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Species richness of plants generally decreases along elevational gradients or peaks at intermediate elevations. Land use including grazing by wild and domestic herbivores also affects plant communities and diversity, but how grazing affects plant diversity along elevational gradients is less clear.
Using a field experiment along an elevational gradient in Norway, we tested whether the impact of grazing on plant diversity interacts with elevation. Vascular plant communities were sampled following 10 years of experimental sheep grazing (decreased/ceased, maintained and increased densities of 0, 25 and 80 sheep km?2) and compared with pre-experimental baseline data.
The impact of grazing on species richness (alpha diversity) was investigated at the species level, and temporal community variance was used to investigate community stability (temporal beta diversity). Spatial community variance (spatial beta diversity) at the enclosure level was used to test whether community homogenization differed between treatments.
Species richness and temporal stability varied between treatments and along the elevational gradient. Where grazing was ceased, species richness declined by up 3.7 species at low elevations and increased by up to 3.5 species at high elevations, whilst where grazing was maintained or increased, changes were less extreme along the elevational gradient.
Temporal stability of plant communities was highest at low elevations and lowest at high elevations where grazing was reduced. There were no clear differences in spatial homogenization between grazing treatments, although spatial species turnover increased in heathlands where grazing decreased.
Synthesis. This study shows that the effect of grazing on plant diversity varies with elevation, and grazing herbivores can thus directly affect elevational patterns of plant diversity. Grazing can buffer changes in plant communities and species richness, even in the face of other environmental drivers such as climatic warming.