Standardized sampling from many sites worldwide was used to address an important ecological problem.

Exotic species dominate many communities; however the functional significance of species' biogeographic origin remains highly contentious. This debate is fuelled in part by the lack of globally replicated, systematic data assessing the relationship between species provenance, function and response to perturbations. We examined the abundance of native and exotic plant species at 64 grasslands in 13 countries, and at a subset of the sites we experimentally tested native and exotic species responses to two fundamental drivers of invasion, mineral nutrient supplies and vertebrate herbivory. Exotic species are six times more likely to dominate communities than native species. Furthermore, while experimental nutrient addition increases the cover and richness of exotic species, nutrients decrease native diversity and cover. Native and exotic species also differ in their response to vertebrate consumer exclusion. These results suggest that species origin has functional significance, and that eutrophication will lead to increased exotic dominance in grasslands.

Abstract. One significant unanswered question about biotic responses to climate change is how plant communities within topographically complex landscapes will respond to climate change. Alpine plant communities are strongly influenced by topographic microclimates which can either buffer or compound the effects of more regional climatic changes. Here, we analyzed species changes over 20þ years in a complex alpine landscape with pronounced gradients in microtopography and consequently large variation in temperatures, snow depths, and nitrogen availability across small (10 m) scales. Using data from long-term monitoring plots from six community types, we asked how species composition and functional diversity changed over time in these different areas of the landscape, and whether fine-scale heterogeneity allowed species to move in response to temporal changes in the environment. We found sitewide patterns of increasing species and functional diversity. However, the majority of variability in composition over time was non-directional, both within and between community types. Within community types, Carex-dominated snow banks and wet meadow communities were the most variable in composition over time, while Sibbaldia-dominated snow banks, fellfield, dry meadow and moist meadow exhibited moderate change. Over forty percent of the plots also transitioned between community types during the census intervals, but these also were largely transient, with a shift occurring in one time interval and then shifting back in the next interval. Thus, even with evidence of directional change over time in climate, N deposition, and release from grazing, vegetation is tracking finer-scale variability both in time and space. Environmental heterogeneity may allow vegetation to track this finer-scale variability and enhance resilience to underlying directional changes in alpine and other topographically-complex environments.