Biological invasions describe the proliferation and persistence of exotic species to the detriment of indigenous organisms. Consequences of invasions are manifested at all levels of biological organization. Invaders may directly compete with native species, duplicating their functional role and resulting in their marginalization or extirpation. Despite the negative impacts of exotics on native plant diversity, few studies have focused on the potential co-existence between exotic and native species. Potential co-existence of exotic and native species promises a richer insight into the functional biology of invaders and the restoration of invaded communities. The objective of this study is to examine the consequences of smooth brome invasions on the structure and composition of northern fescue prairies. Using examples from Riding Mountain National Park, Canada, our study examines the response of prairie communities to invasions of smooth brome, a Eurasian grass, introduced to the Great Plains during the 1800's. We used 1 x 1 m quadrats to evaluate the proportional cover of native plants inside and outside of invading clones of smooth brome, and examined the persistent impacts of invasions on the diversity of northern fescue prairies using indices of community diversity and equitability. Lastly, we used redundancy analysis to compare the composition of prairie communities among areas inside the clones and the adjacent native prairies.
Despite the decline in the cover of smooth brome at the centre of invading clones, its impacts on the composition of northern fescue prairies was persistent. Compared to areas in the adjacent native prairie, the centres of clones were characterized by significantly lower community richness, diversity, and evenness. The persistent impact of smooth brome invasion on the prairie community was well illustrated along the first axis of the redundancy analysis ordination. Clearly separating areas of native prairie along with the fringes of invading clones from the centres of clones, the first ordination axis illustrated displacement of native species following invasion. Despite this impact, several native species, including Cirsium drummundii, Lathyrus ochroleucus and Potentilla fruticosa were characteristic of clone centres. Our results demonstrate that even declining populations of exotic invaders can leave a legacy that is likely to hinder restoration. Although we observed a steep decline in the diversity of invaded prairie communities, the persistence of several native species in the centre of invading clones, suggests that future restoration of invaded communities may be possible based on the complementarity of traits of native and exotic organisms.