Optimising the long-term control of invasive American mink from catchment to coast: how many mink is too many?

Invasive Non-native Species (INNS) threaten biodiversity, ecosystem functions and services and impose costs to national economies. Management efforts are underway globally to reduce these impacts, but little attention has been paid to situations where INNS are impossible to eradicate, and therefore population reduction and containment are the only feasible solutions. This is particularly true in mainland situations where removal of the final individuals is often prohibitively expensive or technically not feasible. In such circumstances, effective long-term management must optimise scarce available resources for effective control to address INNS impacts. Critically, this requires knowledge of the relationship between the abundance/density of managed INNS and the continued presence of threatened native species, and recovery of previously decimated native species populations in areas prioritised for management. However, even for long-standing projects, the evidence for the outcome of INNS control efforts on native species is mostly lacking. This project will contribute to remedying this global knowledge gap by collating and analysing valuable data on 16 years of INNS control effort over 23,000 km2 and new data on native species recovery.

The American mink has invaded most of the UK following historical escapes from fur farms. Mink have invaded all habitats, particularly thriving along coastlines where they exploit both marine and terrestrial prey. Their escalating impacts on riparian and coastal biodiversity is well documented. Multiple mink control projects have been initiated in the UK and beyond, many motivated by the desire to protect water voles, severely affected by American mink predation.

As is often the case with prey naïve to a novel predator, there is no local coexistence when mink invade a riparian water vole colony; local extinction always ensues. When this process is repeated across space and time, metapopulation collapse gradually occurs. Coordinated mink control over 23,000 km2 delivered substantial reduction in mink abundance but reinvasion from outlying areas necessitates ongoing management. Evaluating what amount of mink control is sufficient to prevent declines and allow the recovery of target species in a cost-effective manner is a key step in adaptive management, with applications to other INNS species that cannot be eradicated.

In mainland Scotland, mink control was achieved by mobilising the enthusiasm of volunteer citizen conservationists coordinated by a handful of local fisheries trusts' staff. Building on these efforts, we will relate mink control intensity to estimates of the extent of recovery of water voles. Colloquially, we can find out "how many mink is too many mink?"

Our research questions include:
1. What control effort allows the persistence of vulnerable native prey species? What is the shape of the mink control effort - mink impact relationship, as measured by the extent of water vole recovery.

2. Is more intensive mink control required for a time to allow the recovery of non-equilibrium metapopulations that rely on dispersal for spread into habitats where populations are locally extinct due to prior mink predation?

3. How do topography, land cover and adjacency to uncontrolled catchments impact the resilience of mink to control effort and their impact on native prey?