Cyclic and non-cyclic populations of red grouse: a role for parasitism?

Abstract

The hypothesis that parasitism is a key element influencing the pattern of fluctuations in red grouse numbers was investigated. Data were collected from estates in the north of England which exhibited either cyclic, intermediate or non-cyclic bag records. Cyclic populations were associated with greater worm burdens of Trichostrongylus tenuis and more rainfall than non-cyclic but there was no difference in the nitrogen content of food or mean bag size. It is proposed that low humidity reduces the survival of free-living stages resulting in lower levels of infection and reduced effects on host fecundity in non-cyclic populations. By modelling, it was shown that the interaction of a long-lived free-living stage and parasite induced reduction in fecundity could be of prime importance in producing the cycles in grouse density. These findings are discussed in relation to previous hypotheses and studies on grouse population cycles.