Can raptor predation limit red grouse populations?

Abstract

Whether vertebrate predators can limit their prey has long been a controversial question in ecology. This paper summarises a study of predation by Hen Harriers Circus cyaneus and Peregrines Falco peregrinus on Red Grouse Lagopus lagopus scoticus populations in Scotland. Harrier breeding densities were highest in areas and years where passerines and microtines were most abundant. Peregrine breeding densities were highest on southern moors, probably as a result of high pigeon densities. Harriers showed a Type III functional response to grouse chicks. Peregrines showed a Type II response to adult grouse. The implication of these findings is that the greatest proportion of grouse will be removed by raptors from low density grouse populations on southern, grassy grouse moors were alternative prey are abundant. On a moor where breeding densities of harriers and peregrines were high, predation by these raptors during April to September reduced autumn grouse densities by 50% within a single breeding season. On this moor, an increase in raptor densities coincided with a large decline in the numbers of grouse shot. Our data were strongly suggestive that predation by generalist raptors limited Red Grouse populations at low density.