Biological invasions: A growing threat

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

An army of invasive plant and animal species is overrunning the United States, causing incalcuable economic and ecological costs.

To the untrained eye, Everglades National Park and nearby protected areas in Florida appear wild and natural. Yet within such public lands, foreign plant and animal species are rapidly degrading these unique ecosystems. Invasive exotic species destroy ecosystems as surely as chemical pollution or human population growth with associated development.

In July 1996, the United Nations Conference on Alien Species identified invasive species as a serious global threat to biological diversity. Then in April 1997, more than 500 scientists called for the formation of a presidential commission to recommend new strategies to prevent and manage invasions by harmful exotic species in the United States.

Already, many states attempt to maintain their biological heritage, and a number of state and federal regulations restrict harmful species. Unfortunately, for a variety of reasons, such tactics have failed. Without greatly increased awareness and coordinated efforts, the devastating damages will continue.

Exotic species have contributed to the decline of 42 percent of U.S. endangered and threatened species. At least 3 of the 24 known extinctions of species listed under the Endangered Species Act were wholly or partially caused by hybridization between closely related exotic and native species. After habitat destruction, introduced species are the second greatest cause of species endangerment and decline worldwide-far exceeding all forms of harvest. As Harvard University biologist E. O. Wilson put it, "Extinction by habitat destruction is like death in an automobile accident: easy to see and assess. Extinction by the invasion of exotic species is like death by disease: gradual, insidious, requiring scientific methods to diagnose."

According to a 1993 report by the (now defunct) congressional Office of Technology Assessment (OTA), lack of legislative and public concern about the harm these invasions cause costs the United States hundreds of millions, if not billions, of dollars annually. This includes higher agricultural prices, loss of recreational use of public lands and waterways, and even major human health consequences. About a fourth of U.S. agricultural gross national product is lost to foreign plant invaders and the costs of controlling them. Foreign weeds spread on Bureau of Land Management lands at over 2,300 acres per day and on all western public lands at twice that rate.

Other effects on private land are more obvious. The spread of fire-adapted exotic plants that burn easily increases the frequency and severity of fires, to the detriment of property, human safety, and native flora and fauna. In 1991, in the hills overlooking Oakland and Berkeley, California, a 1,700-acre fire propagated by Eucalyptus trees planted early in this century destroyed 3,400 houses and killed 23 people.

Over the past two centuries, human population growth has substantially altered waterways and the natural landscape. Once contiguous across the entire United States, wetland and upland ecosystems are often mere remnants that are now being degraded and diminished by nonindigenous species invasions. This exacerbates the problem of conserving what remains of our country's biological heritage.

At the same time, nonindigenous crops and livestock, including soybeans, wheat, and cattle, form the foundation of U.S. agriculture, and other exotic species play key roles in the pet and nursery industries and in biological control efforts. Classifying a species as beneficial or harmful is not always simple; some are both. For example, many imported ornamental plants are used in manicured landscapes around our homes. On the other hand, about 10 percent of these same species have escaped human cultivation, some with devastating ecological or economic results.

Scientists wake up

Until the past decade or so, conservationists were often complacent about nonindigenous species. Many shared the views of Charles Elton in his 1958 book The Ecology of Invasions of Plants and Animals, which introduced generations of biologists to invasion problems. He contended that disturbed habitats, because they have fewer or less vigorous species, pose less "biotic resistance" to new arrivals. Conservationists now realize that nonindigenous invaders threaten even species-rich pristine habitats. The rapidly increasing conservation and economic problems generated by these invasions have resulted in an explosion of interest and concern among scientists.

In the United States, invasive plants that constitute new habitats and dramatically alter a landscape or water body have some of the greatest impacts on ecosystems. On land, this could be the production of a forest where none had existed before. For example, sawgrass dominates large regions of Florida Conservation Area marshes, providing habitat for unique Everglades wildlife. Although sawgrass may be more than 9 feet tall, introduced Australian melaleuca trees are typically 70 feet tall and outcompete marsh plants for sunlight. As melaleuca trees invade and form dense monospecific stands, soil elevations increase because of undecomposed leaf litter that forms tree islands and inhibits normal water flow. Wildlife associated with sawgrass marshes declines. The frequency and intensity of fires change, as do other critical ecosystem processes. The spread of melaleuca and other invasive exotic plants in southern Florida could undermine the $1.5-billion effort to return the Everglades to a more natural state.