Forest-dependent species of mammals can be affected by this reduction and fragmentation of habitat. Species that are mobile and readily traverse a variety of habitat types, such as deer, raccoons, or white-footed mice, still are ubiquitous among patches of forest in these landscapes. However, species with poor dispersal abilities or that are averse to leaving forest cover, such as flying squirrels, chipmunks, and woodland voles, may disappear from all but the largest tracts of forest when habitat patches become small and isolated.
Gray squirrels (Sciurus carolinensis) and fox squirrels (Sciurus niger) seem to respond differently to forest fragmentation. Historically, gray squirrels were widespread throughout Illinois and outnumbered fox squirrels in forested areas. As forested areas were cut, thinned, or grazed, fox squirrels increased in abundance and gray squirrels often declined. Now, gray squirrels are absent from many parts of their former range, although they have adapted well to many urban areas. Gray squirrels can be very abundant in some urban areas, as anyone with a bird feeder or a garden planted with tulip bulbs can attest, but they have been replaced by fox squirrels in many rural areas.
In east-central Illinois, gray squirrels are currently found only within some urban areas and continuous riverine forests, and are absent from isolated non-urban areas. In contrast, fox squirrels are numerically dominant to gray squirrels throughout the countryside. Although gray squirrels have been successfully reintroduced to some urban areas and a few large, forested areas, they have not spread out from these sites to fill their former range. What biological mechanisms might account for the different distributions of these two similar species?
Dan Rosenblatt with a fox squirrel and a gray squirrel ready to be radio-
collared and relocated.
Habitat fragmentation can lead to changes in the physical environment in remnant patches. In spite of the behavior of gray squirrels in urban areas, in non-urban settings they tend to prefer interior parts of forest where there is a closed canopy of mast-producing trees, such as oaks and hickories, and where there is a brushy understory. Fox squirrels seem to prefer forest edges and forests with more open canopies and little understory. This difference in habitat preference is not absolute, however, and both species can sometimes be found in the same place. Perhaps fragmentation has altered the environment in small woodlots to such an extent that gray squirrels no longer consider them suitable habitat.
Fox squirrels are slightly larger than gray squirrels, and female fox squirrels defend core areas around their nest sites. Fox squirrels also seem to be able to exploit waste grain in agricultural fields better than gray squirrels. Perhaps these factors, combined with the opening up of forest habitat, have given fox squirrels a competitive edge over gray squirrels, and the presence of fox squirrels prevents gray squirrels from colonizing and maintaining populations in rural woodlots in east-central Illinois.
Finally, gray squirrels and fox squirrels differ in some aspects of their dispersal and predator avoidance behaviors. In non-urban settings, fox squirrels tend to spend more time on the ground than gray squirrels. Fox squirrels may run on the ground for a considerable distance when pursued, whereas gray squirrels more quickly take to the trees. Perhaps these behaviors make fox squirrels better at dispersing over open habitat. In landscapes such as east-central Illinois where wooded patches are often small and isolated, high rates of movements among patches may be required to keep all patches occupied. If gray squirrels are hesitant to disperse across open landscapes, they may not immigrate frequently enough into small woodlots to "rescue" dwindling populations, or recolonize sites quickly after local extirpations.
Such a system, where several small subpopulations occupying different habitat patches are connected by dispersal, is called a metapopulation. If dispersal between the patches becomes disrupted or decreases to a low rate, because of habitat fragmentation for example, the system becomes unstable. If subpopulations in local patches disappear and the patches are not recolonized quickly enough, a species could disappear from a region entirely. Thus, if fox squirrels are better cross-country dispersers than gray squirrels, they could persist in a landscape consisting of small wooded fragments while gray squirrels do not.
Urban gray squirrel on the UI campus.
To evaluate these three possible mechanisms, we are conducting a series of controlled introductions of radio-collared squirrels. The radio collars allow us to track the fates and movements of the squirrels. If gray squirrels introduced into rural woodlots from which fox squirrels have been removed survive and reproduce, then we can assume that the habitat was suitable and something else was responsible for their absence. If gray squirrels also fail to persist in woodlots where fox squirrels have not been removed, it suggests that competition may be important. If gray squirrels persist in rural woodlots both with and without removal of fox squirrels, it indicates that differences in dispersal abilities may explain the different distributions of these two species. Additional experiments are being conducted to quantify differences in dispersal behavior.
These studies are important for more than just answering questions about squirrels. As humans dominate more and more of the landscape, more and more habitats become fragmented, and the intervening areas may restrict or impede dispersal movements. How different types of plants and animals will respond to this fragmentation is an important concern for conservation biologists and managers. Are dispersal corridors needed for some species? Are there habitat modifications that should be done in the remnant patches to help some species persist? Are there thresholds of isolation (distance between patches) or area (size of patches) that lead to extirpation of some species? It will take the gradual accumulation of information from a variety of species to practically address these questions.
And we'll be able to tell you just why those squirrels in your woodlot or at
your bird feeder are gray and white or gray and red.
Dan Rosenblatt and Ed Heske, Center for Wildlife Ecology
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