Grazing and browsing mammals effect islands in such pervasive ways that it is difficult to see how native ecosystems can be protected unless they are eliminated. Studies of whole islands and of exclosures have clearly demonstrated that ungulate populations affect erosion, soil fertility, and the success of invasions by alien plants (Loope and Scowcroft, 1985; Merlin and Juvik, in press; Mueller-Dombois and Spatz, 1975; Vitousek, 1986). Island plants often lack defenses, such as thorns and toxic chemicals, against herbivores, and herbivority reduces total plant cover and selects for better defended alien plants. Moreover, feral pigs (which are widespread on many oceanic islands) directly disrupt soil structure in the course of their feeding. Efforts to eliminate mammals are expensive and difficult, but they have been highly successful in a number of areas (Bramwell, 1979; Stone et al., in press b). In many cases, the removal of grazing animals has been followed by the recovery of native plants and even by the discovery of entirely new species of native plants (Bramwell, 1979; Mueller-Dombois and Spatz, 1975).
Alien vertebrate and invertebrate predators can have significant effects on island ecosystems both directly, by eliminating natives, and indirectly, by altering community structure. For example, rats and feral cats affect the breeding success of ground-nesting birds in many areas (Clark, 1981; King, 1984; Wace, 1986). Alien ants altered invertebrate communities in the Hawaiian lowlands years ago, and other ant species are now threatening to do so at high elevations (Medeiros et al., 1986). Invertebrate predators are particularly problematic in that they may eliminate important native pollinators from island faunas.
Any alien species that alters ecosystem-level characteristics (such as primary productivity, nutrient availability, hydrological cycles, and erosion) of the area it invades alters the living conditions for all organisms in that area (Vitousek, 1986). It may also alter the kind or quality of the services that natural ecosystems provide to human societies (Ehrlich and Mooney, 1983). Alien animals clearly alter ecosystem properties in a number of ways (as described above), and it is becoming clear that alien plants can do so as well. In Hawaii, for example, the exotic nitrogen-fixing fire tree (Myrica faya) increases the availability of the soil nitrogen in nitrogen-limited volcanic ash deposits (Vitousek, in press). Similarly, the alien grasses Andropogon virginicus and A. glomeratus provide fuel for fires and also sprout rapidly following fires, thereby greatly increasing both their abundance and the overall frequency of fires to the detriment of native species not adapted to fire resistance (Smith, 1985).
Alien animals are frequently (not invariably) able to invade intact native ecosystems, but plants species that can do so are not common. Most often, alien plants invade undisturbed native ecosystems in association with alien animals. In Hawaii,