Extrinsic factors influencing the population dynamics of Red Grouse Lagopus lagopus scoticus.
The population dynamics of Red Grouse Lagopus lagopus scoticus vary both temporally and spatially, even within the birds' relatively small geographical range of heather moorland in Britain. Both intrinsic and extrinsic mechanisms have been shown to play some part in the population processes that influence breeding density and breeding production (reviewed by Lawton 1990). This paper concentrates solely on extrinsic factors, in particular the impacts of parasitism and the interaction between parasitism and predators. Detailed population data coupled with intensive radio-tracking, field experiments and detailed mathematical modelling have described the roles of these different factors on the population dynamics of Red Grouse. This paper summarises these findings.
Red Grouse have been killed as game on private sporting estates in Britain for several centuries, Intensive grouse management and driven grouse shooting started in the middle of the 19th Century. Many of these sporting estates keep detailed daily and annual records of the numbers of grouse shot and have done so for more than 100 years, thus providing a detailed description of changes in the abundance of the birds both spatially and temporally. Most of these time series data exhibit statistically significant cyclic fluctuations in numbers (Potts et al. 1984, Hudson 1985, Hudson 1986a), although bag records from estates in Scotland have also shown marked longterm declines over the last 40 years (Hudson 1992).' Populations of grouse in the north of England tend to show five-year cycles (Potts et al. 1984) and the frequency of the cycles increases to around 10 years in the north of Scotland (Hudson 1992). Grouse populations which show no statistically significant tendency to cycle are located either on the dry freely drained areas of heather moorland in the east of the country or are on relatively small areas of moorland (Hudson 1992).