Competitive interactions and associations of the invasive thistles Carduus nutans and C. acanthoides

As the number of biological invasions continues to increase, the degree to which communities can resist invasions and the interactions between multiple invasive species will become increasingly important. This will have important consequences for existing communities, as the combined impact of multiple invaders may not simply be additive. Using experimental, observational and modeling approaches, we investigated the direct competitive interactions between Carduus nutans and C. acanthoides, two congeneric invasive thistle species. It was hypothesized that their observed spatial segregation was due to interspecific competition. Spatially-explicit simulation models of competitive interactions between these species were developed for both the landscape and the field levels, to explore the range of behaviors predicted. We examined direct interspecific competition using three response surface field experiments. Additionally, the thistle distribution patterns were quantified at two resolutions: the regional level in the area of overlap, and the field level in four fields of natural co-occurrence. Their interactions with the existing vegetation were examined in two ways: by quantifying their vegetative associations in four fields of co-occurrence, and by experimentally examining their germination and establishment response to microsite characteristics. The competition experiments did not demonstrate strong competition or density dependence. The simulation models do generate spatial segregation, but not for parameters corresponding to the results of the competition experiments; a gradual decay of the segregation pattern with eventual coexistence is more likely. The landscape-scale observational studies indicate negative associations between these species, but at the field scale, which is the scale at which competition would occur, C. nutans and C. acanthoides are positively associated, implying that competition between them does not lead to exclusion. While there were significant differences in the plant communities in areas with and without thistles, there were no significant differences in communities associated with C. nutans versus C. acanthoides. Both species were found to be sensitive to microsite characteristics, with generally better establishment in larger gaps, and better survival in gaps with less disturbance. Our results reject the hypothesis that resource competition between these two species strongly influences their distributions; instead, our results suggest that interactions with other community members, combined with the spread history of these species, likely most strongly influence the current distribution. Although strong competitive interactions were not observed between these two congeneric invaders, interactions between invasive species will only continue to increase, and the combined empirical and theoretical approach presented here will be critical to assessing how such interactions influence the success and impact of invasive species.