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

CASE HISTORY:

Dynamics of Weed Populations in the south Okanagan

Herbarium records indicate that sulphur cinquefoil was first discovered in the Okanagan Valley in 1940, only one year after diffuse knapweed was first documented. Dalmatian toadflax was recognized a mere twelve years later. Diffuse knapweed rapidly dominated the natural landscape, in both disturbed and undisturbed habitats (Myers and Berube 1983; Taylor 1995), and has been particularly devastating to the ranching economy. However, sulphur cinquefoil and Dalmatian toadflax, two extremely aggressive perennials, appear to have exhibited a rapid expansion phase only during the last 10-15 years (J. Pethybridge, Range Officer and P. Youwe, Range Resource Agrologist, Ministry of Forests, pers. comm., 1997). These two species also appear to be successfully invading well-established diffuse knapweed sites (Rice 1993; Powell 1996; P. Youwe, Range Resource Agrologist, Ministry of Forests and C. Hinkley, Tonasket Forest Service, pers. comm., 1997).

Weed invasions often remain unrecognized until populations are well-established and consequently, herbarium records may be a poor source of information. However, a significant amount of anecdotal information supports these seemingly large differences in length of the lag periods. Several possible explanations have been explored in an attempt to elucidate this important issue.

As non-native species have evolved under dissimilar conditions, they will take different amounts of time to adapt to a new environment and the timing of events is difficult to predict. Time lags between introduction and the beginning of an invasion are generally longer for perennials compared with annuals (Toney et al. in press); this can potentially be expanded to include biennials as also having a shorter time lag. This could help to explain the apparent differences between the two perennials and diffuse knapweed which usually exhibits a biennial behaviour, although weed specialists do not consider the time lag observed for sulphur cinquefoil and Dalmatian toadflax to be very long (C. Toney, Division of Biological Science, University of Montana, in lit., 1997). The important point however, is that the species that start invasions most quickly are not necessarily the most successful ones (Kowarik 1995).

Another consideration is dispersal mechanisms. Forcella (1985) found that those species which spread fastest in northwestern United States were those that dispersed as contaminants of cereal and forage seed. Diffuse knapweed is believed to have arrived to BC as a contaminant in crop seed (BC Ministry of Agriculture, Fisheries and Food, undated pamphlet). Since knapweed establishment, movement of hay from knapweed infested areas to uninfested areas has been considered a serious problem, in addition to the spread of seeds and plant fragments by vehicles, trains, machinery, wildlife and livestock (BC Ministry of Agriculture, Fisheries and Food, undated pamphlet). Diffuse knapweed is much more mobile than the other two species and will therefore have moved to new sites more rapidly.

One final thought relates to the active biological control program that was initiated on diffuse knapweed in the early 1970s. The complex of bioagents are presumably stressing the knapweed, thereby influencing the plant’s ability to establish and complete with the more aggressive perennials. This allows sulphur cinquefoil and Dalmatian toadflax to successfully invade diffuse knapweed sites. Additionally, preferential grazing of knapweed species in habitats containing sulphur cinquefoil (Rice 1993), may also promote the decline of the knapweed and the dominance of sulphur cinquefoil. Sulphur cinquefoil is unpalatable to most livestock and wildlife due to a high concentration of phenolic compounds in the leaves and stems (Rice 1993). The same may hold true for Dalmatian toadflax which is grazed by few mammals and is not a preferred species (Robocker 1974). Eventually, each species will display a different ecological amplitude, and weed communities will most likely contain all three species rather than one completely displacing the others (R. Old, Weed Identification Specialist, XID Services Inc., in lit., 1997).