Conservation management within strongholds in the face of disease-mediated invasions: red and grey squirrels as a case study.
Abstract
There is increasing evidence that disease-mediated invasions are widespread across a range of vertebrate, invertebrate and plant systems. We therefore need a better understanding of the role of disease in managing conservation threats due to introduced and invasive species. Here, we develop a general theoretical model framework to assess the impact of disease-mediated invasion on the viability of conserving native species through refuges taking into account explicit spatial and stochastic processes. The model techniques are applied to the well-documented red and grey squirrel conservation system in the UK as a case study. By combining general and specific modelling approaches, we are able to make management predictions while also gaining an understanding of the processes that underlie population outcomes leading to more robust conservation practice. Model results indicate that in the absence of control of the invading species, native populations are driven to extinction both in the absence of disease (through competition) and more rapidly when the disease is included (through competition and disease processes). When control is applied to reduce the abundance of the invading species, there is a threshold in the level of control, above which the invading population can be prevented from establishing and the native species can be protected. Highly virulent infections - squirrelpox in red squirrels - lead to periodic outbreaks of disease in the native population due to continual invasion attempts from the disease-carrying invader. Infections with low virulence may become established at endemic levels in native populations. Therefore, an important finding is that the disease can spread through the native species even when the invading species is prevented from establishing. The benefits of increased density may be countered by an increased risk of disease outbreaks. Therefore, a critical message is that there is a correlation between native density (and therefore habitat quality) and the impact of disease 'harmful' to native species. Control of the invading species to prevent it establishing in strongholds can protect the native species from exclusion, but may not protect it from disease outbreaks. Synthesis and applications. Disease outbreaks in the absence of the invading species can result in significant population crashes and therefore represents a serious threat because it contributes to the risk of population extinction by suppressing the size of the population making it more vulnerable to extinction through stochastic processes.