditions. They may not find suitable habitats. Food chains they depend on might be missing. Even if they manage to gain a foothold, their limited numbers make them more susceptible to extinction . Chances of success are better for highly adaptable species and those that are widely distributed beyond the island. Wide distribution increases the likelihood a species on the verge of extinction may be saved by the rescue effect, the replenishing of a declining population by another wave of immigration.
Challenging established theories
Many biogeographers point out that isolated ecosystems are more than just collections of species that can make it to islands and survive the conditions they encounter there. Several other contemporary theories of island biogeography build on MacArthur and Wilson's theory; other theories contradict it.
Equilibrium theory suggests that species turnover is constant and regular. Evidence collected so far indicates MacArthur and Wilson's model works well in describing communities of rapid dispersers who have a regular turnover, such as insects, birds, and fish. However, this model may not apply to species who disperse more slowly.
Proponents of historical legacy models argue that communities of larger animals and plants (forest trees, for example) take so long to colonize islands that changes in their populations probably reflect sudden climactic or geological upheaval rather than a steady turnover. Other theories suggest that equilibrium may not be dynamic, that there is little or no turnover. Through competition , established species keep out new colonists; the newcomers might occupy the same ecological niches as their predecessors. Established species may also evolve and adapt to close off those niches. Island resources and habitats may also be distinct enough to limit immigration to only a few well-adapted species.
Thus, in these later models, dispersal and colonization are not nearly as random as in MacArthur and Wilson's model. These less random, more deterministic theories of island ecosystems conform to specific assembly rules—a complex list of factors accounting for the species present in the source areas, the niches available on islands, and competition between species.
Some biogeographers suggest that every island—and perhaps every habitat on an island—may require its own unique model. Human disruption of island ecosystems further clouds the theoretical picture. Not only are habitats permanently altered or lost by human intrusion, but anthropochory also reduces an island's isolation. Thus, finding relatively undisturbed islands to test different theories can be difficult.
Since the time of naturalists Charles Darwin and his colleague, Alfred Wallace, islands have been ideal "natural laboratories" for studying evolution. Patterns of evolution stand out on islands for two reasons: island ecosystems tend to be simpler than other geographical regions, and they contain greater numbers of endemic species , plant, and animal species occurring only in a particular location.
Many island endemics are the result of adaptive radiation—the evolution of new species from a single lineage for the purpose of filling unoccupied ecological niches. Many species from mainland source areas simply never make it to islands, so species that can immigrate find empty ecological niches where once they faced competition. For example, monitor lizards immigrating to several small islands in Indonesia found the niche for large predators empty. Monitors on these islands evolved into Komodo Dragons, filling the niche.
Conservation of biodiversity
Theories of island biogeography also have potential applications in the field of conservation . Many conservationists argue that as human activity such as logging and ranching encroach on wild lands, remaining parks and reserves begin to resemble small, isolated islands. According to equilibrium theory, as those patches of wild land grow smaller, they support fewer species of plants and animals. Some conservationists fear that plant and animal populations in those parks and reserves will sink below minimum viable population levels—the smallest number of individuals necessary to allow the species to continue reproducing. These conservationists suggest that one way to bolster populations is to set aside larger areas and to limit species isolation by connecting parks and preserves with wildlife corridors.
Islands with greatest variety of habitats support the most species; diverse habitats promotes successful dispersal, survival, and reproduction. Thus, in attempting to preserve island biodiversity , conservationists focus on several factors: the size (the larger the island, the more habitats it contains), climate , geology (soil that promotes or restricts habitats), and age of the island (sparse or rich habitats). All of these factors must be addressed to ensure island biodiversity.
[Darrin Gunkel ]
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Quaman, David. Song of the Dodo: Island Biogeography in an Age of Extinction. New York: Scribner, 1996.
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