The presence of sheep leads to increases in plant diversity and reductions in the impact of deer on heather

1. Grazing has been implicated in the decline of heather‐dominated moorlands in Britain, but there has been little consideration of the effects of different species of herbivores on plant diversity. Sheep stocks have recently decreased in Scotland, and quantification of how this affects biodiversity is essential for understanding how different grazing regimes modify upland habitats. 2. We investigated the effects of grazing on plant biodiversity in heather/grass mosaics at 16 upland sites in Scotland. Red deer Cervus elaphus L. were present at all sites, but sheep Ovis aries L. had been removed from half of the sites. Our experimental design incorporated replication at three spatial scales, from a landscape level down to 10 × 10 m plots. 3. We quantified the relative effects of different herbivore species, vegetation structure and rainfall on heather utilisation, species richness and evenness (alpha diversity; Shannon–Wiener index) and beta diversity. 4. At all spatial scales, deer dung counts were higher and heather was shorter when sheep were absent. Furthermore, utilisation of heather was positively correlated with the amount of deer dung, the amount of grass present and smooth grass height. 5. Alpha diversity was consistently positively correlated with the relative amount of grass, but was also positively related to the amount of sheep dung at the largest spatial scale. At the finest scale, alpha diversity was negatively correlated with the amount of deer dung. Beta diversity was higher when sheep were present at all scales. 6. Synthesis and applications. Mixed grazing by sheep and deer appears to be beneficial for increasing both alpha and beta diversity and minimizing damage to heather in the uplands. The absence of sheep is likely to result in expanding deer populations and greater impact on heather. Management of grazing herbivores is an important tool for maintaining biodiversity in many ecosystems. Our results indicate that reducing livestock may alter the impacts of wild grazers on their habitats and drive changes in diversity, whereas mixed grazing can enhance habitat quality and maintain plant diversity. Therefore, this effect should be considered when devising policy recommendations.