Causes and Implications of Fox Population Dynamics in Central Southern England.

Abstract

The red fox (Vulpes vulpes) is a generalist mesopredator and the world’s most widespread terrestrial carnivore. As a major predator of game, livestock, and wildlife, foxes are culled throughout their range to limit their impact, including in the UK, where data suggests they occur at higher overall densities than many other European countries. However, owing to source-sink dynamics, numerous studies have demonstrated that individuals removed by lethal methods can be replaced rapidly through compensatory immigration. Therefore, given that culling is not a long-term solution, identifying the underlying causes of such high fox immigration pressure is needed to provide support for alternative, more sustainable means of controlling foxes. In this thesis, I investigated the population ecology of foxes in central southern England, a region identified as having exceptionally high fox densities in some areas. The region also boasts important areas for conservation, including the New Forest National Park (NFNP), where foxes are culled by professional wildlife managers to conserve threatened populations of ground-nesting wading birds, such as the Eurasian curlew (Numenius arquata). Compensatory immigration is the primary mechanism that happens in response to culling in population sinks, and occurs because culling in restricted-areas generally fails to account for the spatial scale of dispersal. Due to extensive gene flow, the areas contributing to compensatory immigration are expected to show no genetic differentiation to the population sink, together forming part of the same ‘management unit’. The first objective of this study was to provide an initial investigation into the extent of the management unit relevant to the NFNP using a population genetic analysis. As foxes are highly mobile, genetic structure was not expected on mainland locations surrounding the NFNP, whereas differentiation was expected between these populations and an adjacent insular population on the Isle of Wight, given the relative isolation of the latter. A 653 base-pair fragment of mitochondrial DNA (mtDNA) was obtained from 53 foxes sampled throughout the study region and aligned to a database of published homologous sequences across Europe. These data were used to provide insight into the geographic origins of foxes on the Isle of Wight and test for differences in haplotype frequencies using pairwise FST. The findings showed that foxes on the Isle of Wight are the only population consistently differentiated from discrete areas on the mainland, supporting previous findings that foxes exhibit a general lack of population genetic structure across large swaths of mainland areas, although large bodies of water such as The Solent can isolate individual populations. Compensatory immigration undermines the effect of culling on fox populations, therefore the factors that could potentially exacerbate the rate of replacement into population sinks are important to understand from a management perspective. One example is anthropogenic subsidisation. Population sinks can function as ‘ecological traps’ due to dispersers being deceived by an attractive feature of sinks that belies the true reduction in fitness incurred by inhabiting them. For instance, anthropogenic subsidisation could increase the productivity and carrying capacity of the sink, promoting higher rates of immigration, despite the high mortality rate caused by culling. Accordingly, the second main objective was to examine the diet of foxes for evidence of anthropogenic subsidisation in the population sink (i.e., the NFNP). The occurrence of anthropogenic subsidisation here was postulated given the absence of secure wheelie bins in the waste disposal system, potentially providing foxes with easier access to human food waste. To identify the local food resources supporting foxes in the NFNP, a stomach content analysis of 447 culled foxes was conducted. Stomach contents were separated into broad categories and quantified using electronic weighing scales. Foxes exhibited a highly varied diet, including anthropogenic food, which comprised at least 14% of overall stomach volume. Access to this anthropogenic food was associated with human infrastructure. The volume of anthropogenic food consumed annually and the number of foxes this could support in isolation was estimated, using Monte Carlo simulations to handle uncertainty. The output of these simulations equated to a significant proportion of the annual number of foxes culled in the NFNP. The conclusion was that that improved waste management should be included as a more holistic conservation strategy. The size of adjacent source populations contributes to the rate of compensatory immigration into population sinks. Access to food resources is a limiting factor for population growth among all organisms (i.e., bottom-up control) including mesopredators, especially in the absence of top, apex predators. Therefore, large-scale anthropogenic subsidisation of source populations could also exacerbate the rate of compensatory immigration by elevating regional average fox density. A potentially widespread and prevalent form of anthropogenic subsidisation of source populations is the release of free-roaming gamebirds that lack anti-predator behaviours having been reared in captivity. Nevertheless, there has been a paucity of studies examining fox diet on a management unit scale. Stable isotope analysis is one method that could be utilised, which quantifies the assimilated diet of consumers and overcomes some of the disadvantages of traditional macroscopic analyses. Using stable isotope analysis of 162 fox ear hair samples and potential food items, the diet of foxes throughout the study region was quantified using Bayesian Stable Isotope Mixing Models (BSIMMs). However, these BSIMMs did not produce precise estimates of fox diet owing to the large number of overlapping source categories, such that firm conclusions regarding anthropogenic subsidisation could not be made. The limitations of this method are highlighted and ideas for future work are suggested. The impact of compensatory immigration means that the effectiveness of restricted-area fox control could be negated to a large extent, therefore such a programme needs to be rigorously evaluated and alternatives tested to support its implementation. Hitherto, a tool that can be readily used to examine how varying the intensity and timing of fox control by lamping has been missing. The final objective was to adapt and parameterise an agent-based model (ABM) to simulate several alternative management strategies that could be implemented in the NFNP or elsewhere where lamping is the principal mode of fox control. An existing, spatially explicit ABM – ‘FoxNet’ – was used to investigate how different management scenarios at local and regional scales might influence fox density within the NFNP, testing how the number of hours spent shooting foxes at night during the culling season, and the timing of the culling season, affected the resulting fox density during the breeding season for ground-nesting birds (March- July). To build on the existing model framework, a lamping procedure was written into the underlying code, with its efficiency determined by a modifiable log-normal distribution for the rate of successful search. These simulations showed that intensifying fox control effort above existing levels, or extending it back into the fox dispersal season, would not necessarily reduce spring fox density any further in the long-term. Instead, reducing regional habitat productivity would be useful. Overall, these findings support that the prolific mobility of foxes undermines the long-term efficacy of restricted-area fox culling in the NFNP, potentially exacerbated by local anthropogenic subsidisation. The results are not in favour of more intense fox control in the NFNP, instead highlighting that a change of land management should be considered to contribute to more sustainable fox control and enhance species conservation. In striving for sustainability and ethical responsibility, the results of this thesis have direct implications for the management of foxes in the UK lowlands.