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The fact that biological invasions decrease diversity on islands is paradoxical, because, as pointed out by Lugo in Chapter 6, the introduction of alien species generally increases the total number of species on an island, often spectacularly. However, most of the introduced species are cosmopolitans that are in no danger of global extinction, whereas most species on isolated islands are endemic. Biological invasions can therefore cause a net loss of species worldwide and a homogenization of the biota of Earth (Mooney and Drake, 1986).
SCOPE OF THE PROBLEM
The disproportionate effects of human colonization and attendant biological invasions on island ecosystems are well known (Carlquist, 1974; Darwin, 1859; Elton, 1958; Wallace, 1880); they can be demonstrated even on large islands such as Madagascar and Australia (Carlquist, 1974). The most severe consequences are experienced on old, isolated, mountainous, tropical, or subtropical islands or archipelagos. Islands located near continents receive organisms from those continents and rarely develop unique species. Truly oceanic islands have rates of evolution and speciation greater than those of immigration; hence, their biota contains many endemic species. Low islands (such as atolls) lack the range of environments that permits evolutionary radiation, while islands at high latitudes are subjected to strong climatic fluctuations (Bramwell, 1979), which prevent radiation.
Together these factors suggest that the Hawaiian Islands, the most isolated archipelago in the world, should have a large number of exotic species and a large potential for loss of endemic species as a consequence of biological invasions. The very large number of endemic species on these islands is well documented (Carlquist, 1974); the importance of biological invasions can also be demonstrated. For example, a survey of exotic plants on National Park Service lands (Loope, in press a) shows that island parks have a much larger proportion of alien species in their flora than do continental parks (Table 20–1). Moreover, in most continental parks alien species are largely confined to roadsides and areas occupied by humans before the park was established. In contrast, Channel Islands National Park in California, Everglades National Park (an island of tropical vegetation at the tip of the Florida peninsula), and the Hawaiian parks contain alien species that establish themselves in otherwise undisturbed native ecosystems and change the nature of the sites they occupy (Ewel, 1986; Stone and Scott, 1985; Stone et al., in press a).
The problems in the Hawaiian parks reflect in part the overall abundance of exotic species in Hawaii. As many as 1,765 native species of vascular plants (probably fewer as taxonomic revisions take hold) existed in the islands when the Polynesians arrived, and 94 to 98% of them were endemic (Kepler and Scott, 1985). Polynesians brought additional species, perhaps 30 of them (Nagata, 1985), when they colonized Hawaii and journeyed among the Pacific islands. The advent of more rapid transportation from distant areas and especially the occupation of Hawaii by people from diverse western and eastern cultures, each with its distinctive food, medicinal, and ornamental plants, greatly increased the number of species present. More than 4,600 species of introduced vascular plants are now known to