Tree squirrel introduction: A theoretical approach with population viability analysis

Reintroduction efforts require knowledge of how many animals are needed for successful establishment. Population viability analysis can be used to predict trajectories of introduced populations and tree squirrels provide an ideal model system to investigate this challenge. Conservation action is needed because more than 80% of species of tree squirrels are of precarious conservation status in some portion of their range. We combined data from closely related species of tree squirrels and used VORTEX to determine how many squirrels are needed to successfully establish populations of 6 species (Sciurus aberti, S. carolinensis, S. niger, S. granatensis, S. vulgaris, and Tamiasciurus hudsonicus). We ran multiple simulations to account for between-patch differences in breeding success (resource availability) and variation between years in different habitats. In the best-case scenarios, populations could be successfully established with fewer than 35 individuals for all species and as few as 15 for a subset of species. Empirical evidence from introductions of tree squirrels supports our simulation results, with 93% of populations of greater than 10 squirrels surviving more than 50 years. With relatively few individuals needed for establishing new squirrel populations, reintroductions are feasible and useful as a buffer for imperiled species.