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Environmental change can result in substantial shifts in community composition. The associated immigration and extinction events are likely constrained by the spatial distribution of species. Still, studies on environmental change typically quantify biotic responses at single spatial (time series within a single plot) or temporal (spatial beta diversity at single time points) scales, ignoring their potential interdependence. Here, we use data from a global network of grassland experiments to determine how turnover responses to two major forms of environmental change - fertilisation and herbivore loss - are affected by species pool size and spatial compositional heterogeneity. Fertilisation led to higher rates of local extinction, whereas turnover in herbivore exclusion plots was driven by species replacement. Overall, sites with more spatially heterogeneous composition showed significantly higher rates of annual turnover, independent of species pool size and treatment. Taking into account spatial biodiversity aspects will therefore improve our understanding of consequences of global and anthropogenic change on community dynamics.

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.

Human alterations to nutrient cycles and herbivore communities are affecting global biodiversity dramatically. Ecological theory predicts these changes should be strongly counteractive: nutrient addition drives plant species loss through intensified competition for light, whereas herbivores prevent competitive exclusion by increasing ground-level light, particularly in productive systems. Here we use experimental data spanning a globally relevant range of conditions to test the hypothesis that herbaceous plant species losses caused by eutrophication may be offset by increased light availability due to herbivory. This experiment, replicated in 40 grasslands on 6 continents, demonstrates that nutrients and herbivores can serve as counteracting forces to control local plant diversity through light limitation, independent of site productivity, soil nitrogen, herbivore type and climate. Nutrient addition consistently reduced local diversity through light limitation, and herbivory rescued diversity at sites where it alleviated light limitation. Thus, species loss from anthropogenic eutrophication can be ameliorated in grasslands where herbivory increases ground-level light.

The content and ratio of nutrients in plants can be constrained by a wide array of factors, including nutrient supply, light intensity, herbivory, infection or intrinsic growth rate and can, in turn, affect many ecosystem processes including photosynthesis, decomposition, resource limitation and nutrient cycling. Studies of plant stoichiometry and stoichiometric homeostasis have focused primarily on the role of nutrient supply as a constraint on plant tissue chemistry, yet recent work suggests that local plant diversity, plant species composition and consumers may change the nutrient composition of whole plant communities., By experimentally removing insects, foliar fungi and soil fungi from a long-term experiment manipulating plant diversity, we found that the foliar stoichiometry of individual plant species depends on biotic context., Local plant diversity and the composition of the consumer community each altered foliar tissue carbon and nutrient chemistry of four different grassland species. The greatest impacts of consumers on foliar chemistry occurred at low plant diversity, and these changes induced by altering the food web were of a similar magnitude to the effects of fertilization or drought found in previous work. Consumers and plant diversity acted primarily on foliar carbon and nitrogen, whereas changes in foliar phosphorus were associated with the productivity of the local plant community. Thus, changes in whole-community stoichiometry that have been documented in response to alterations of the consumer food web or plant community are underlain by stoichiometric changes in individual species as well as plant species compositional changes., Synthesis. These results suggest a new pathway by which loss of consumer or plant diversity may significantly impact the wide variety of ecosystem processes that depend on foliar nutrient content. [ABSTRACT FROM AUTHOR]