The effect of habitat fragmentation on forest mammals: An experimental analysis of three squirrel distributions in the agricultural landscape of east central Illinois

I investigated mechanisms responsible for generating distribution patterns of tree squirrels in the agricultural landscape of eastern Illinois. I surveyed mammalian species richness in 10 forest remnants and found that 7 of 16 species were present in all sites, with as many as 13 of the 16 species probably occurring within all sites on occasion. The southern flying squirrel, eastern chipmunk, and eastern gray squirrel were consistently absent from isolated forest fragments. All three species rely on mast as their main food source and are thus dependent on forested areas for their survival. However, the fox squirrel, a species with similar requirements, was present across all sites. I rejected three hypotheses for the observed distribution of fox and gray squirrels by performing a series of introductions. Introduced gray squirrels survived longer and were more successful at reproducing than introduced fox squirrels, rejecting the hypothesis that fox squirrels possess superior colonization ability. Gray squirrel introductions were successful in two of the three isolated woodlots, including the woodlot with the highest density of resident fox squirrels, indicating that if gray squirrels can reach these sites, they are capable of persisting within them. This finding was contrary to the hypotheses that gray squirrel absences were due to poor habitat suitability or competitive exclusion by resident fox squirrels. Radio-telemetry data from juvenile females of both species that were released into the agricultural landscape demonstrated that fox squirrels have superior dispersal ability, as fox squirrels moved greater distances and visited a greater number of habitat patches over the 30-day tracking period. An analysis of fox squirrel population genetic structure using RAPDs was consistent with high rates of interpatch movement. A comparison of escape behavior between fox and gray squirrels demonstrated that the escape strategy of the gray squirrel is more suited to an urban environment, providing a possible explanation why gray squirrels displace fox squirrels in certain urban areas. This study illustrates that species-specific responses to habitat fragmentation can result in differences in species distributions, with dispersal ability and escape behavior being the main factors leading to differences in distributions.