Temporal scales of density-dependent habitat selection in a large grazing herbivore

Habitat selection is a density-dependent process, but little is known regarding how this relationship may vary across different temporal scales. Over long time scales, grazing shapes the structure, diversity and functioning of terrestrial ecosystems, and grazing-induced changes in forage production over time are likely to affect the level of density dependence in habitat selection. In this fully-replicated, landscape-scale experiment, we investigated how density-dependent habitat selection by a large grazing herbivore, sheep Ovis aries, develops over the time scale of a decade. We also address an often-neglected challenge in habitat selection studies; namely, whether there is variation in use within a particular habitat or vegetation type and why. We found clear evidence of density dependence in habitat selection, with a wider use of habitats at high density. Despite a change in the standing biomass of high-productivity vegetation at high herbivore density over the years, with herb biomass declining and graminoid biomass increasing, there was no clear evidence that these grazing-induced changes in habitat over the years were strong enough to affect the level of density-dependent habitat selection. The difference in selection for high versus low-productivity habitats remained similar, despite annual fluctuations in the strength of selection. We found strong variation in selection within each vegetation type, even when vegetation types were mapped at a fine-resolution scale. Our study shows that despite the interactive effects of herbivores and habitats, they are not always sufficiently strong enough to affect the level of density-dependent habitat selection.