Red grouse population cycles and the population dynamics of the caecal nematode Trichostrongylus tenuis.

Abstract

(1) The relationship between the parasitic nematode T. tenuis and its host the red grouse, have been examined through analysis of population data, experimental reduction of parasite numbers, and the development of mathematical models.
(2) Analysis of bag records showed that the majority of grouse populations exhibit cyclic patterns of abundance. Intensive population studies in northern England found that both adult and breeding losses were greater when mean intensity of parasite infection in breeding adults was high.
(3) Adult grouse from cyclic populations carried significantly higher worm intensities than grouse from non-cyclic populations. Mean parasite intensity was high throughout much of the geographical range of red grouse.
(4) Theoretical analysis has suggested that three processes could generate population cycles in a nematode-host system. All three were present in a red grouse population studied in northern England. First, parasite-induced reduction in grouse breeding production was demonstrated through experimental reduction of T. tenuis numbers. Second, T. tenuis exhibited a statistical distribution in the host population characterized by low degrees of aggregation. Third, developmental time-delays in winter infection were described.
(5) The between-year dynamics of the grouse-T. tenuis system were explored by considering the grouse, adult parasites and free-living populations in a mathematical model. Grouse numbers in the model showed cyclic oscillations when the life expectancy of the free-living stages was long and the impact of worms on grouse reproductive rate was greater than on grouse mortality.