Linking sheep density and grazing frequency to persistence of herb species in an alpine environment

Large herbivores are important drivers in ecosystems worldwide. Changes in herbivore densities are predicted to especially affect herbs that are strongly preferred by herbivores. The persistence of herbs could be challenged by enhanced grazing, but also grazing cessation may affect persistence, especially for prostrate herbs, which might be out-competed. To test how different herbivore densities (high, low, and no sheep) affect grazing frequency and plant responses (plant height, flowering frequency, and plant density) at the herb community and species level, we conducted a fully replicated, landscape-scale experiment in an alpine environment. We found that none of the herb species changed their densities after 5 years with experimental changes in grazing pressure. Sheep density affected grazing and flowering frequency at the herb community level. Eight herb species were more grazed at high sheep density as compared to enclosures with no sheep. Herb height decreased at high sheep density as compared to no sheep for the two species with the highest grazing frequency. Increased height and flowering frequency were found for small herbs at high sheep density. Our experiment clearly shows that herbs do not constitute a homogeneous functional group and that, in particular, tall and small herbs are affected in contrasting ways but all species (n = 15) tolerated changes in grazing regimes as densities were maintained at both enhanced grazing and grazing cessation.