The Interaction Between Demography and Harvesting in Red Grouse.

Abstract

Many animal populations are threatened by human activity, including habitat loss and harvesting but recent advances in population ecology show that the age- and sex-structure are important when aiming to understand population dynamics. However, research on population dynamics often focuses on species that experience relatively little human disturbance and human caused mortality is often assumed not to affect population dynamics. An increasing number of studies shows short-term and long-term evolutionary and ecological consequences of harvesting. This has not only implications for the understanding of population dynamics but also for the management and conservation of species in human-dominated landscapes.
This thesis aims to investigate the interaction between the demographic structure and harvesting in a fluctuating species, the red grouse (Lagopus lagopus scoticus) using a combination of empirical and theoretical approaches. The relative roles of age-related parasite burden, fecundity and shooting in driving population dynamics are assessed, and empirical data are used to parameterise a model examining the effect of harvesting on the population dynamics.
The empirical part of the thesis starts with the investigation of the intensity and distribution of the parasitic nematode Trichostrongylus tenuis for three age-classes of red grouse before the breeding season. The intensity of the parasite increased with host age. Parasites in two-year-old grouse were more aggregated than in one-year-old and 3+-year-old grouse. This has implications for population dynamics because the aggregation of parasites within the host influences mortality rates. Females of the oldest age class (3+-years-old) were less fecund than younger grouse and interactions with nesting habitat and year were found to affect female fecundity. The age- and sex-structure of shot birds was compared with the age- and sex-structure of the population before shooting. More old birds were shot at small bag sizes but this bias decreased as more birds were shot. Old males were highly susceptible to shooting at the beginning of the season but susceptibility decreased with number of shooting events. A relationship between bag numbers and population density was found.
An age- and sex-structured population model was used to examine the effect of harvesting on red grouse population dynamics. Selective harvest for or against a certain age class led to a skew in the sex ratio of the population and to a decrease in fecundity and therefore to a drop in population size and harvest yield. Parasites and the exclusion of young birds through aggressiveness affected the population dynamics and led to a skew in the harvest rate at which the maximum sustainable yield was attained. Shooting early in the season resulted in a higher yield because individuals were removed from the population before they contributed to the availability of free ranging stages of the parasite. The population model demonstrated that parasites, aggressiveness and harvesting interact and that harvesting is a significant factor in population dynamics. Overall, the current practice of shooting rarely more than 50% of the population seems to be a good precautionary principle. Uncertainty in the harvesting rate increased the probability of local extirpation of the population. Therefore, reliable estimates of the population, including the age and sex structure, are invaluable parts of red grouse management. This study showed that harvesting and the age and sex structure of the population are important drivers of red grouse population dynamics and an understanding of their interactions is important for sustainable management of red grouse.