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What We've Learned

Project Squirrel was originally created by Wendy Jackson and Joel Brown, and has been operating since 1997. During this time, over 1000 people have participated, provided observations, and filled out the survey. We have been able to learn a great deal about these squirrels, particularly in the Chicago Metropolitan Region. Observations from other parts of the country have also been welcome and interesting. Many participant's comments are shared under What People Have Said.

The Coexistence of Fox (Sciurus niger) and Gray (S. caroliniensis) Squirrels
in the Chicago Metropolitan Area.
From van der Merwe, M., J.S. Brown, and W.M. Jackson. 2005. Urban Ecosystems 8:335-347.

Fox squirrels were more likely to be observed in the western and especially southwestern suburbs. There were 27 zip codes where only gray squirrels were recorded, compared to only two zip codes where only fox squirrels were recorded (60104 in the village of Bellwood and 60402 in the village of Berwyn). There were 85 zip codes for which both fox and gray squirrels were recorded.

Results include more fox and fewer gray squirrels than expected in areas with single-family homes. The incidence of both species together was also higher than expected in these areas. This is in contrast to multiple-family units, highrise buildings, parks and campuses, where the pattern was reversed and where more than the expected number of gray squirrel observations and fewer than expected fox squirrels observations were made. Amongst multiple-family units, high-rises and on campuses, fewer than the expected number of observations that included both species was recorded. Results for multiple-family units and high-rises were in accordance with the pattern of gray squirrels being more common in areas of higher population density. However, parks and campuses are not always situated in areas of high population density, and the gray squirrel association with these areas may be more closely tied in with the results for different tree species.

Gray squirrels were found more often than expected in association with oaks and pines, whereas fox squirrels were associated with elms and maples. The acorns from oak trees are an important food source for tree squirrels, and as a general rule, areas with large numbers of oak trees can probably support higher squirrel densities than areas with few oak trees. The cacheability of acorns is especially important for winter survival (Husband, 1976; Havera and Nixon, 1980; Thompson and Thompsen, 1980; Korschen, 1981). The noticeable abundance of oaks in parks and campuses of the Chicago Metropolitan area may be tied in with the preponderance of gray squirrels in these land use types. Pines, though not as important as oaks, also provide cacheable food. Trees like maples and elms provide food in the form of new spring growth, but the lack of cacheability of these food types restricts their importance to shorter time periods. Taken together, the data for land use and tree species variables seem to support the notion that gray squirrels are strongly associated with areas of high human population density and an abundance of oak trees. Fox squirrels are more likely to be associated with lower human population densities in areas with single-family homes and where elms and maples are the dominant tree species.

Apart from high human population density and oak trees being associated with gray squirrels, therewas also a significant pattern with cat abundance. The number of observations where only fox squirrels were observed was different from the expected number. Fox squirrels were more likely to be observed in areas of high cat abundance and less likely to be observed in areas of lowcat abundance. Though not significant, the same trendwas found for dogs: the public made more fox squirrel observations than expected in areas of higher dog abundance. Fox squirrels are larger than their gray squirrel counterparts, and differences in body size have been suggested to result in trade-offs that could promote coexistence (Kotler, 1984; Lima et al., 1985). Larger and more mobile prey is more difficult to subdue by predators, whereas smaller foragers can be more efficient due to lower metabolic foraging costs. This mechanism is supported by all the observed patterns from our study. In areas where predation risk is relatively minor and where food is abundant, gray squirrels may have the competitive advantage due to being more efficient foragers. In areas where pets (as potential predators) are more prevalent, the larger fox squirrels may have a competitive advantage over gray squirrels. This is in accord with the favored natural habitats of the two species, where fox squirrels do better in the more risky open areas and forest edges, and gray squirrels prefer denser forest.

Gray squirrels were associated with densely populated areas, parks and campuses, fox squirrels with suburban areas. Compared to gray squirrels, fox squirrels were more likely to be observed in areas of high cat density. In the suburb of Oak Park, the current trend seems to be an extension of gray squirrel distribution and a decrease in fox squirrel distribution. Our study provides support for the idea that fox and gray squirrel coexistence is facilitated by a trade-off between managing the cost of predation and foraging efficiency, gray squirrels out-competing fox squirrels in areas of high food and low predator (or pet) density.

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