alien birds and mammals consume and disseminate the fruit of the aggressive alien plants strawberry guava (Psidium cattleianum) and banana poka (Passiflora mollissima) throughout native forest areas. Interactions between feral pigs and these invading plants are particularly severe: pigs disseminate seeds of these fleshy-fruited aliens, mix them with organic fertilizer, and deposit them into seedbeds, which are cleared by the pigs’ rooting activity. The pigs’ descendants then use fruit of the daughter plants as a major food source (Smith, 1985; Stone, 1985). Similar interactions between cattle and common guava (Psidium guajava) occur in the Galapagos (Bramwell, 1979). These interactions between alien plants and animals further illustrate why control of alien animals is fundamental to protecting the native ecosystems of islands.
A second strategy for limiting the effects of biological invaders is to control manageable alien species in selected critical habitats. This process is expensive and time-consuming, but it does lead to the maintenance of areas as close to their natural state as possible (although birds, flying insects, and microorganisms are of course difficult or impossible to control). Management in “Special Ecological Areas” of Hawaii Volcanoes National Park has been designed to protect areas that represent the major ecosystems in the park by minimizing the influence of alien species. These areas can then act as refugia for threatened native biota and as areas for ecological study and education (Stone et al., in press a; Tunison et al., 1986).
Control over habitat destruction is also essential to protecting biological diversity on oceanic islands. Land clearing or fire in native systems can both destroy individuals of threatened native species and lead to the establishment of alien-dominated successional ecosystems. Conflicts in achieving this objective are inevitable; most islands are neither museums nor biological preserves, and one person’s “habitat destruction” will certainly be another’s source of food or income. Destruction of critical habitat on islands is perhaps most severe on Madagascar, but it is not a problem confined to developing countries. Nearly half of Hawaii’s largest native-dominated lowland rain forest was cleared during 1984 and 1985 in a subsidized endeavor to generate electricity from wood chips.
Controlling the effects of biological invasions on islands is paramount, but there is also a great deal to be gained from studying their effects carefully. The relative simplicity of the biota of many islands perhaps enables invading species to have greater effects on native communities than they would in continental areas; it certainly facilitates a much more complete evaluation of those effects. Better understanding of biological invasions and their consequences for biological diversity on islands will contribute to the development and testing of basic ecological theory