Strangers in Paradise: Alien Species Disrupt the Ecology of Hawaii

Article excerpt

Hawaii is Earth's most isolated archipelago. For 70 million years, newcomers to this chain of islands had to fly or float to reach their new home. Ecologists estimate that a new species arrived in Hawaii from afar only once every 100,000 years, when a bird got blown off its migratory course or when plant debris washed ashore from the nearest continental landmass roughly 2,500 miles away.

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. …