Strangers in paradise: alien species disrupt the ecology of Hawaii.
That leisurely pace of immigration gave Hawaii's existing residents plenty of time to recover from the ecological shock of new arrivals and to adapt to life alongside them. For example, last year researchers reported genetic evidence that the silversword - a dramatic Hawaiian plant with gray-green, saber-like leaves - evolved from the homely california tarweed, probably borne to Hawaii tens of thousands of years ago as a seed in the features or gut of a bird (SN:4/27/91, p.264). With the arrival of the Polynesians 1,500 years ago, however, this incremental immigration picked up speed.
Today, it has become a full-scale alien invasion. Floods of tourists and the capacious cargo holds of ships and jetliners offer easy paths to paradise for hitchhicking plants, animals, and insects. Bird-eating snakes curl up in the wheel wells of airplanes arriving from Guam, only to slither out into the rain forest once the plane lands. Seeds from exotic ornamental plants imported by well-meaning homeowners jump backyard fences to disperse and take root in dry forestland already disrupted by human habitation. Exotic insects emerge from the wrappers of tourists' carry-on snacks and end up noshing on the vulnerable moths that pollinate some of Hawaii's distinctive flora.
Aliens are also smuggled into the state, despite regulations against importing many non-indigenous animals, such as snakes and carnivorous fish. Hawaii's agricultural officials and postmasters report numerous instances in which packages labeled "Fragile: Handle with Care" turn out to contain someone's prospective pet python or piranha, mailed from a pet store on the mainland.
Ecologists are now finding that the stepped-up influx of alien species has far outstripped the Hawaiian ecosystem's ability to deal with such change. By eating, competing with, or changing the habitat of native species, alien wildlife and plants disrupt the intricate, interdependent network of Hawaii's flora and fauna -- 10,000 species of which exist nowhere else on Earth.
Hawaii's diversity draws tourists, provides sources of new medicines, and yields plant species that agricultural researchers can cross with existing crops to give them new characteristics, such as resistance to specific diseases or pests. Although some of Hawaii's native species -- such as its huge array of insects -- may not at first seem important to preserve, ecologists caution that they know so little about Hawaii's ecology that they often cannot predict which species is expendable and which is not. As more and more species disappear, they add, the remaining ones become even more vulnerable to extinction because of the changing habitat.
Hawaii is particularly unprepared for some alien species -- such as tree snakes, rats, and Argentine ants -- because the islands have no native snakes or ants and only one native land mammal, a cave-dwelling bat. Such aliens "have the capacity to undo all other conservation efforts in Hawaii," says Francis G. Howarth of the Bishop Museum in Honolulu. "Alien species are a major cause of extinctions here," he adds.
Accordingly, scientists have boosted their efforts to understand and counter the effects of introduced species, which can gain footholds following habitat-devastating natural disaster such as last September's Hurricane Iniki. They have also joined with environmental groups to apply pressure to strengthen state and federal measures to stem the tide of exotic invaders (see p. 316).
Alien insects present especially serious problems for Hawaii's ecosystem, in part because they disrupt the normal interdependence of particular insects and plants. For example, most of the 354 species of fruit fly, or Drosophila, discovered in Hawaii only lay their eggs on the fruits or vegetation of specific plants, says Hampton L. Carson of the University of Hawaii in Honolulu.
Many of these fruit flies - particularly the so-called picture-wing species, each of which has a characteristic pattern of dark splotches on its wings -- are now rare, in part because of predation by the western yellow-jacket wasp, Vespula pennsylvanica. Since its inadvertent introduction in 1981, this alien marauder has decimated native Drosophila populations, methodically stripping off their wings and sucking out their fruit-sweetened bodily fluids, Carson says.
Because the fruit flies existed for eons with few predators -- and accordingly had the opportunity to thrive and develop into hundreds of species -- carson says they provide a near-perfect means for researchers to retrace the steps of the evolutionary process. "They give us clues to evolutionary change, which is of worldwide importance," he asserts. "The Hawaiian Drosophila are one of the best data sets we have anywhere in the world."
Carson adds that the fruit flies may play an important role in the Hawaiian ecosystem because their larvae eat decaying fruit and plant matter, helping to recycle forest nutrients. Some of the fruit flies might also act as pollinators, he says.
