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Why Are Weeds Bad?

Overview
Economic Losses
Effects on Natural Areas
Government and Other Organized Involvement
The Spread of Weeds
Overview

Human activity moves many species across long-standing biogeographic barriers; the consequent biological invasions blur the regional distinctiveness of Earth's biota (Vitousek 1994). One consequence of this activity is weeds. They damage natural areas, cause economic losses to cropland and forage production, alter ecosystem processes, displace native species, hybridize with natives and change their genetic makeup, and support other non-native plants, animals and pathogens (Randall and Marinelli 1996). According to a recent survey by the U.S. Department of the Interior, noxious weeds have invaded over 17 million acres of public rangelands in the West, more than quadrupling their range from 1985 to 1995 (Westbrooks 1998). On National Forest Service (NFS) lands, an estimated six to seven million acres are currently infested by weeds, which have the potential to increase at a rate of 8 to 12 percent per year (USDA 1998).

Economic Losses

Weeds cause economic losses. Based on research by the U.S. Congress, Office of Technology Assessment (OTA) (1993), from 1906 to 1991, 15 problem plants caused documented losses of $603 million based on cost of control. Another $134 billion in future economic losses could be caused by 15 potentially invasive species. Total losses from weed competition with major crops produced in the United States and Canada were approximately $7 billion per year in 1984 (Chandler 1985). This same survey method was repeated in 1991 and estimated a monetary loss caused by weeds in 46 crops to be $4.1 billion for that year (Bridges 1992). This did not include cost of control. If herbicides were not available, this loss was estimated to be 19.6 billion (Bridges 1992). Total expenditures for herbicides in the United States were approximately $2.1 billion for 1984 and application of these herbicides cost an additional $938 million (Chandler 1985).

Effects on Natural Areas

While the threat of losses caused by weeds in agricultural production has been recognized for centuries, there currently is an increased awareness and concern about weeds in natural areas. For example, the Refuge Manager for the U.S. Department of Interior Fish and Wildlife Service, opined that noxious weeds may be the greatest long term threat to the productivity of the Monte Vista and Alamosa National Wildlife Refuges, which have been documented as some of the most productive wetlands in North America (Blenden 1998). Dr. Dennis Childs, Department Head, Rangeland Ecosystem Science, Colorado State University, conducted a survey to determine the concerns of rangeland managers in Colorado. They identified noxious weed management as one of the greatest research priorities (personal communication, May to Childs 1999).

Government and Other Organized Involvement

Legislation, administration, and planning initiatives reflect the increased concern about weed invasions. For example, based on recommendations from the Task Force on Invasive Upland Exotic Plants, the Florida legislature recently amended State statutes to include the control of invasive upland exotic plants, such as cogongrass [Imperata cylindrica (L.) Beauv.], on public lands (USDA 1998). In 1996, the Colorado Legislature created a statewide noxious weed management fund to provide financial assistance to communities throughout the state to stimulate and support their weed management efforts.

Recently, there has been the development of cooperative weed management areas (WMA's) with neighbors through programs such as the "Pulling Together" partnerships that help to pool resources and expertise to accomplish more in prevention, treatment, and control than any single party could achieve (USDA 1998).

The Spread of Weeds

Most data on the spread of weeds indicates that they are increasing in abundance and distribution. The Federal Interagency Committee for the Management of Noxious Weeds (FICMNEW) (1998) warned that if weeds continue to spread at the estimated annual rate of 14%, 33 million acres of western wildlands will be dominated by weeds in year 2000 (Westbrooks). Some sources argue weeds are increasing due to an increase in land disturbance, such as overgrazing and land use changes such as development (Westbrooks 1998). However, it is difficult to determine to what extent data on increasing weed distributions are actually due to spread of weeds versus increased awareness of weeds. Westbrooks (1998) recognizes the possible error between estimated economic losses caused by weeds and the actual losses. In his opinion, estimates are considered conservative because careful inventories have not been carried out in many regions (Westbrooks 1998). Others believe that weed supervisors have a tendency to overestimate weed acreages and yet others believe that figures tend to be in greater error when acreages become widespread, such as Canada thistle.

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