The implications of grazing and predator management on the habitats and breeding success of black grouse Tetrao tetrix

1. Data on black grouse densities and breeding success were collected from five blocks of moorland, each consisting of four moors, between 1991 and 1993. Moors within a block differed in grazing intensity of either sheep or red deer and the presence of a gamekeeper. Results obtained were related to differences in grazing and predator management. 2. Moors with higher intensities of grazing had vegetation, on average, 32% shorter and had 36% less vertical vegetation cover. Grazing had no significant effect on species composition. 3. Heavily grazed moors supported 41% fewer invertebrates; threefold fewer Lepidoptera larvae and half as many Araneae and Hemiptera. 4. Highest densities of male (2.1 km(-2)) and female black grouse (3.4 km(-2)) were found on lightly grazed moors. Density did not differ between keepered and unkeepered moors. 5. Black grouse breeding success not only differed between years and regions, but also between managements, being 37% lower on heavily grazed moors. The presence of a gamekeeper was not associated with higher breeding success. 6. The presence of a gamekeeper was associated with three times fewer carrion crows. 7. The results suggest that lower numbers of large herbivores allow the development of good ground cover with high numbers of preferred insects, which may permit black grouse to survive in situations where they would otherwise be severely reduced by predators.