Exotic plants threaten natural communities worldwide, yet only a fraction of exotic species behave as strong invaders, overrunning plant communities at the expense of native biota (Ortega and Pearson 2005). Even so, we still do not understand what factors determine the success of invaders, distinguishing the strong from the weak. A leading hypothesis states that exotic plants become strong invaders by escaping the controlling influence of their natural enemies. This model serves as the underlying basis for biological control, a prevalent weed management tool which aims to suppress target invaders by introducing their natural enemies. However, the conditional success of biocontrol efforts suggests that abiotic factors may play a prominent role in determining the invasion strength of exotic plants and the ability of natural enemies to suppress them. To improve the efficacy of weed management programs, research is needed to understand the factors limiting the success of strong invaders.
We are conducting a common garden experiment to isolate key factors potentially affecting the dominance of the strong invader, spotted knapweed. Both the biocontrol agent Cyphocleonus achates and intensified drought conditions are thought to be factors in knapweed's recent declines. Our key objectives are to address the following questions: (1) What are the impacts of C. achates on knapweed populations under drought versus normal precipitation conditions? (2) How is the competitive dominance of spotted knapweed over native bunchgrasses affected by C. achates and drought conditions? (3) Can information about spotted knapweed water stress be used to better deploy C. achates in the field for more effective knapweed control? (4) What are optimal conditions for rearing C. achates for distribution?
Findings from the first three years of the study (Ortega et al. in press) show interactive effects of natural enemies and drought on spotted knapweed populations. C. achates reduced survival of knapweed plants, but only in plots with native grasses under normal precipitation conditions. Drought similarly reduced knapweed survival in mixed species plots. However, the ultimate effect of C. achates on knapweed populations depends not only on the survival of knapweed plants but also on the extent to which dead adults are replaced with new individuals. Indeed, we found that reduced survival of adult knapweeds was correlated with increased recruitment of new adults. Thus, knapweed populations fully compensated for losses attributable to biocontrol herbivory and drought. Additionally, native grasses showed no appreciable benefit from reduced survival of knapweed adults. Our results underscore the importance of both abiotic factors and biotic factors in conditioning biocontrol impacts on individuals, and the primacy of population-level compensation in determining ultimate outcomes.
- The University of Montana
- USFS Forest Health Protection (Region 1)