Invasive Plants: Their Impacts and Control in Changing Environments

While it is frequently stated that invasive species are second only to habitat destruction as a cause of species extinctions, this generalization does not apply to plants. In this paper, we briefly discuss three issues concerning the impacts of exotic plants: the impact of an introduced plant species on a rare native plant species, the potential long-term impacts of exotics on plant communities, and the possible interactions between biological control success and climate change. Purple loosestrife (Lythrum salicaria), is a European plant species that is now widespread in wetland areas of northern North America. We predicted that the invasion of purple loosestrife might reduce the density of a rare plant species, Henderson's checker-mallow (Sidalcea hendersonii), in a tidal marsh habitat in British Columbia; however, this was not the case. Thus, just because a new species becomes common does not mean it will threaten rare native species. Predicting species interactions is difficult. Introduced plants, however, can have long-term impacts on plant communities. For example, legumes can change soil nitrogen, and through soil enrichment, can create long-term changes to invaded communities. Plants that efficiently use soil moisture can also prevent the reestablishment of native species and can reduce biodiversity. In these situations, the invasive plants act as ecological engineers. We discuss two examples of this: the invasion of threatened Garry oak (Quercus garryana) meadows by Scotch broom (Cytisus scoparius) and the persistence of crested wheatgrass (Agropyron cristatum ssp. pectinatum) in dry rangelands. In these situations, the invasive plants could possibly act as ecological engineers. Models of climate change predict increased drought in some regions of Canada. Between 2001 and 2003, rainfall in the Okanagan Valley was well below normal levels, although similar drought conditions have occurred in the recent past. Over this same time period, an invasive rangeland weed, diffuse knapweed (Centaurea diffusa), declined dramatically in density following high levels of attack by a recently introduced biological control agent, a weevil, Larinus minutus. Rainfall and knapweed density are related, and the feeding damage by this weevil likely increased the drought stress of knapweed plants and may have contributed to their demise.

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