Nutrients and herbivores impact grassland stability across multiple spatial scales through different pathways

Qingqing Chen, Institute of Ecology, College of Urban and Environmental Science, Peking University, Beijing, NY, China, Shaopeng Wang, Institute of Ecology, College of Urban and Environmental Science, and Key Laboratory for Earth Surface Processes of the Ministry of Education, Peking University, Beijing, China, Eric W. Seabloom and Elizabeth T. Borer, Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN, Andrew S. MacDougall and Oliver Carroll, Department of Integrative Biology, University of Guelph, Guelph, ON, Canada, Jonathan Bakker, School of Environmental and Forest Sciences, University of Washington, Seattle, WA, Ian Donohue, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland, Johannes M. H. Knops, School of Biological Sciences, University of Nebraska, Lincoln, NE, John W. Morgan, Department of Ecology, Environment and Evolution, La Trobe University, Bundoora, Australia, Mick Crawley, Biology, Imperial College, London, Ascot, United Kingdom, Miguel Nuno Bugalho, Centre for Applied Ecology/ School of Agriculture, University of Lisbon, 1349-017 Lisbon, Portugal, Sally A Power, Hawkesbury Institute for the Environment, Western Sydney University, Richmond, NSW, Australia, Anu Eskelinen, Department of Physiological Diversity, Helmholtz Centre for Environmental Research - UFZ, Leipzig, Germany, Risto Virtanen, Department of Ecology and Genetics, University of Oulu, Oulu, Finland, Anita Risch and Martin Schütz, Research Unit Community Ecology, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland, Carly J. Stevens, Lancaster Environment Center, Lancaster University, Lancaster, United Kingdom, Maria C. Caldeira, Centre for Forestry Research/ School of Agriculture, University of Lisbon, Lisbon, Portugal, Sumanta Bagchi, Centre for Ecological Sciences, Indian Institute of Science, Bangalore, India, Juan Alberti, Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, Mar del Plata, Argentina, Yann Hautier, Utrecht University

Background/Question/Methods
Nutrients and herbivores have independent effects on the temporal stability of aboveground biomass in grasslands; however, their joint effects may not be additive and may also depend on spatial scales. We therefore assess (1) the joint effects of nutrient addition and herbivore exclusion on the temporal stability of aboveground biomass at the local and larger spatial scales (i.e. alpha and gamma stability); (2) the relative contribution of different facets of plant diversity (e.g. species richness, evenness, and community composition in time and space) in mediating the responses of alpha and gamma stability to nutrients and herbivores. We used a globally coordinated grassland experiment (Nutrient Network) replicated at 34 sites with a factorial combination of nutrient addition and herbivore exclusion by fencing (5 years of post-treatment) to answer these questions.

Results/Conclusions
Nutrients and herbivores mainly had additive effects. Nutrient addition consistently reduced stability at the local and larger spatial scales (aggregated local communities), while herbivore exclusion weakly reduced stability at these scales. Moreover, nutrient addition reduced stability primarily by causing changes in local community composition over time and by reducing local species richness and evenness. In contrast, herbivore exclusion weakly reduced stability at the larger scale mainly by decreasing asynchronous dynamics among local communities, but also by weakly decreasing local species richness. Our findings indicate disentangling the influences of processes operating at different spatial scales may improve conservation and management in stabilizing grassland biomass.