Synergies and trade-offs between ecosystem services in an alpine ecosystem grazed by sheep - An experimental approach

Domestic livestock drives ecosystem changes in many of the world's mountain regions, and can be the dominant influence on soil, habitat and wildlife dynamics. Grazing impacts on ecosystem services (ES) vary according to densities of sheep, but an ES framework accounting for these is lacking. We devised an experiment to evaluate synergies and trade-offs of ESs and components of biodiversity affected by sheep density at the alpine landscape scale in southern Norway. We examined the effects of increased (80 per km(2)), decreased (0 per km(2)) and maintained sheep densities (25 per km(2)) on 'supporting', 'regulating' and 'provisioning' services and biodiversity (plants, invertebrates and birds). Overall, I and biodiversity were highest at maintained sheep density. Regulating services, including carbon storage and habitat openness, were particularly favoured by maintained densities of sheep. There was no overall decline in ESs from maintained to increased sheep densities, but several services, such as runoff water quality, plant productivity and carbon storage, declined when grazing increased. Our study provides experimental evidence for a positive effect of grazing on ES, but only at maintained low sheep densities. By identifying ES and biodiversity components that are traded-off at decreased and increased grazing, our study also demonstrates some of the negative impacts on ecosystems that can occur in mountain regions if management does not regulate herbivore densities.