Plant community-specific greening patterns predict population size increases in a temperate herbivore

Climate change-driven impacts on vegetation productivity have been shown to drive mammalian herbivore population dynamics in Arctic and alpine environments. However, there is less evidence for temperate systems. To address this, we examined the contribution of increasing plant biomass in different vegetation communities (measured by NDVI, normalised difference vegetation index) and winter weather on the observed long-term upward trend in the population of the Soay sheep of Hirta, St Kilda, UK. We found that biomass had increased in all vegetation communities present and increased the fastest in vegetation types preferred by the sheep. Specifically, those communities with high specific leaf area and Ellenberg's N, low leaf dry matter content. Peak summer NDVI and either winter average wind speed or winter North Atlantic Oscillation data added to the variance explained by a simple density dependence model of yearly sheep population growth rates. The highest explanatory power was found for preferred vegetation types including maritime cliff communities dominated by Plantago species, but also for both inaccessible (Rumex acetosa-dominated) or unpreferred (Eriophorum vaginatum- or Sphagnum-dominated) communities where seasonal variation more closely reflects productivity due to minimal grazing. Although the climate is getting windier and wetter, it is also getting warmer allowing increased plant productivity and this appears to be behind the long-term increases in the Soay sheep population. Our study indicates that analysing key vegetation communities may reveal these links better than using landscape-level averages, and that oceanic-temperate systems may show similar climate-driven herbivore population trends to those reported in Arctic and alpine systems.