Population fluctuations in red grouse: analysis of bag records and a simulation model.

A time-series analysis of numbers of Lagopus lagopus scoticus shot per annum was carried out for 52 moors in N England where data were available for at least 20 consecutive years during 1870-1977. In the most representative sub-sample of moors, 83% of the series had significant negative coefficients at T + 2 or T + 3 years or both. A mathematical model was fitted to the fluctuations to describe their quasi-cyclic nature. Average 'cycle-length' was 4.84+ or -0.086yr. Field observations and data from trials with captive grouse were combined to construct a simulation model of a red grouse population; its main features were: 1) An inverse logistic curve relating mean numbers of the parasitic nematode Trichostrongylus tenuis in adult red grouse to their breeding success. 2) T. tenuis accumulation by young red grouse to steady state levels. Numbers of worms accumulated per bird varied according to a combined effect of the density of grouse prior to the breeding season and the worm burden of these grouse. 3) A logistic curve relating the proportion of grouse shot with the density of grouse available. 4) An annual survival rate of the non-shot population inversely proportional to the density of old grouse. The simulation model successfully captured mean levels and other population parameters, but it only gave a good fit to observed fluctuation patterns when stochastic elements were introduced to to represent known effects of weather. Red grouse cycles could be caused by effects of T. tenuis working together with stochastic elements and a time delay arising from the uptake of worms. -from Authors