Nuts and Niches: How Two Illinois Squirrel Species Share the Landscape

Eastern gray and fox squirrels are perhaps some of the best examples of the remarkable ability of certain wildlife species to persist and thrive within human-modified landscapes. These charismatic rodents are a common sight across the woodlands, agricultural landscapes, and concrete jungles of the eastern half of North America, inhabiting a broad range of environments. Both species of arboreal tree squirrel are closely related but vary substantially in size and form.

Eastern grays are usually between 14 and 21 ounces as adults, with gray backs, white or light gray abdomens, and a bushy brown and gray tail with white fringes. Though they persist in many habitats, gray squirrels are most abundant in mature forests with dense undergrowth. Hardwood trees, such as oaks, sugar maples, beech and sweet gum, are particularly prized as they provide food and cavities for shelter.

Fox squirrels are larger than grays, usually 16 to 40 ounces, with a rusty yellow body with light yellow abdomen and a long, bushy tail. Fox squirrels also favor mature hardwoods, but prefer more open woodland, savannahs and forest edges where little understory improves sightlines. They are particularly adept at living within agricultural landscapes, making use of fencerows and remnant ear corn.

Because of the widespread range of both species and the great diversity of environments they inhabit, there is considerable variation in the morphologies and ecologies of each species. Differences in the climate, predator communities, habitat structure and available food have led to the evolution of traits that ensure survival in a given environment. For example, melanistic (all-black fur) eastern grays are far more common in southeastern Canada and are more tolerant of colder temperatures, while fox squirrels in Florida have more brown-yellow morphs in areas that are frequently burned (which helps them blend in with their surroundings to avoid predators). Both species are common in urban environments, though often only one species inhabits a given town.

Illinois is home to four species of tree squirrel: fox squirrels, gray squirrels, red squirrels and flying squirrels, but fox and gray squirrels are by far the most numerous and widespread. Since the two species share preferences for similar resources and cooccur throughout most of eastern North America, biologists have been interested in how much the two species compete and how they’re able to coexist. Because of the broad range of environments they inhabit, this dynamic likely varies among regions, but areas like Illinois have received little study. To reduce competition, animals can adopt a strategy of utilizing different resources, or at different times, than their competitor. This phenomenon is known as niche partitioning. Studying the degree of overlap in the use of resources can give biologists a clearer understanding of the competitive pressures among species, which helps predict the impact upon wildlife during times of dwindling resources, in addition to providing a framework for understanding the mechanisms of wildlife interactions as a whole. If a dominant species, for example, uses aggression to scare away subordinate species from prime habitat, the subordinate species may be relegated to suboptimal habitat, which could have subsequent effects upon fitness and longevity. Under certain conditions, the competitive dynamic among species can shift, depending on the availability of resources and how well-adapted each species is to their environment. Generalist species can make use of multiple habitats and food sources, which makes it easier for them to shift niche to reduce competition.

How Much Do Fox and Gray Squirrels Compete in Illinois?

In a recently published study funded by the Illinois Department of Natural Resources, Justin Remmers and fellow researchers at Southern Illinois University and the United States Department of Agriculture sought to investigate the degree of niche overlap and competition among gray and fox squirrels in the 16 southernmost counties of Illinois. Few studies have documented the competition among these species in the hardwood forests that dominate the area. As Remmers commented via email “How these species interact with their habitat and each other can change based on regional differences and differences in forest types. Illinois allowed for a great opportunity to look at these species in an area that had not been as thoroughly investigated.”

The researchers made use of an extensive array of heat and motion-triggered camera traps, active between January and April of the years 2008 through 2015. They used photographic data of the occurrences of both squirrel species and the time of day in which they were detected. They applied statistical models to determine what factors drive their distributions, co-occurrences, activity patterns, and how much of their activities overlapped.

Results of the Study

Gray squirrel occurrences were associated with abundant hardwood trees, while fox squirrel occurrences were associated with areas with less forest cover, farther from roads, but also with abundant hardwood trees. The researchers suggest that the relationship between gray squirrel occurrences and hardwood trees could be explained by the increased availability of food and tree cavities for rearing winter litters found in hardwoods such as sugar maple, black oak, white oak, and northern red oak.

Fox squirrel occurrences, on the other hand, were associated with hardwood trees but also less forest cover. The authors think that this may be because fox squirrels favor more open and fragmented forests but use hardwoods for food and shelter. Thus, they prefer hardwood trees that are not part of a large, dense, contiguous tract of forest.

Since fox squirrels often inhabit urban environments, their decreased occurrences closer to roads is peculiar. The researchers suggest that roads may represent increased predation risk from predators and increased risk of death from vehicle collisions. Cooccurrences of both species were also more likely to arise farther from roads, suggesting areas far from roads may offer both species significant benefits, such as increased food and decreased mortality risk.

The researchers surprisingly did not find a relationship with ground cover, which is an important factor determining squirrel occurrences elsewhere. This may be because during Illinois winters, squirrels may be using areas with less ground cover that are more ideal for caching their food, where higher predation risk means decreased likelihood of their stores being stolen. Both gray and fox squirrels are scatter-hoarders, meaning they deposit food (like nuts) in many small caches scattered across the landscape. This behavior ensures that squirrels have adequate food supplies during the scarcity of winter, and by dispersing their stockpiles in many different areas, they minimize the chance of their whole supply being wiped out by flooding. Or pilfered by scavengers.

The researchers found that both species had moderate overlap in their activity periods, but while gray squirrels were more crepuscular (active at dawn and dusk), fox squirrel activity peaked during the middle of the day. Interestingly, these activity patterns did not change in areas where one of the species was absent. This is surprising, since the absence of a potential competitor could mean easier access to resources and individuals would be free to broaden their activity to other parts of the day.

As Remmers commented “Similar to how a person may spend more time in a store when there are less people around also shopping, I expected squirrel activity patterns to be influenced at least in part by the presence or absence of other squirrel species also trying to find food and resources.”

The authors suggest that the twilight activity of gray squirrels may be a strategy to avoid diurnal (day-active) predators, as they are much smaller than fox squirrels and vulnerable to more diurnal predators, but it could also be a result of the ‘ghost of competition past.’ This evolutionary relic may be a trait left over from a time when the two species competed more for resources and has stuck around in the gray squirrel’s behavioral roster.

Conclusion

While both species seem to overlap in activity periods and their preference for hardwood trees, the broader habitat context appears to be an important factor in the coexistence of gray and fox squirrels. While fox squirrels are edge-specialists, preferring forest edges and areas with a mix of habitats, gray squirrels prefer more contiguous forest. As Remmers commented, “where these hardwood trees are located, what other landcover is around them, and how hardwood forests are distributed on the landscape likely helps these species partition resources.”

Tree squirrels perform pivotal roles within Illinois’ ecosystems. Not only do they serve as prey for many avian predators (such as hawks and owls), mesopredators (such as bobcats, coyotes, and red foxes), and snakes, but also act as important seed dispersers through their scatter-hoarder caching behavior. Therefore, it is important that we do not take these animals for granted. While commonplace, gray and fox squirrels are keystone species, and their remarkable ecologies warrants admiration.

The research project was provided funding by the U.S. Forest Service and Illinois Department of Natural Resources via Federal Aid in Wildlife Restoration Project W-135-R. The Cooperative Wildlife Research Laboratory; the College of Agriculture, Life and Physical Sciences; and Forestry Program at Southern Illinois University provided further support.

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Nathan Proudman is a Postdoctoral Research Associate at the Illinois Natural History Survey. His research has primarily focused on the ecology of mammals. Currently, he is working on a statewide monitoring program for mammals in Illinois.