Modeling Impacts and Costs of Gray Squirrel Control Regimes on the Viability of Red Squirrel Populations

Abstract

The introduced gray squirrel (Sciurus carolinensis) is a principal threat to the populations of the red squirrel (S. vulgaris). We used a spatially explicit population dynamics model to investigate the changes of red and gray squirrel populations in landscapes to assess the effects of gray squirrel population control methods on red squirrel population viability. We analyzed 2 methods of gray squirrel population control: (1) trapping where adults were killed on an annual basis, and (2) immunocontraception where reproduction was suppressed. The model was run under a range of efficiency and intensity of control as well as red and gray squirrel life-history scenarios created by Latin Hypercube Sampling of the realistic parameter range for both demographic and control variables. The viability of squirrel populations in 2 contrasting landscape scenarios was investigated: (1) Thetford Forest, East Anglia, United Kingdom (UK), where a small isolated population of red squirrels is surrounded by a large population of gray squirrels; and (2) Redesdale Forest, Northumberland, UK, where red squirrels are still in abundance and the first gray squirrels were observed to disperse into the area from Scotland. In the absence of population control of gray squirrels, red squirrel populations were predicted to become extinct in both landscapes within 15 years at Thetford and 6 years at Redesdale. Immunocontraception did not lead to persistent populations at Thetford but allowed red squirrels to persist in Redesdale. Removal of gray squirrels by trapping at both sites was predicted to lead to red squirrel population persistence. Our findings suggest that immunocontraception is unlikely to be effective unless it is applied to a large portion of the target and surrounding population. Our results also indicate that an integrated control strategy, incorporating both trapping and immunocontraception, may be the best option for controlling gray squirrels. This integrated strategy needs to include the relative costs of control, and it may be that immunocontraception provides the best overall solution in areas where gray squirrels are low in abundance.