Biological invasions: A growing threat

Issues in Science and Technology, Summer 1997 by Schmitz, Don C, Simberloff, Daniel

Throughout the world, such invasions threaten biodiversity. In Australia, invasion by Scotch broom led to the disappearance of a diverse set of native reptiles and to major alteration of the composition of bird species. On the island of Hawaii, the tall Atlantic shrub Myrica faya has invaded young, nitrogen-poor lava flows and ash deposits on the slopes of Mauna Loa and Mauna Kea. Because it fixes nitrogen, it inhibits colonization by native plants, favoring other exotic species.

Plant communities offering little forage value ultimately lower wildlife abundance or alter species composition. Invading plant species often exclude entire suites of native plants but are themselves unpalatable to native insects and other animals. Two Eurasian plants-spotted knapweed, which infests 7 million acres in nine states and two Canadian provinces; and leafy spurge, which occupies 1.8 million acres in Montana and North Dakota alone -provide poor forage for elk and deer. Likewise in Florida, the prickly tropical soda apple from Brazil and Argentina excludes native palatable species. Losses to the local cattle industry are over $10 million per year, or about 1 percent of gross revenues.

Bird, reptile, and amphibian invasions may also devastate individual native species but generally do not cause as much damage as exotic plants. Herbivorous mammals and insects are often far more troublesome. In Great Smoky Mountains National Park, feral pigs descended from a few that escaped from hunting enclosures in 1920 devastated local plant communities by selectively feeding on plants with starchy bulbs, tubers, and rhizomes and by greatly changing soil characteristics. In parts of the southern Appalachians, two related insects, the hemlock woolly adelgid and the balsam woolly adelgid, defoliate and kill dominant native trees over vast tracts. Host trees have not evolved genetic resistance, and native predators and parasites of the insects are ineffective at slowing their advance.

The zebra mussel from the former Soviet Union has clogged the water pipes of many electric companies and other industries, particularly in midwestern and mid-Atlantic states. It also threatens the existence of many endemic native bivalve molluscs in the Mississippi Basin. Infestations in the midwest and northeast cost power plants and industrial facilities nearly $70 million between 1989 and 1995.

Death by disease

Introduced animal populations can also harm their native counterparts by competing with them, preying on them, and propagating diseases. For example, a battery of introduced Asian songbirds are host to avian pox and avian malaria in the Hawaiian Islands; native birds are especially susceptible. Introduced species can also gradually replace native species by mating with them, leading to a sort of genetic extinction.

Pathogens are among the most damaging invaders. Plant pathogens can change an entire ecosystem just as an introduced plant can. The chestnut blight fungus, which arrived in New York City in the late l9th century from Asia, spread in less than 50 years over 225 million acres of the eastern United States, destroying virtually every chestnut tree. Because chestnut had comprised a quarter or more of the canopy of tall trees in many forests, the effects on the entire ecosystem were staggering, although not always obvious. Several insect species restricted to chestnut are now extinct or endangered.