Differential responses of heather and red grouse to long-term spatio-temporal variation in sheep grazing

Abstract

During the last century, afforestation and intensification of sheep grazing in the British uplands have led to widespread declines in globally rare heather moorland. We quantified changes in heather cover over 70 years in relation to changes in sheep grazing on Langholm Moor, and examined the impact on red grouse, a gamebird inhabiting heather moorland. Between 1948 and 2009, when grazed heavily by sheep, heather-dominated vegetation declined from 53 to 14% cover. Large-scale sheep reductions from 2011 then allowed increase of heather-dominated vegetation cover to 18% by 2015. However, changes in heather cover were associated with changes in grouse abundance only where heather-dominated cover was reduced below thresholds of 27% (95% CL 18–36%; pre-breeding) and 17% (95% CL 13–20%; post-breeding). The number of grouse shot between 1951 and 1992 remained high where 37–65% of dominant heather cover was retained, but then declined between 1992 and 1996 following increased predation by raptors, leading to the cessation of shooting. Subsequently, grouse densities fluctuated in relation to periodic management by gamekeepers (1992–1999 and 2008–2016), but heather loss continued possibly until 2011, and predation in this context prevented sustained increases sufficient for 'driven' shooting. Grouse shooting provides an economic incentive to maintain and restore heather moorland. On Langholm Moor, however, afforestation in the surrounding landscape and isolation from other heather moors may have led to a grouse population less well buffered against growing predation pressure, especially outside keepered periods. As grouse shooting could not be restored, the future management of the moor remains uncertain.