The direct and indirect effects of the caecal nematode Trichostrongylus tenuis on red grouse

Examines the population dynamics of Trichostrongylus tenuis, a parasite of moderate pathogenicity, that is lethal at high worm intensities but has a significant sublethal impact on the breeding production of red grouse Lagopus lagopus scoticus. Evidence is presented which supports the hypothesis that population cycles in red grouse are causd by reduced fecundity and survival through the delayed density-dependent effects of parasites and that these effects are sufficient to cause population cycles in grouse. Populations that did not show cycles carried lower levels of infection. While such effects do not preclude the action of other factors causing population cycles the evidence for other extrinsic factors or intrinsic changes in spacing behaviour is considered weak. -from Authors