OVERCOMING THE COMPENSATORY RESPONSE OF AN INVASIVE PREDATOR

Many animal species have been transported across the globe by human activities. Some of these have become established and damage the species that naturally live in the area they invaded. Invasive predator species able to use a wide range of prey are particularly problematic and are held responsible for the extinction or catastrophic declines of some native species. If those native species are to be preserved, conservationists must intervene, often by actively removing individuals of the invasive species. Removing the last individual however is only possible on islands. Elsewhere, it will be necessary to continue controlling those invasive species for the foreseeable future. This is expensive and can only be done in areas selected for their conservation value, such as national parks or reserves. It is also important that conservationists learn to control those alien species as effectively as possible, so that the native species really benefit, but also in order to be able to protect as large areas as possible for a given amount of resources. Academic ecologists have been studying the interactions between predators and their prey. They have learned about the impact of that fragmenting habitats may have on the likelihood of species not causing the extinction of other species. Ecologists now realise that indirect interactions between two species mediated by a third species, may play an important role in the success and mitigation of invasions. They have also devised equations to predict how the rate at which a predator remove prey from a population varies with the abundance of a prey population. They have also gained a better understanding of the conditions that cause individuals to disperse away from the area where they were born. Our proposal is for a partnership between academic ecologists, conservationist and a users of wildlife resources that will use predator-prey theory and new empirical data to devise a science-based management strategy to safeguard populations of a protected and rapidly declining mammal, the water vole that is subjected to predation by the invasive American mink. The project will take place in the Cairngorms National Park, on the fringe of the uplands. First, we need to know how fast the number of immigrants arriving in an area where control has taken place declines, as this area is more distant from uncontrolled areas. This is important at it will tell us how large an area must be controlled to keep the mink population low in its centre. For this, we will remove mink from sections of rivers for a time and monitor recolonisation as it takes place. Second, we will consider how the success of trappers changes as mink become scarce. We will find out whether there is a density below which trappers are no longer interested in trapping or are ineffective. Given the different motivations of participant to the project, we will also find out whether groups of trappers will have different trapping success. Third, we will ask if the presence of rabbits in some valleys does subsidies mink populations that prey on them, and may then move on and prey on water voles elsewhere in a valley. This may result in rabbits and water voles not being found in the same valley, even though rabbits certainly do not eat water voles! If mink require rabbits to breed successfully in upland Scotland, concentrating mink control effort in those areas may be very effective. Finally, we will integrate our results in models that can be used to explore the relative merits of different management strategies including 1. Increasing the number of trappers that remain motivated and effective in trapping mink even when they are scarce 2. increasing the number of those trappers only in the centre of the area we wish to protect 3. expanding the area where mink are trapped downstream from the national park, even though fewer people are interested in trapping mink in those areas 3. controlling rabbits in key sites in the core area.