Many exotic species have become more abundant outside their native ranges and exert greater impacts on native species in those ranges. However, in many cases we do not know whether such behavior is only characteristic for the non-native range, or if they also respond similarly to different ecological variables (e.g. disturbance, competition) in their native range.
The fundamental objective of our research was to use a biogeographic approach to study the performance of cheatgrass (Bromus tectorum) in its native and non-native range. Cheatgrass originates from Eurasia and causes problems all over North America. We correlated the frequency of cheatgrass with associated plant species richness in plots measured over large areas of the Northwestern US and Europe. We also tested the effect of disturbance on cheatgrass performance along a gradient in both ranges.
Our results show that increasing frequency of the invader correlated with significant decreases in total species diversity and native species diversity in the non-native range, but not in the native range. Cheatgrass plants grew bigger when occurred with higher frequency. We found a large number of Eurasian species in North America co-occurring with cheatgrass that might also contribute to its invasive success. Cheatgrass itself and many of these co-occurring species were annuals germinating in the fall. This way they fill in an empty niche that is underrepresented in the flora of the Northwestern US. Disturbance had a significant effect in the spread of cheatgrass in both ranges, however, it was more pronounced in the non-native range. Along the gradient we found that in the less disturbed sites cheatgrass invasion decreased and gave way for secondary invasion.
Our results are consistent with a growing body of quantitative results demonstrating a strong biogeographic context to exotic plant invasions. This work indicates that the biogeographic origin of species can affect community organization, as well as individual performance in the native and non-native ranges.
The research was supported by the Montana Tech New Faculty Seed Grant (R.W.P.).