Population dynamics in wild boar Sus scrofa: ecology, elasticity of growth rate and implications for the management of pulsed resource consumers.
Abstract
In terrestrial ecosystems many species show large population fluctuations caused by pulsed resources, such as mast seeding. A prime example of a mammal strongly affected by mast seeding of trees is the wild boar Sus scrofa, a species that has become a pest in many parts of the world. We investigated the population dynamics of wild boar to assist the development of effective management strategies for this species and possibly for other pulsed resource consumers. We analysed published vital rates of wild boar using Leslie matrix projection models and elasticity analysis. Models were based on vital rates of animals under poor, intermediate and good environmental conditions, which represent combinations of differences in food availability (particularly mast of beech Fagus sylvatica and/or oak Quercus spp.) and winter climate. Interestingly, we observed a crossover in the ranking of elasticities (e; the relative impact of each vital rate on population growth rate λ) when comparing different conditions. While the elasticity of λ to adult survival was highest in poor environments [e(Padult)=0.36, e(Pjuvenile)=0.22], the elasticity of λ to juvenile survival was highest under good conditions [e(Padult)=0.16, e(Pjuvenile)=0.28]. Thus juvenile survival becomes increasingly important for population growth as habitat conditions improve. Our analysis of empirical beech mast records gave some indication of an increase of full masts over the last few decades. Modelling different beech mast scenarios showed that an increase in full mast frequency will lead to a rapid increase in λ. The availability of alternative food resources, namely agricultural crops, may also contribute to an expansion of wild boar populations. Synthesis and applications. We suggest that, whenever possible, management strategies should be based on separate elasticity analyses for different environmental conditions, especially for species dependent on pulsed resources. For wild boar we suggest the following principal management strategies to stop further population increases: (i) supplementary feeding should be strictly avoided; (ii) under good environmental conditions, reducing juvenile survival will have the largest effect on λ, whereas strong hunting pressure on adult females will lead to most effective population control in years with poor conditions.