REGULATION AND STABILITY OF A FREE-LIVING HOST-PARASITE SYSTEM - TRICHOSTRONGYLUS-TENUIS IN RED GROUSE .1. MONITORING AND PARASITE REDUCTION EXPERIMENTS

1. Intensive population studies were conducted for 10 years on red grouse (Lagopus lagopus scoticus) and the parasitic nematode, Trichostrongylus tenuis, in northern England. Winter loss was the key factor determining changes in grouse numbers, although breeding losses were also important. 2. T. tenuis had an aggregated distribution within the adult grouse population, even though the degree of aggregation was relatively low compared with other parasite systems. Recruitment of parasites into the adult worm population was dependent on grouse density. 3. Both winter loss and breeding losses were correlated with the intensity of parasite infection. 4. Experimental reduction in parasite burdens consistently increased breeding production and winter survival of grouse thus demonstrating that parasites cause increased winter and breeding losses. 5. The red grouse-T. tenuis system exhibits three conditions that will generate population cycles: (i) parasite-induced reduction in grouse breeding production, (ii) a low degree of parasite aggregation within the grouse population, and (iii) time delays in parasite recruitment.