Islands may be regarded as closed ecosystems. Although this is not true in every case—witness the island-hopping of species on the Hawaiian-Emperor island chain—or at all times for any given island, the relative isolation of islands has made them an ideal setting in which to explore theories of evolution and adaptation.
Two words frequently used in relation to island environments are equilibrium and change. Ecosystems in equilibrium are assumed to have reached steady state, with very slow rates of change. No more is taken out of the ecosystem than is replenished; predator-prey relationships remain constant, and die-offs are balanced by new colonization. Whereas the individual species involved in these interactions may change, the overriding patterns do not. The equilibrium model of insular biogeography was formally stated in the 1960s by R. H. MacArthur and E. O. Wilson and has been used to, among other things, establish and manage natural preserves on both islands and mainlands.
Environments in equilibrium are, of course, subject to change. A catastrophic storm can destroy a large sector of the biota; land bridges come and go. The changes introduced by the entry of other life forms into the closed system of an island, however, can have dramatic immediate effects. Island environments may have permissive or controlling effects on organisms that attempt to establish a presence. Among the permissive effects, or opportunities for colonization, is the availability of unoccupied biological niches. Unfilled niches appear to hasten organismal radiation, the evolutionary branching of species. On the Hawaiian islands, for example, the native, or endemic, family of birds known as honeycreepers has branched into 23 species, many adapted to different feeding niches—seeds, insects, and so on. Further, the beaks of the birds have adapted to extracting the different diets from different tree species—a remarkable series of adaptations.
Among the controlling effects that islands have on would-be immigrants is simple inhospitality. Volcanic or coral islands lacking sufficient layers of sediment to grow plants, for example, would not be attractive or even feasible as a home for many kinds of animals, including agricultural humans.
Once a breeding pair of immigrants has successfully penetrated the isolation of an island and taken up residence there, it can profoundly affect the existing dynamics of the island's ecosystem. Human-induced change is particularly devastating to islands. The domestic goats and rabbits introduced by human colonizers can denude a small island of succulent vegetation in less than a year, and dogs can turn every small mammal into prey. If plants to a goat's liking are not available or if steep ravines effectively corral dogs' activity, the ecosystem effects may be finite—if one does not consider the ticks and other disease carriers that may be introduced with the immigrants. Thus, the interactions between islands and migrant species are a two-way street; migrant species propose entering (and potentially changing) the closed system of an island's environment, and the environment permits or controls the success of such entry.
The question of closed versus open systems becomes highly interesting in the case of endemic island species—species native to an island, and perhaps found nowhere else. Did they evolve in place from an extinct ancestor? Did they island-hop from now drowned islands? If a land bridge was ever available, were the species around to use it? Local conditions rather than grand theories are usually called on to answer such questions, although the answers may in turn support grand theories. In many cases the answers remain perplexing. For example, it is estimated that more than 40% of the species of marine molluscs on the shores of Easter Island and a neighboring island are endemic. This is a startlingly high figure, for the islands are considered too young for evolution alone to have resulted in such prolific and successful branching. The molluscs may have originated from the shores of drowned islands in the region. The finding of old endemic species on young islands has led to some changes in the temporal boundaries of the geologic time scale, which are linked to index species.
The closed-system model of islands is useful for making inferences about evolution and adaptation but does not necessarily agree with reality. Immigrant species do colonize islands, with the colonization rate correlated with closeness to the mainland and size of the animal. Animals reach islands by swimming, rafting (on floating logs or matted leaves), flying, transport by carriers (a tick on a dog), or walking on frozen ice. The success rate need not be very high to develop thriving animal populations on islands. It is estimated that the arrival of one breeding pair every few hundred thousand years would have been enough to build the rich species diversity of the Philippine islands. Over geological time, that amounts to a large number of accidental tourists.
A second metaphor has therefore arisen, that of islands (more specifically, interisland distance) as a filtering mechanism. The filter has, again, permissive and controlling effects. In a hypothetical series of five islands extending outward in a line from a species-rich mainland, large mammals may never get beyond the first island, small mammals and tortoises may be filtered out by the third island, and the fifth island may be colonized only by fliers—birds, bats, and insects. Such a filtering effect has been recorded in islands extending eastward from New Guinea; the last wallabies and marsupials occur on the close islands New Britain and New Ireland, the last frogs on the Solomon islands, the last snakes on Fiji, and the last lizards on the island of Tonga. Biologists studying the biota on filtering islands consider the energy expenditure needed for animals to reach a distant goal and the adaptations species may have had to make to consume local food.
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