Do habitat characteristics influence predation on red grouse?
Abstract
Predation is not only an important ecological process in the population dynamics of red grouse Lagopus lagopus scoticus, but also has conservation implications for their predators such as hen harrier Circus cyaneus and peregrine falcon Falco peregrinus. It has been suggested that habitat management might reduce the susceptibility of grouse to predation and thus reduce conflicts between grouse management and raptor conservation. We investigated whether habitat characteristics influenced predation on red grouse on a managed moor near Langholm in southern Scotland during 1992-96. We combined demographic studies of the grouse population with radio-telemetry of individual grouse to assess the influence of habitat on mortality rates. Systematic observations of hen harriers were also used to assess the effect of habitat characteristics on their encounter rates and strike success with grouse and other prey. There was no evidence that habitat characteristics directly influenced grouse mortality rates at the scale of the grouse population. However, grouse densities were higher and overwinter losses of grouse were lower on areas with greater cover of heather Calluna vulgaris. The most likely explanation for the observed pattern of winter loss was that grouse dispersed into areas with more heather, to some extent locally compensating for losses to predators. Individual radio-tagged grouse that survived the winter had more blaeberry Vaccinium myrtillus in their home ranges than grouse that were killed in winter by predators. There was, however, no effect of heather cover, vegetation height or vegetation density on the likelihood of individual grouse survival. Hen harriers were more likely to encounter grouse broods in a mixture of heather and grass than expected from the observed distributions of grouse broods on moorland. However, having encountered a grouse brood, there was no effect of habitat type or vegetation height on the strike success of the harriers. We conclude that the direct effects of habitat on the susceptibility of red grouse to predation are limited under current predator control regimes on managed moorland. Habitat management aimed at mitigating conflicts between raptor conservation and grouse management should focus on reducing the availability of passerine and small mammal prey for hen harriers and thus reducing harrier abundance on grouse moors. Further research is required, however, to assess the consequences of such management for biodiversity.