A Spatially Explicit Population Model To Compare Management Using Culling And Fertility Control To Reduce Numbers Of Grey Squirrels

In the UK the now widespread non-native grey squirrel produces problems for the forestry industry through damage from bark stripping and threatens the survival of native species most notably the red squirrel which, mainly as a result of resource competition and transmission of infection, has suffered dramatic declines. Reducing grey squirrel numbers is essential to decrease this species' ecological and environmental impact.Using an individual-based model operating in a fine scaled landscape of well-mixed woodlands with reliable seed production we develop novel effort-based mechanisms explicitly representing the probabilistic interaction of individual squirrels with either traps or bait hoppers to capture the density-dependant efficiency of culling and fertility control respectively. We also integrate a habitat-based resistance to inter-patch movement to describe more realistically the source-sink dynamics in regional scale populations.We use this new framework to compare the relative effort of population management at a landscape scale using both culling and fertility control, alone and combined as part of an integrated, sequential, approach. We also exploit our spatially-explicit framework to demonstrate how we might identify neighbourhoods within our study area where management may be easier (quicker and cheaper) or more difficult (sub-populations resistant to management), to enable the prediction of an optimal spatial and temporal deployment of management effort.Our results agree with previous studies on the relative efficiency of culling in eradicating squirrels, as well as on the substantial "costs" of this approach. Despite an assumption of lower deployment costs, our findings suggest that, at least for the initial squirrel densities assumed, fertility control alone is unlikely to achieve rapid enough reduction to prove a viable cost-effective alternative to completely replace culling. However, when applied to the low density populations following short-term culling, eradication could be achieved within the same timescales as continuous culling alone but with substantially lower costs.