EXPERIMENTAL PREVENTION OF A POPULATION-CYCLE IN RED GROUSE

Red Grouse (Lagopus lagopus scoticus) show unstable population dynamic s. The number shot for sport at Rickarton moor in northeast Scotland, for example, has cycled with 10-11-yr periodicity since 1946. Here, de mographic and other causes of a population cycle were documented from 1979-1989, and an experiment tested the prediction that removing some cocks during the increase phase would prevent a subsequent cyclic decl ine, Throughout the study, sport shooting was stopped on the area wher e the main work was done. During 1979-1982, before the experimental re moval of cocks began, numbers over the whole moor rose from a trough a t the start of the study. On the control area, the cyclic peak in 1983 was followed by a decline until 1988, as predicted in advance from mo dels derived from a previous study elsewhere. On the experimental area , enough territorial cocks were removed each spring from 1982 to 1986 to prevent the population from attaining peak densities for five succe ssive years, and no cyclic decline occurred. The removal of cocks resu lted in similar numbers of hens being lost from the breeding populatio n. The main demographic cause of population change on control (cycling ) and experimental (cycle broken) areas was variation in the recruitme nt of young cocks to the spring population. On the control area, recru itment was related to cycle phase and breeding success. Changes in foo d, nitrogen metabolism, and parasite burdens could not explain the cyc le. Demographic patterns were consistent with a model in which changes in age structure affected recruitment. These and previous results ref ute four hypotheses as necessary causes of population cycles in Red Gr ouse: (1) maternal nutrition, (2) a version of Chitty's genetic hypoth esis, (3) host-parasite (caecal threadworm), and (4) predator-prey rel ationships. The hypothesis that age structure changes and associated b ehavior cause cycles by affecting recruitment and, thus, population ch ange, remains unrefuted..