Of the 2,500 species of Arthropoda -- the phylum of interverbarates that includes insects --currently in Hawaii, roughly 600 were introduced by humans, says Howarth. Lloyd L. Loope of theHaleakala National Park on Hawaii's island of Maui and his colleagues have studied one of the most voracious newcomers, the Argentine ant, Iridomyrmex humilis.
The Argentine ant "eats every insect it can, including pollinators," says Loope. At the joint annual meeting of the American Institute of Biological Sciences and the Ecological Society of America in Honolulu last August, Loope discussed the results and implications of a study he and colleagues conducted on the detrimental effects of the Argentine ant.
Loope and Haleakala National Park co-worker Arthur C. Medeiros teamed up with F. Russell Cole and William W. Zuehlke of Colby College in Waterville, Maine, to sample insect populations in three shrubby areas on the western slope of the relatively dormant Haleakala volcano, the highest peak on Maui. They set out 55 partially buried "pitfall" traps -- large jars filled with an automobile antifreeze solution -- and returned after two weeks to count the insects captured and preserved. They also recorded the insect species present under 622 rocks.
Loope says he and his colleagues captured many fewer species of native insects than expected in areas in which they caught large numbers of Argentine ants. Moreover, he says, they found that other alien insects were more prevalent at sites that had lots of ants, suggesting that the ants pave the way for further alien invasions. The researchers present a detailed description of their study in the August ECOLOGY.
"If this ant spreads, it's going to decimate the native fauna wherever it goes," Loope predicts. Because the Argentine ant eats insects that pollinate many of Haleakala's native plants, its presence could indirectly lead to the eradication of the only native plants that can exist at high elevations, he says. His team also has evidence that the ants are usurping the ecosystem's normal top-of-the-food-chain predators -- wolf spiders and carbid beetles -- an event that they may allow populations of native and alien insects further down the food chain to grow unchecked.
Some of the most destructive alien species were introduced to the Hawaiian islands intentionally. Michael G. Hadfield and Stephen E. Miller of the University of Hawaii's Kewalo Marine Laboratory in Honolulu are studying the fate of spectacularly striped Oahu tree snails from the genus Achatinella following the introduction of the cannibal snail, Euglandina rosea.
In the 1950s, ecologists imported the cannibal snail from Florida to the Hawaiian island of Oahu in an attempt to control another alien species, the giant African snail (Achatina fulica), a garden pest. However, when the carnivorous snails arrived, they turned out to prefer the taste of native Achatinellae to that of the African pests they were intended to eradicate.
"[Cannibal snails] are devouring the last of the Oahu tree snail species," Hadfield warned the August ecology conference. Of the 41 species of Oahu tree snails recorded since 1900, he says. "only 19 or 20 at the very best" remain. And many of the remaining species, he adds, are represented by fewer than 100 individuals.
Rats, whose ancestors traveled to Hawaii on 18th century ships, also prey on the Oahu tree snails, Hadfield says. He adds that shell collectors and the increasing development of Oahu have further contributed to their decline.
Alien species don't have to be predators. They can disrupt established ecosystems if they meet at least one of three other criteria, according to Peter M. Vitousek of Stanford University. He writes in the February 1990 OIKOS that invaders that alter the food chain, acquire or use resources differently than native species, or change fundamental characteristics of the ecosystem will have detrimental effects on native flora and fauna.
One of Vitousek's colleagues, Carla M. D'Antonio of the University of California, Berkeley, reported last August that alien grasses in particular fulfill the last condition. At the August ecology conference, she discussed the fire-promoting dangers in Hawaii of grasses originating from Africa or North America.
These alien grasses, such as the beard grass, Schizachyrium condensatum, now constitute 93 percent of the standing dead biomass in Hawaiian dry forests, D'Antonio says. Between 1920 and 1967 - about the time the grass was introduced to Hawaii by ranchers or imported livestock - the island experienced only 27 major fires, each covering an average of four hectares. In contrast, between 1967 and 1989, Hawaii's forests were swept by 58 large fires, each covering more than 200 hectares. D'Antonio attributes the surge in fire incidence and damage to the tinder-box qualities of S. condensatum.
