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

ANALYSIS FOR TRENDS IN SPECIES OCCURRENCE AND ABUNDANCE

Terrestrial Species

The detrimental impact of the Centaurea species on the BC’s rangeland resource prompted the provincial government to develop an effective weed control program. The BC Ministry of Forests instigated a biological control program in concert with a chemical containment program in the early-mid 1970s to combat knapweed (P. Youwe, Range Resource Agrologist, Ministry of Forests, pers. comm., 1997). While populations of diffuse knapweed have evidently stabilized, spotted knapweed infestations appear to be expanding, particularly at higher elevations on isolated sites (J. Pethybridge, Range Officer and P. Youwe, Range Resource Agrologist, Ministry of Forests, pers. comm., 1997). Apparent hybrids of spotted and diffuse knapweed may increase the ecological amplitude of the knapweed species, although research has not investigated the implications of hybridization.

Although the Sunflower family is the largest family of weeds occurring within the Montane Cordillera, some extremely invasive species in the Figwort, Spurge and Rose families are rapidly spreading over large geographic areas. New colonies of these species are establishing at a phenomenal rate within this Ecozone, and many infestations are apparently reaching environmentally severe size and density. Unlike many other introduced plant species, these weeds are invading and displacing native vegetation in undisturbed areas (White et al. 1993; Taylor 1995; Powell 1996; Whitson et al. 1996). In the south Okanagan, the rapid invasion and spread of sulphur cinquefoil and Dalmatian toadflax pose a threat of significant environmental and economic impact.

Wetland and Aquatic Species

Although the geographic distribution of purple loosestrife continues to increase, established populations do not appear to be expanding peripherally (A. Peatt, Habitat Protection Branch, Ministry of Environment, pers. comm., 1997). A biological control program has been initiated for purple loosestrife infestations in southern BC and is expected to have a significant impact on the loosestrife infestation in the near future (D. Ralph, Provincial Weed Technician, Ministry of Agriculture, Fisheries and Food, pers. comm., 1997). Evidence from the United States suggests that it may be possible to reduce the impact of purple loosestrife in large areas with an effective biocontrol program (G. Piper, Washington State University and M. Hudson, Washington Department of Fish and Wildlife, pers. comm., 1997).

Physical control of Eurasian watermilfoil through mechanical harvesting was initiated in the 1970s to reduce plant biomass. However, the plant can quickly regrow and the artificial creation of a large number of fragments presumably enhances the spread of the plant (BC Department of Environment, Water Resources Service 1976; BC Ministry of Environment, Water Investigations Branch 1977; White et al. 1993). Recent observations indicate that Eurasian milfoil populations have stabilized and recent reports suggest that previously dense infestations are reverting back to historical macrophyte populations (B. Nijman, Water Management Branch, BC Environment, in lit., 1997).