Sheep grazing, invertebrates and black grouse

Key findings

• More invertebrates were found in plots with restricted grazing.
• Numbers of sawfly larvae did not differ between grazing regimes.
• Invertebrate abundance responded quickly to grazing regime change.
• Breeding black grouse densities were higher where grazing was restricted.

With the RSPB, we are lead partners responsible for restoring black grouse numbers in the UK as part of the Government's Biodiversity Action Plan. In northern England, through our combination of research and subsequent management, black grouse numbers have increased from about 800 displaying males in 1998 to an estimated 1,200 males in 2007. Thus we have achieved our first major target of stabilising numbers ahead of our target date. How was this achieved? Our research has shown that black grouse respond well to habitat improvement along the moorland margin. In the North Pennines, we found that improved breeding success increased numbers of black grouse following reductions in sheep grazing, particularly in the autumn and winter. The key to this has been agri-environment payments (notably Countryside Stewardship Schemes), which pay farmers to restore heather on the moor edges through livestock reductions.

Grazing restrictions not only provide better cover from predators and more diverse herbs which adult black grouse eat but also, we suspect, more insects needed by growing black grouse chicks. By inspecting chick droppings from the North Pennines, we found that sawfly larvae comprised two-thirds of all invertebrates eaten by chicks, with moth caterpillars, beetles and spiders comprising most of the remainder.

To ascertain how reduced sheep grazing affected invertebrates important to black grouse chicks we sampled invertebrates at 10 sites in the North Pennines over two years. At each site, we selected two plots: one where sheep were either permanently or seasonally excluded, the second where sheep grazing continued at locally typical densities and duration. Eight of the 10 plots where sheep grazing was restricted were in Government funded agri-environment schemes to promote recovery of ericaceous vegetation.

Reduced sheep grazing was associated with, on average, 34% more invertebrates when sampled by sweep nets, and 23% more when sampled by a vacuum method. Plant bugs, spiders, harvestmen and moth caterpillars were significantly more numerous where grazing had been restricted, but flies were fewer. Notably, the abundance of sawfly larvae did not differ between grazing treatments (see Table 1), but moth caterpillars, beetles and spiders all preferred by chicks increased. Differences in invertebrate abundance between paired plots in relation to grazing did not increase over time since entry into the reduced grazing scheme, indicating that invertebrate abundance responded quickly to grazing changes.

Table 1: Mean number of invertebrates (95% CL) caught by sweep-netting and by vacuum sampling in
exclosures where sheep grazing was reduced, or in grazed areas without change

Significant relationships are given in bold

Breeding densities of female black grouse were on average 41% higher where grazing had been restricted. However, differences in breeding hen densities between grazing treatments declined with time after the first three years of grazing reduction. Densities of black grouse were not related to the abundance of sawfly larvae, the primary invertebrate food of their chicks in this region. Instead, differences in vegetation structure were a better predictor of differences in hen densities, with decreased hen densities being associated with structurally less diverse vegetation.