What's worse, she says, alien grasses recover from a fire more quickly than do native plant species and effectively choke them out. This disruption, in turn, increases the ecosystem's vulnerability to further alien invasion. One of these secondary alien species, molasses grass (Melinis minutiflora), is even more fireprone than S. condensatum, because it burns even in a slightly moist environment, says D'Antonio. The presence of molasses grass therefore increases the likelihood that a particular stretch of land will burn a second time.
Furthermore, "these sites don't appear to recover," says D'Antonio. She notes that, on average, previously burned areas contain one-tenth the number of native species that unburned regions do. Fires also deplete nitrogen from Hawaii's already nitrogen-poor soils, she adds, so twice-burned areas have only roughly half the usual amount of this important plant nutrient.
The end result of this alien grass invasion is the conversion of the Hawaiian dry forest - the home of many birds and pharmaceutical-bearing plants found nowhere else on Earth - into treeless, grassy savannas swept periodically by wildfires, D'Antonio concludes.
Pigs are one of the oldest habitat-disrupting alien species in Hawaii. First brought to the Hawaiian islands by the Polynesians, the pig's proclivity for eating bark and roots has wrought major damage to many areas, according to David Foote of the Hawaii Volcanoes National Park on the island of Hawaii.
Foote and U.S. National Park Service colleagues Charles P. Stone and Linda W. Cuddihy have studied the ecological effects of exclosures - areas surrounded by pig-proof fences - in Volcanoes National Park. The park contains many species-rich enclaves, called kipukas, that exist between the fingers of recent lava flows.
Foote told the August ecology conference that exclosures built in kipukas during the early 1980s contain more species of native arthropods and fewer species of alien arthropods than do similar exclosures constructed two years ago. He attributed the difference to the elimination of feral pigs, which uproot large tree ferns and gnaw the bark of hardwood trees, killing and toppling them. This, in turn, exposes the forest floor to more sunlight, which can disrupt the entire forest ecosystem.
Foote's group found that only one genus of native arthropods - damselflies - benefited from the presence of pigs. The delicate insects thrived in the more recent exclosures, the researchers discovered, by taking advantage of water-filled pig wallows as sites for laying eggs.
Further studies of the effects of feral pig populations and other alien species are among the seven goals of the Hawaii Conservation Biology Initiative, a project organized by the Nature Conservancy of Hawaii in Honolulu and initially funded by a $500,000 grant from the John D. and Catherine T. MacArthur Foundation. The initiative - drafted by representatives from 19 universities, federal and state agencies, environmental groups, and botanical gardens - seeks to coordinate ecological research in Hawaii.
"There's a great deal of research being done," says one of the initiative's chief organizers, Colin Bassett of the Nature Conservancy of Hawaii and the University of Hawaii. "But even more is needed if we're going to understand and protect the unique ecosystems here," he says.
Besides feral pig research, the initiative aims to stimulate studies of alien plant species, native forest birds, the ecology of rare native plants and animals, and the vegetation dynamics of selected plant communities. It also plans to facilitate the monitoring of native and alien organisms, as well as studies of the best ways to restore Hawaii's ecosystems to a "self-sustaining, natural condition," according to an outline of research priorities.
Patrick Dunn, an ecologist at the Nature Conservancy of Hawaii, says the initiative will dispense money for a small number of "seed" grants to get researchers started on enterprising projects - especially those that cut across academic disciplines. Initative administrators will then help such researchers obtain further funding from government and private organizations.
The initiative plans a system of biological field stations throughout the Hawaiian islands to serve as outposts for researchers working in the field. Two such stations have already been built - at Pelekunu on the island of Molokai and in the Haleakala National Park on Maui. The initiative also has established a Secretariat for Conservation Biology at the University of Hawaii to organize scientific conferences and serve as ongoing headquarters for the project.
Dunn hopes the research initiative will help target studies in areas useful to conservation managers, the keepers of Hawaii's great diversity. "Managers are faced every day with the possibility of species going extinct," he says. "The threat is tremendous, but we expect [the initiative] to make a real difference."
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|Article Type:||Cover Story|
|Date:||Nov 7, 1992|
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