Summary1 Invading plant species often alter community structure, composition and, in some instances, reduce local diversity. However, the community dynamics underlying these impacts are relatively unknown. 2 Declines in species richness with invasion may occur via displacement of resident species and/or reduction of seedling establishment by the invader. These two mechanisms differ in the demographic stage of the interaction. 3 We document turnover dynamics using long-term permanent plot data to assess the mechanism(s) of invasion impacts of four exotic species on a mixed community of native and exotic species. These mechanisms were evaluated at both the neighbourhood (1-m 2 plot) and population (individual species) scales. 4 During invasion, species richness declined with increasing invader cover for three of the four invaders. All invaders reduced colonization rates, but had no effect on extinction rates at the neighbourhood scale. Populations differed in their susceptibility to invasion impacts, with significant reductions in colonization for 10 of 25 (40%) species and increases in extinction for only 4 of 29 (14%) species. 5 At neighbourhood and population scales, influences of invasion on community dynamics were essentially the same for all invaders regardless of life-form. While individual resident species had some increase in extinction probability, community richness impacts were largely driven by colonization limitation. 6 The consistency of invasion impacts across life-forms suggests establishment limitation as a general mechanism of invasion impact. This common causal mechanism should be explored in other systems to determine the extent of its generality.

Exotic plant invasions are often associated with declines in diversity within invaded communities. However, few studies have examined the local community dynamics underlying these impacts. Changes in species richness associated with plant invasions must occur through local changes in extinction and/or colonization rates within the community. We used long-term, permanent plot data to evaluate the impacts of the exotic vine Lonicera japonica. Over time, species richness declined with increasing L. japonica cover. L. japonica reduced local colonization rates but had no effect on extinction rates. Furthermore, we detected significant reductions in the immigration of individual species as invasion severity increased, showing that some species are more susceptible to invasion than others. These findings suggest that declines in species richness associated with L. japonica invasion resulted from effects on local colonization rates only and not through the competitive displacement of established species.

Summary Species interactions in diverse plant communities affect community‐scale functions such as above‐ground biomass production, diversity and invasion resistance. While the strength of these formative interactions can be affected by the balance of inter‐ and intraspecific interactions among the resident species, it is unclear over what distances individuals typically interact in grasslands and whether or not species interactions at seeding can be effectively manipulated to improve these responses. In a three‐year study, we tested whether manipulating the size of seeded species patches affected above‐ground biomass, diversity and invasion resistance in experimentally restored grassland plots (16 m2). Plots were divided into patches that were 1, 0.5, 0.25 or 0.125 m on a side, and the equivalent of 1 m2 in each plot was seeded with one of 16 grassland species (1728 seeds m−2). A final treatment involved mixing and broadcasting all seeds into an undivided plot to mimic typical restoration approaches. Using successively smaller initial seed patches resulted in plots that were less diverse and initially more productive (˜25% difference in second‐year biomass production) than larger patch counterparts. Smaller patch plots also had a greater selection effect and experienced increases in resident species connectivity, suggesting that diversity declines and productivity gains resulted from the enhanced establishment and spread of more productive seeded species. Plots seeded with larger patches of the resident species were initially more invaded than those seeded with smaller patches, but this effect diminished over time. This likely reflects the ability of the non‐seeded species to more effectively colonize larger patches with poor seeded species establishment. Mixed seeding plots were most similar to the smallest patch plots in their metrics of resident species spatial pattern, diversity and invasion resistance. However, these plots were initially less productive and had a weaker selection effect, which reflects a different compositional outcome, than the structured seeding plots. Synthesis. Species interact over sub‐metre scales in establishing tallgrass prairie and, once established, their patterns may change over time. Given these dynamics, structured seeding approaches may be used over traditional mixed seeding approaches to control species dominance and preserve seeded species diversity within grassland systems.