Interspecific competition in fox (Sciurus niger) and gray squirrels (Sciurus carolinensis)

Background/Question/Methods

In the United States, populations of fox squirrels (Sciurus niger) are declining, yet gray squirrel (Sciurus carolinensis) populations are expanding.  Both species utilize similar food sources, but gray squirrels are smaller, faster, and more aggressive than fox squirrels.  Gray squirrels might outcompete fox squirrels via interference competition or exploitation.  To determine if interspecific competition is occurring in central Illinois, I observed direct encounters between the fox and gray squirrels when acorns (low energy food) and walnuts (high energy food) were provided.  I also estimated population density and examined vertical space use of each species to document their relative population sizes and whether niche partitioning may occur. I expected gray squirrels to be most aggressive during direct encounters and to have relatively large populations.  The two species were expected to utilize different vertical strata of the forest.

Experimental feeding trials(n = 32) were conducted on private land(Greene County, IL) from Dec 2013 – Feb 2014 to determine the number of squirrels that handled walnuts and acorns and to observedirect encounters (chasing and physical contact) between the squirrel species.  Density and space use were calculated through systematic observations of squirrel location (ground, lower tree, upper tree) using four transects (120m).

Results/Conclusions

Gray squirrels occur at greater densities (8.02/ha) than fox squirrels (4.76/ha) in Greene County, Illinois.  The mean proportion of observed squirrels that were fox squirrels during the feeding experiment (0.26) was similar to that observed during transect walks (0.37)(t = -1.68, n = 16, p = 0.104). Both species handled acorns and walnuts equally.  During direct encounters (n = 20), gray squirrels were most often the aggressor (x² = 4.13, df= 1, p = 0.042).  Gray squirrels tended to utilize the ground and lower half of trees more so than fox squirrels, but the results were not statistically significant.

Exploitation competition is unlikely in this squirrel community because the relative number of both species during the feeding experiment was similar to that observed in the natural population during transect walks. Niche partitioning may occur, however, because the species appear to use vertical space differently.  Contrary to previous research, these data indicate that both species handle low and high energy foods equally.  Future studies should examine a greater number of squirrels across multiple seasons in order to determine if season alters foraging behavior and vertical strata use.