Lisa Scott. 1998. Weed Invasion in Smith, I.M., and G.G.E. Scudder, eds. Assessment of species diversity in the Montane Cordillera Ecozone. Burlington: Ecological Monitoring and Assessment Network, 1998.

WEED INVASION

Lisa Scott

THE IMPACT OF INVASIVE PLANTS

Non-native plant species are adapted to a wide range of environmental conditions but occur most often in disturbed soils and depleted rangelands (Robocker 1974; Upadhyaya and Cranston 1991; White et al. 1993; Harding 1994; Hobbs and Humphries 1995). Some species, however, are able to invade areas with little or no soil disturbance (Myers and Berube 1983; Taylor 1995). Weeds can invade a broad variety of habitat types, compromising land values for wildlife, livestock, agricultural crops, recreation and aesthetics. The range of a species is governed by the availability of suitable habitats and the presence of dispersal barriers (Cousens and Mortimer 1995). When conditions are opportune, dense stands of tenacious weeds can displace native flora and alter the biological structure and possibly the functions of natural ecosystems (Hoglund et al. 1991; Kummerow 1992; White et al. 1993; Harding 1994; Powell et al. 1994; Cousens and Mortimer 1995; Kay et al. 1995; Bureau of Land Management 1996; USDA Forest Service 1997).

The highly competitive nature of weeds is partly due to a lack of natural enemies or parasites to keep populations in check. Prolific seed production, varied dispersal strategies and an ability to tolerate severe stress conditions such as drought, soil compaction and frequent disturbances contribute to the ecological success of weeds and their often rapid colonization of habitats. In the Montane Cordillera Ecozone, as elsewhere, weeds frequently become established in highway rights-of-way, gravel pits, utility corridors, recreation sites, heavily grazed areas, mining sites, logged areas and other areas of soil disturbance. Their often close association with human-managed habitats affords them a powerful means of dispersal; seeds are readily spread by vehicles, livestock, wildlife, humans, contaminated seeds, hay or topsoil, and through agricultural practices.