Abstract
The effect of herbivores on nutrient cycling is a central process for understanding the effects of herbivores for ecosystem functioning and the sustainability of grazing regimes. Herbivores either directly ? by returning nutrients in the form of faeces and urine ? or indirectly ? by altering plant litter quantity and quality ? influence the rate and type of nutrients that are returned to the soil. The dominating conceptual model suggest that herbivores have predominantly positive effects on nutrient availability and primary production in nutrient-rich environments dominated by plants of high quality, and mainly negative ones in nutrient-poor environments dominated by plants of low quality. However, new scientific findings, novel empirical data, and theoretical modelling challenge this model, and point out the need to in-cooperate the type of herbivory, plant traits, microbial immobilization, plant uptake of organic nitrogen and potential stoichiometric shifts in the nutrient limiting plant growth. The Scandinavian tundra offers unique possibilities to do this since plants with contrasting traits coexist at small spatial scales, and we have access to more than 50 large and fairly old exclosures (15-20 years old). I am going to test how the effect of herbivores on nutrient cycling varies across gradients in ecosystem productivity, and in communication with practitioners evaluate new criteria for sustainable grazing regimes based on these novel findings.
Sweden