Predicted threats to a native squirrel from two invading species based on citizen science data

Abstract

Invasive species are a major threat to biodiversity worldwide and tree squirrels (Sciuridae) are among the most successful mammalian invaders. Two tree squirrels native to the eastern United States, the eastern grey squirrel (Sciurus carolinensis) and eastern fox squirrel (Sciurus niger), have been repeatedly introduced into the western United States over the past century. In that time, the non-native species have expanded their ranges extensively and have negatively impacted the only native species of large tree squirrel in the western United States, the western grey squirrel (Sciurus griseus). While numerous studies have documented the impacts of these invasions on S. griseus, few have assessed the potential for future expansion and none have evaluated all three squirrel species simultaneously. In this study, we address: whether the predicted distributions derived from observations in the western states differ from those derived from the native eastern distribution; to what extent do the predicted distributions overlap with the native S. griseus; what role does urbanization play in invasion potential of the introduced squirrels. We use citizen science data from the iNaturalist database to model the species distributions in the western United States for all three Sciurus species. We generated models based on occurrences in the squirrels’ native ranges, compared to models generated from their introduced ranges, and evaluated current and predicted conflict zones. We determined that the greatest potential conflict with the native squirrel is currently in areas adjacent to regions of high human footprint. As human development expands, the invasive squirrels are likely to expand into previously inaccessible areas, increasing conflict with and potentially displacing the native western grey squirrel.