Harvesting unstable populations: red grouse Lagopus lagopus scoticus (Lath.) in the United Kingdom

The optimal harvesting strategies for unstable populations are explored usi ng first discrete time models and second a continuous time model specifical ly applied to the destabilising effects of the caecal nematode Trichostrong ylus tenuis on the dynamics of red grouse Lagopus lagopus scoticus. In disc rete time models, with overcompensation generating either cyclic or chaotic fluctuations in abundance harvesting can act as both a stabilising and a d estabilising process. Maximum yields occur at the harvesting rate that coin cides with the point where the harvesting stabilises the overcompensation. Optimal harvesting rates increase with the degree of overcompensation altho ugh these are more vulnerable to overharvesting. Harvesting in the continuo us time model provides similar results, although observed hunting records d o not appear to be stabilised by harvesting. Empirical data on the mortalit y caused by other natural enemies of red grouse, the hen harrier Circus cya enus and the louping ill virus, show that these mortalities do stabilise gr ouse dynamics. One explanation is that both hen harriers and louping ill vi rus cause significant mortality to chicks before the infective stages of T. tenuis are laid down on the ground, whereas shooting takes place after the infective stages are laid down and thus do not stabilise the populations.