Dispersal and compensatory population dynamics in a harvested mammal

Populations of wild birds and mammals are often harvested for sport, subsistence or commerce. Sustainable exploitation is an important aspect of environmental management and is critical to human wellbeing. However, our inability to sustainably harvest even well studied populations is often due to poor demographic data and a lack of understanding of critical biological processes. A key component of sustainable harvesting is the density-dependant feedback between vital rates (births, deaths, immigration and emigration) and population density, as this mechanism provides populations with the capacity to compensate for harvest mortality. The aims of this thesis are to investigate density-dependence, compensation and sustainable harvesting in a traditional Scottish quarry species, the mountain hare, Lepus timidus scoticus, using a combination of replicated field experiments, cross-sectional studies and modelling. The mountain hare has been traditionally harvested for centuries and continues to be an important source of revenue for some land owners. Over the past decade the motive for killing hares has switched from predominantly sport shooting, to culling to reduce ticks and the tick-borne disease Louping-ill virus (LIV). Louping-ill virus causes high mortality, and can supress harvest, in economically important red grouse, Lagopus lagopus scoticus. Sustainable management of the mountain hare is further complicated by cyclic, or unstable, population dynamics shown throughout their circumpolar distribution. The thesis begins with a review of the evidence that culling mountain hares can lead to a reduction in ticks and LIV prevalence, resulting in increased red grouse harvest. The empirical studies follow with the aim of providing evidence for compensatory survival and dispersal, and density-dependent reproduction. Two field study approaches were adopted; the first was a live-capture and radio-telemetry study which compared birth dates, survival probabilities and dispersal distances and rates of different age and sex classes between two populations under different harvesting regimes (harvested and non-harvested). The second empirical study used a cross-sectional design whereby population density of ten independent populations were estimated using Distance sampling, which preceded hare harvesting. Tissue samples of killed hares were collected, enabling the effects of population density on female fecundity and juvenile recruitment to be assessed. Finally, an age-structure, female-only matrix population model was parameterised using data from the empirical studies. Sustainable harvest rates were determined as the maximum harvest that produced a positive population growth rate and an extinction probability of less than 5%. By manipulating age-specific harvest and initial population size, the effects of age-biased harvest and changing population size on population viability could be investigated. Results of the cross-sectional study revealed evidence for negative density-dependent juvenile recruitment. However, this finding was not translated into compensatory survival in either juveniles or adults, although birth date of leverets was significantly later in the harvested population. Overall, dispersal distances and rates were low. Dispersal distance was greater in the harvested population, although when distance was scaled to account for differences in observation time, to give a dispersal rate, no difference was found. Projections from the matrix model suggest that 40% annual harvest rate is sustainable, although extinction probability increases with decreasing population size and as harvesting becomes increasingly yearling biased. Overall, no evidence for compensatory survival or dispersal was found, although density-dependent juvenile recruitment was identified and may be important in population persistence under exploitation. Survival and timing of breeding may be influenced by behavioural and physiological effects of harvesting, or environmental variation, which may have implications for unstable dynamics of mountain hare. I identify and discuss key areas for future research aimed at increasing our understanding of the effects of harvesting on mountain hare population dynamics and demography. In conclusion, we found little evidence that culling mountain hares can increase red grouse harvest, and therefore, cannot justify culling mountain hares for tick and LIV control. The low dispersal rates and distances, combined with limited evidence for compensatory mechanisms, imply that local blanket culls may succeed in reducing hare numbers with unknown implications for mountain hare population persistence and the wider biodiversity of the Scottish uplands.