Modelling the effects of mink and habitat fragmentation on the water vole

1. The decline of the water vole Arvicola terrestris in the UK has been attributed to the spread of the introduced American mink Mustela vison. Understanding the causes and dynamics of this decline is vital to water vole conservation. We investigated the dynamics of water voles in relation to habitat fragmentation and mink predation using an individual-based spatially explicit model of population dynamics on the River Windrush, Oxfordshire, UK. 2. A sensitivity analysis was undertaken using values for life-history parameters drawn from known ranges using Latin hypercube sampling. Partial correlation coefficients were used to estimate how the predicted size of water vole population and extinction were determined by the life-history parameters. The model was then validated by comparing model predictions with observed distributions of water voles. 3. The effects of mink predation and habitat fragmentation on the future viability of water vole populations on the River Windrush were analysed after artificially manipulating habitat fragmentation on the river and running the model in the presence and absence of mink predation. 4. The match between predicted and observed distributions was significantly related to home range requirement and high reproductive success. At low fragmentation, home range requirement was the most important influence on the number of populations. Reproductive output, and adult and juvenile mortality, became increasingly important with increased fragmentation. At high levels of fragmentation demographic stochasticity had a large influence on population size. 5. We deduce that the importance of demography in determining population persistence will depend on the level of fragmentation. Additionally, life-history parameters that are crucial to the viability of water vole populations can only be identified in the context of the landscape in which populations are found. 6. The extinction of water vole on the River Windrush became more likely as habitat fragmentation and mink predation increased. Mink predation effectively doubled the probability of extinction over that arising from fragmentation alone. 7. These simulations indicate that extant populations on the Windrush are now so fragmented that populations may not be viable even in the absence of mink predation. We assessed the extent of habitat restoration necessary to ensure population persistence on the River Windrush and considered developments of the model for use in water vole conservation.