SCIENCE AND RESEARCH
Invasive Exotic Plants Are Destroying the Naturalness of U.S. Wilderness Areas BY JERRY E. ASHER AND DAVID W. HARMON Introduction
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IKE A THUNDERHEAD building on a hot summer day, there is a growing peril looming over the wilderness landscape—the rapid expansion of invasive exotic plants into wilderness ecosystems. These invaders are beginning to destroy natural ecosystems in far too many wilderness areas.Without major increased control efforts these aggressive plants will continue expanding through and degrading the lands we value most: designated wilderness and wilderness study areas. Wilderness managers and the public must first understand more about the proliferation of exotic plants and their often permanent damage to ecosystem health. If we act quickly, however, on a large scale, cooperative exotic plant management can be economical and effective.
Exotic Plant Invasions on Western Wildlands Since exotic plants arrived in this country without their natural enemies, they are typically very aggressive.They prefer disturbed sites, but they commonly invade relatively undisturbed plant communities as well. “Several exotic, noxious, perennial weeds, including spotted, diffuse and Russian knapweeds, leafy spurge, and yellow star thistle, are moving into excellent condition stands of native vegetation” (Harris 1991). Tyser and Key (1988) reported that spotted knapweed invaded and reproduced in rough fescue communities in Glacier National Park. Forcella and Harvey (1983) documented Eurasian weeds dominating relatively undisturbed grasslands in Montana. “Several exotic weeds will invade undisturbed climax communities and can become significant components of a community” (Bedunah 1992). While discussing the ecological equilibrium of native communities, Bedunah also noted “that the introduction of exotic plants can throw this balance off, possibly forever.” Weeds are spread by vehicles, humans, horses, livestock, wind, water, and wildlife. For example, S. Dewey (personal comment 1993) reported that birds spread weed seeds, and leafy spurge is being spread by elk and deer in the Naomi wilderness area in Utah. According to D. Lange (personal comment 1993) Elk have been reported to eat knapweed seedheads. It also has been shown that over 13% of spotted knapweed seeds pass undamaged through the digestive tracts of mule deer (Wallander et al. 1992). D. Lange also noted that ground squirrels have also been observed carrying knapweed seedheads into their burrows (personal comment 1993). To address the management of exotic plants in wilderness we must understand the exotic plant invasions in adjacent lands. Exotic plants increased on the U.S. Bureau of Land Management (BLM) lands from 2.5 million acres in 1985 to over 8 million acres in 1994.When exotic plant populations on the U.S. Forest Service,
Dyer’s woad, subject to transport by people and wildlife along a trail in the Wellsville wilderness in Utah.
National Park Service, and the US. Fish and Wildlife Service lands are included, that acreage nearly doubles. Recognizing that exotic plants typically increase at about 14% per year if unchecked, the increased infestation rate on these federal lands is now approximately 4,600 acres per day, an “explosion in slow motion.” A few examples of recent exotic plant increases will serve to demonstrate the rapid invasions that are underway. Spotted knapweed, first reported in Montana in 1920, has increased to over 4 million acres in that state alone. Similarly, there are over 600,000 acres of leafy spurge in Montana. During the last 30 years, leafy spurge has increased from 200,000 acres into over 1 million acres in North Dakota. Since 1977 yellow star thistle infestations in northern California have spread from 1 million acres to over 10 million acres today. In Idaho, rush skeleton weed has expanded from 40 acres in the early 1960s to over 4 million acres today. Exotic plants typically spread more slowly in wilderness but these areas are not immune. For example, parts of the Selway– Bitterroot Wilderness in Idaho are severely infested with spotted knapweed, and acreage estimates range up to 40,000 acres. Sheep and cattle have not grazed in this area for over 50 years and logging and mining have been absent.The exotic plant introduction has been attributed to recreational activities.
Exotic Plants Destroy the Natural Integrity of Wilderness The Wilderness Act of 1964 mandates that wilderness be administered so its community of life is untrammeled by man, its primeval character is retained, and its natural conditions are preserved (PL88– 577). Policy directs that wilderness be managed to ensure that its ecosystems and ecological processes function naturally (USDI BLM
INTERNATIONAL JOURNAL OF WILDERNESS / Volume 1, Number 2, December 1995
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