Foraging of gray squirrels on an urban-rural gradient: Use of the GUD to assess anthropogenic impact

Responses of organisms to urbanization may involve adjustments in behavior. To qualify such behavioral plasticity we measured the degree to which gray squirrels (Sciurus carolinensis) exploited sunflower seeds in pans distributed over an urban-rural gradient of 78 sites in Virginia. Our objective was to use squirrel GUDs as a functional, relativistic measure of the effects of urbanization. Results showed that a higher proportion of pans were foraged from and that the GUDs were lower (more seeds were removed) in relatively high-density urban and suburban areas than in more rural agricultural areas, or in relatively human-free forest controls. For sites near or within human settlements, GUDs were lower nearer to human-occupied structures than at a greater distance from them, where more squirrels were observed, and where the density of trees was higher; GUDs were higher where there was substantial ground cover and where domestic pets (i.e., cats/dogs) were present nearby. Squirrels living in close proximity to humans appear to be either more limited by food or less sensitive to predatory risk than those living in more natural areas. We argue that the GUD represents a valuable metric with utility for measuring the separate and combined impact of anthropogenic actions at the individual and population levels.