Ecological traps for large-scale invasive species control

Predicting recolonisation areas favoured by American mink is a step towards invasive species control, according to this new article by Melero et al. Ecological traps for large-scale invasive species control: Predicting settling rules by recolonising American mink post-culling is published today in Journal of Applied Ecology.

Mink in trap Jamie 2006
Mink in trap on mink raft, Image credit: Jamie Urquhart

Invasive species present daunting challenges to conservationists because of the scale over which their impact on native biodiversity plays out. Despite the current emphasis on prevention and early action, many damaging invasive species have become established. Their spread can overwhelm the limited resources of conservation agencies, leading to the accumulation of more non-native species and the erosion of native biodiversity.

It’s not all is doom and gloom however as, on oceanic islands, conservation practitioners have become increasingly adept at eradicating rats, mice, rabbits and feral cats. The response of birds and vegetation have been spectacular, so it is possible to reverse the spread of invasives and regain what was once lost!

While the impacts of invasive species may take longer to become evident in mainland areas than on islands, they are still serious. Despite this, eradication of mobile species is not feasible in invaded mainland because there will always be a flux of dispersers arriving from outside and seeking to settle in vacant attractive territories. Thus, rather than eradication, the aim of invasive management on mainland is to reduce the density of the most damaging invasive species sufficiently so as to minimize their impact on native species.

Ongoing invasive management on mainland entails the removal of incomers as they arrive.  It must be designed to last in perpetuity, hence optimising cost-effectiveness is crucial. One way to achieve this is to better-understand the rules that re-invading dispersers use when deciding where to settle in an area largely depleted of conspecifics and this is what the research in our paper sets out to do.

It is the product of a close collaboration with a partnership of conservation practitioners who sought to push back the invasion of Scotland by the American mink, one of the world’s most nefarious invasive mammals) over 10 years. Mink escaped from fur farms and now devastate bird and mammal species that live along waterways, including the water vole, a species with high cultural value in the UK. Citizen conservationists supported by project staff operated floating mink raft devices used to detect, trap and remove mink in their local area. Their combined effort severely and durably reduced mink density over a staggering 20,000km2 area, the size of Wales – a third of the area of Scotland. The scale and the topographic barriers surrounding the management area contributed to reducing the flux of dispersers.  Despite this, a steady flow of dispersers had to be dealt with to preserve the achievements of the project.

We used DNA forensics to reconstruct the pedigree and place of birth of mink culled by conservation volunteers, and estimated the proportion of young mink who settled away from home. We found that there is ample potential for mink dispersal to compensate for the control effort. Seventy seven percent of mink had dispersed from their natal area, and 20 % of these travelled > 80 km. Thus, few places, if any, are beyond reach of such an effective invader.

Next, we asked whether all parts of the management area were at equal risk of reinvasion or whether some parts were more attractive and preferentially re-settled by mink and should receive heightened vigilance. We found it was possible to predict where re-invading mink would settle down by considering all options open to immigrants. We found that distance from the natal area was a poor predictor of settlement choice and that both male and female mink were discerning in their settling decisions and preferred high-quality river sections. Two metrics of quality, one based on habitat features, the other derived from the settlement decision of other mink gave broadly consistent predictions.

We were encouraged to find data obtained during management interventions largely carried out by citizen conservationists were sufficiently information-rich to predict patterns of recolonization. Our results are important because they provide an evidential basis for targeting control to patches attractive to immigrant mink and creating “ecological traps” where long term control effort might be concentrated.

The full article, Ecological traps for large-scale invasive species control: Predicting settling rules by recolonising American mink post-culling is available in Journal of Applied Ecology.

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