COS 278-1 - Evolutionary ecology of variable biological control of spotted knapweed in British Columbia

Background/Question/Methods

Invasive species remain a leading cause of biodiversity loss globally. Spotted knapweed (Centaurea stoebe ssp. micranthos) is a widespread and problematic invader of rangelands across the Pacific Northwest. Together with diffuse knapweed (C. diffusa), spotted knapweed has been the subject of a North American biocontrol programme since the 1970s, resulting in the release of 13 insect agents. Despite effective biocontrol of diffuse knapweed in BC, and widespread reductions in the density and fecundity of spotted knapweed across much of the province, pockets of abundant spotted knapweed remain with considerable ecological costs, as well as substantial costs to rangeland tenure holders. We have previously demonstrated overall declines under biocontrol, but these effects are highly variable among sites, a common pattern in widespread invasive weeds. To explain this pattern in spotted knapweed, we sampled 23 long-term monitoring sites across southern BC for abundance and density of spotted knapweed and its control agents. We dissected >650 individual knapweed plants, and >6300 seed heads to quantify plant morphology, fitness, and control agent presence and abundance, and used a mix of interpolated and directly quantified measures of climate and environment to explain the high variability of control efficacy across BC’s diverse landscape.

Results/Conclusions

Collating spotted knapweed density data from the last 50 years, we re-affirmed the long term decline in spotted knapweed density under biological control. We augment this finding with demonstrated fitness reductions due to biological control agents, in most cases lowering fecundity below self-sustaining population levels. However, a number of our study sites continue to support robust spotted knapweed populations. Using an evolutionary ecology lens, we identified spotted knapweed life history traits, short-growing season sites as refugia from a key control agent species, and diverse land use practices as key predictors of knapweed success. Taken together, the longer term successful control of spotted knapweed in BC will require an integration of control efforts and revised land use to reduce this species’ abundance to non-problematic levels province-wide. Our study has applicability continent-wide as spotted knapweed continues to spread across USA and Canada. Moreover, the methods employed herein can be applied to investigate similarly distributed invasive plant species where control is variable, to reduce the ecological and economic impacts of such species.