Abstract
Lethal control is widely employed to suppress the numbers of target wildlife species within restricted management areas. The success of such measures is expected to vary with local circumstances affecting rates of removal and replacement. There is a need both to evaluate success in individual cases and to understand variability and its causes. In Britain, red fox (Vulpes vulpes) populations are culled within the confines of shooting estates to benefit game and wildlife prey species. We developed a Bayesian state-space model for within-year fox population dynamics within such restricted areas and fitted it to data on culling effort and success obtained from gamekeepers on 22 shooting estates of 2 to 36 km2. We used informative priors for key population processes-immigration, cub recruitment and non-culling mortality-that could not be quantified in the field. Using simulated datasets we showed that the model reliably estimated fox density and demographic parameters, and we showed that conclusions drawn from real data were robust to alternative model assumptions. All estates achieved suppression of the fox population, with pre-breeding fox density on average 47% (range 20%-90%) of estimated carrying capacity. As expected, the number of foxes killed was a poor indicator of effectiveness. Estimated rates of immigration were variable among estates, but in most cases indicated rapid replacement of culled foxes so that intensive culling efforts were required to maintain low fox densities. Due to this short-term impact, control effort focussed on the spring and summer period may be essential to achieve management goals for prey species. During the critical March-July breeding period, mean fox densities on all estates were suppressed below carrying capacity, and some maintained consistently low fox densities throughout this period. A similar model will be useful in other situations to quantify the effectiveness of lethal control on restricted areas.
Highlights
Lethal control of predator numbers to benefit prey species is controversial and there is increasing demand for objective analysis of its benefits and costs [1,2]
The informative prior for r developed in S3 Appendix had a lower coefficient of variation (CV) (0.34) than the >0.5 value recommended for the CV of informative priors in population dynamics models [64], so we examined sensitivity of the results to this prior by using a vague uniform prior for r with a lower bound at zero and an upper bound at 6.0 cub fox-1yr-1
We applied our model to data from 22 estates, which not representative of circumstances across Britain, being in only a few regions and landscapes, resulted in parameter and density estimates that were revealing about the relationships between the cull and within-year fox population dynamics
Summary
Lethal control of predator numbers to benefit prey species is controversial and there is increasing demand for objective analysis of its benefits and costs [1,2]. Benefits may be evidenced by showing increased prey survival or breeding success where predators are culled, but because of variation in these parameters by site and year, unequivocal demonstration of the effect of predator control requires a costly experimental approach [3,4]) Because of their cost and difficulty, such field experiments are rare, and do not sample a range of situations or operators. They are limited in both generality and relevance to any different situation. With few exceptions (e.g. mink [10]) almost nothing is known of the dynamics of predator numbers during control efforts
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