The impacts of grazing pressure by large herbivores on short-tailed field vole (Microtus agrestis) population cycles: results from a long-term upland experiment

Understanding how large herbivores influence cyclic small-mammal populations is essential for predicting community dynamics in grazed ecosystems. Using a 23-year upland grazing experiment in Scotland, United Kingdom, we test how varying livestock grazing pressures affect population cycles of the short-tailed field vole Microtus agrestis, a keystone herbivore and prey species of birds of conservation concern. We combined long-term vole sign indices (VSI), vegetation surveys, climate data and used wavelet analyses and mixed-effects models to quantify changes in VSI (a proxy for abundance), cycle amplitude and periodicity across four grazing treatments and three spatial blocks. Vole abundance and cycle amplitude declined with increasing grazing pressure, yet the frequency of vole population oscillations remained strikingly consistent across all treatments, with a dominant ~3.5-year cycle persisting throughout most of the study. Ungrazed plots supported the highest mean VSI and the largest amplitude oscillations. We found a pronounced, spatially-restricted cycle collapse occurred between 2009-2016 in one experimental block. This breakdown coincided with unusually cold winters and lower vegetation density, suggesting that local microclimate and habitat structure interact to destabilise vole cycles. Vegetation density (but not height) predicted VSI in the more intensively grazed treatments, indicating that grazing-driven changes in habitat structure indirectly mediate vole dynamics. Our findings demonstrate that large herbivores can substantially modify vole population dynamics through both direct and indirect pathways, altering cycle amplitude, habitat suitability and resilience to climatic perturbations. These results highlight the need to consider interactions between grazing, vegetation structure and climate variability when predicting small-mammal population dynamics and their cascading ecological effects.