Associate Professor, Department of Biological Sciences, Stanford University, Stanford, California
To date, human-caused species extinctions are more an island-based than a continental phenomenon. Of the 94 species of birds known to have become extinct worldwide since contact with Europeans, only 9 were continental (Gorman, 1979). Currently, more endemic Hawaiian bird species are officially listed as endangered or threatened than are listed for the entire continental United States. Where information is available on other groups of animals, it indicates that human-caused extinctions are invariably more frequent on islands.
Heywood (1979) summarized the causes of extinction on islands as deforestation and fire, the introduction of grazing mammals, cultivation, and the introduction of weedy plants. All these factors can be important on continents as well, but species introductions (deliberate or accidental) are disproportionately important on islands (Elton, 1958). Isolated islands and archipelagos often lack major elements of the biota of continents, and their native species often lack defenses against grazing or predations.
Biological invasions are not the only factor leading to elevated extinction rates for island species. Extinction rates are also higher on islands because island species generally have small populations, restricted genetic diversity, and narrow ranges prior to human colonization, and because human alterations of land through use destroy an already-limited critical habitat. The plant and animal hitchhikers and fellow travelers who accompany humans to isolated islands interact with these other causes of extinction, however, and biological invaders endanger native species in reserves and other protected lands.