The impact of large herbivores on plant diversity and landscape structure: Potential for restoration

Large mammalian herbivores can have a strong impact on grassland plant diversity. However, the effects of grazers vary with habitat fertility. We performed a large scale cross-site field experiment where we compared the impact of herbivores on plant species richness over a range of grassland productivity in North-America and Europe. Herbivore impact on species richness varied with grassland plant production: herbivores significantly reduced species richness in unproductive grasslands whereas they increased it in productive grasslands. The negative effects at low production were mainly due to small herbivores, whereas the positive effects at high production were caused by large herbivores. We conclude that the conservation of large mammalian herbivores in productive grasslands is crucial for grassland plant diversity.

At the landscape level, large herbivores may have a structuring effect on the dynamics of grassland, shrubs and tree mosaics. The presence of light-demanding trees and thorny shrubs in temperate fertile grasslands and woodlands may reflect adaptations to now-extinct large grazers. In an experiment, we found that regenerating Oaks profit from grazing through enhanced light availability, but need protection from grazing damage through associational resistance with thorny shrubs. Extensive grazing by large introduced grazers in temperate fertile grasslands may induce a shifting mosaic of grassland, shrubs and trees with a high conservation value.

In conclusion, large mammalian herbivores have positive effects on biodiversity in productive grasslands. Where large herbivores went (locally) extinct, the re-introduction of these or similar grazers will positively affect grassland biodiversity.