WHY ARE ISLANDS SUSCEPTIBLE?

The reasons why biological invasions are disproportionately successful on islands, and why island species seem more likely to become extinct, have long been debated. Loope (in press b) summarized this discussion with seven possible explanations for the observed patterns:

  • Reduced competitive ability due to repeated “founder effects,” i.e., chance events during colonization by small initial populations

  • Disharmony of functional groups and relative lack of diversity

  • Small populations and genetic variability; restrictive specialization

  • Relative lack of adaptability to change; loss of resistance to consumers and disease

  • Loss of essential co-evolved organisma

  • Relative lack of natural disturbance, especially fire, in the evolutionary history of many island biotas

  • Intensive exploitation by humans

He also pointed out that the apparent lack of vigor of island species can be overstated, sometimes with negative consequences. For example, Lyon (1909) interpreted a decline of native óhiá (Metrosideros polymorpha) in Hawaii as reflecting that species’ inability to survive in the modern world, and spearheaded the introduction of many alien species to replace it. In fact, periodic diebacks of natural populations of Metrosideros are a natural feature of forest dynamics in Hawaii and elsewhere in the Pacific (Mueller-Dombois, 1983), and Metrosideros naturally recolonizes most of these areas. More generally, many native island species maintain themselves quite successfully in mixed native/exotic ecosystems (Mueller-Dombois et al., 1981).

At the other extreme, it has been argued that alien species are merely temporary components of island ecosystems, certain to be replaced by natives in the course of ecological succession (Allan, 1936; Egler, 1942). In fact, some aliens invade intact native ecosystems, whereas others alter the course of succession in already disturbed sites (Smith, 1985) and seem capable of persisting in those altered sites.

Although biological invasions clearly have contributed to the extinction of native species on islands, the importance of direct competition between native and exotic species in causing these extinctions is uncertain. Habitat destruction by humans and feral animals, alterations in basic ecosystem properties caused by newly introduced species, grazing and predation pressure from introduced consumers, and exotic animal diseases (such as avian pox and malaria) appear to be at least equally important.

The importance of grazing and predation by alien animals deserves special emphasis. Most isolated oceanic islands originally lacked whole groups of organisms; mammals were especially sparse. Even ants were nearly or entirely absent on some islands, including Hawaii (Medeiros et al., 1986).

The introduction of mammals has had enormous effects on island ecosystems throughout the world. Comparisons of islands with introduced ungulates and those without such animals in widely separated Pacific island groups (the Hawaiian



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