The beautiful birds of Hawaii have weathered many storms of invasions. The usual culprits—humans, dogs, pigs, cats, and rats— have all left their footprints on the Hawaiian ecosystem. But in this fragile paradise, the smallest invaders—mosquitoes, protozoa, and viruses—have had an impact disproportionate to their size. In fact, these tiny invasives may have dealt the final blow to several extinct species, such as the 'O'o (Moho nobilis) and the grosbeak finch (Psittirostra kona). Other species are teetering on the verge of extinction under the continued threat of invasive species.
When the first humans, voyagers from Polynesia, arrived in Hawaii around 300 C.E., they found lush tropical forests. There were no terrestrial predators; in fact, there were no mammals (except one species of bat), no snakes, and (as befitting a paradise) no mosquitoes. There were, however, an abundance of birds, demonstrating the fabulous species radiation that often occurs in isolated populations.
Like all settlers, the Hawaiians modified and exploited their new home to suit their needs. Trees were harvested for wood, and land was cleared for settlements. The native birds were hunted for food as well as for their colorful feathers. The Hawaiians brought some domesticated animals with them, including Asian pigs, dogs, and fowl. The Polynesian rat (Rattus exulans) most likely arrived as a stowaway and quickly became established as an invasive species. When the first Europeans arrived in Hawaii at the end of the eighteenth century, the islands' biota had already changed dramatically: most of the lowland forest had been cleared, the human population was burgeoning, and many species of flightless and ground nesting birds had already become extinct.
The arrival of Europeans brought new dangers for the native forest birds. Cattle, goats, sheep, and English pigs were introduced, and these free-ranging animals wreaked havoc upon the native vegetation. More aggressive species of rats (Rattus rattus, and R. norvegicus) arrived with the Europeans, and mongooses (Herpestes auropunctatus), introduced in an attempt to control the rats, turned out to have a taste for eggs and nestlings. A variety of songbirds, such as the Japanese white-eye (Zos-terops japonica), and gamefowl were released for pleasure and sport. These birds, or perhaps domestic poultry, brought with them some passengers of their own: the avian pox virus and the blood parasite (Plasmodium relictum), which causes avian malaria.
We do not know exactly when introduced diseases began to affect Hawaii's native birds, but naturalists noticed forest birds with lesions attributable to avian pox at the end of the nineteenth century. Avian malaria probably went undetected at first, because it does not produce externally visible lesions and because the P. relictum parasite was not recognized as the cause of the disease until the twentieth century (van Riper et al., 1986). These diseases probably did not begin to spread among the native birds until after 1826, when a ship arriving from Mexico dumped barrels of fetid water containing mosquito larvae (Culex quinquefas-ciatus)—a vector for P. relictum. The Hawaiian birds, which had evolved in isolated safety, were immunologically naive to these diseases and therefore particularly susceptible to them. For some species, already burdened by the stresses of habitat loss, invasive predators, and competitors, introduced disease may have been the final straw. By the beginning of the twentieth century, an absence of birds, even in seemingly undisturbed parts of the forest, had become apparent (Warner, 1968).
Introduced diseases have most significantly affected the lowland forest birds. Mosquitoes breed in pools of standing water, especially those commonly found near human habitations. In the forest, feral pigs make wallows and break open logs in which water collects, creating prime locations for mosquito reproduction. At present mosquitoes are restricted to the warmer, lower altitudes, but if global climate change leads to warming, the range of the mosquitoes may expand into higher elevations. Likewise, the introduction of new mosquito species adapted to more temperate climates might provide a vector to carry avian malaria to high altitudes. In the meantime, remnant populations of some species are now found only outside of their original range and faced with the added stress of coping with a new habitat.
Like all ecosystems, the Hawaiian forests are dynamic. A single change results in a cascade of effects. The extinction of one species may lead to geographical and ecological shifts for other species. The extinction of the 'O'o opened the flowers of the ohia tree (Met-rosideros polymorpha) as a food source for another nectivorous bird, the i'iwi (Vestiaria coccinea). It was fortunate for the i'iwi that this niche became available, because the lobelioid flowers that had been its primary food source were undergoing a wave of extinctions as well. In some cases, introduced species may step in to fill a gap left by the disappearance of native species. The ie'ie vine (Freycinetia arborea) was rescued by the Japanese white-eye when its native pollinators went extinct or became too scarce to perform their duties (Cox, 1983).
Assaults upon native birds by hunters, habitat destruction, introduced predators, competitors, and disease have all contributed to the demise of several species. One biologist, Jack Jeffrey of the U.S. Fish and Wildlife Service, has aptly described this as "negative syner-gism." To date, half of the recorded Hawaiian bird species have become extinct since the arrival of humans on the islands, and 40 percent of the remaining species are endangered (Youth, 1995). We may never know how many species succumbed to avian malaria and avian pox. Disease ecology along with habitat protection and predator control must all be taken into consideration in order to save other species from the same fate.
See also: Adaptive Radiation Bibliography
Atkinson, I. A. E. 1977. "A Reassessment of Factors, Particularly Rattus rattus L., that Influenced the Decline of Endemic Forest Birds in the Hawaiian Islands." Pacific Science 31: 109-133; Cox, Paul Alan. 1983. "Extinction of the Hawaiian Avifauna Resulted in a Change of Pollinators for the Ieie, Freycinetia arborea." Oikos 41: 195-199; Smith, Thomas B., et al. 1995. "Evolutionary Consequences of Extinctions in Populations of a Hawaiian Honeycreeper." Conservation Biology 9: 107-113; van Riper, Charles III, et al., 1986. "The Epizootiology and Ecological Significance of Malaria in Hawaiian Land Birds." Ecological Monographs 56: 327-344; Warner, Richard E. 1968. "The Role of Introduced Diseases in the Extinction of the Endemic Hawaiian Avifauna." Condor 70: 101-120; Youth, Howard. 1995. "Hawaii's Forest Birds Sing the Blues." ZooGoer 24, no. 1.
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