Propagule pressure, insect herbivory, and spatio-temporal variation in the establishment of an exotic thistle

Background/Question/Methods

A current challenge in ecology is to better understand the role of biotic resistance in limiting plant invasions. We know little about the spatial extent of locally-documented native herbivore-exotic plant interactions or about how propagule pressure and herbivory jointly influence non-native plant establishment in new environments. In this study, we quantified the effect of native insect herbivory on experimentally introduced populations of the non-native thistle, Cirsium vulgare, for two temporal seed cohorts across eight sites in western tallgrass prairie, where this exotic thistle is not increasing invasively. For each year and site combination, we established insecticide-treated versus control plots across seven levels of seed addition. 

Results/Conclusions

Insect herbivores consistently reduced seedling survival and establishment across years and sites. Additionally, herbivores shifted the establishment probability curve, dramatically decreasing the rate at which increases in propagule pressure increase C. vulgare plant density. There was positive-density dependence in establishment probability when herbivory was present but not absent, a potential insect-mediated Allee effect. The magnitude of the herbivore effect on seedling establishment probability varied widely across sites and years, from nearly preventing any seedling recruitment by this exotic plant to reducing establishment probability by only 20%. These results provide a unique data set on the spatio-temporal variation in magnitude of biotic resistance to invasion, as well as substantiating the dynamic role of native herbivores in repelling invasion by some exotic plant species.