Monitoring Chicago’s Urban Small Mammals

About

Since 2010, Lincoln Park Zoo has monitored large- to medium-sized mammal populations in parks, cemeteries, golf courses, and forest preserves along three research transects radiating from downtown Chicago to the surrounding suburbs. However, one important group rarely captured on motion-activated field cameras is small mammals (mice, shrews, voles, etc.).

These species form crucial components of grassland and forest ecosystems as a source of prey for many species, including endangered raptors, such as short-eared owls (Asio flammeus) and northern harriers (Circus cyaneus). They also influence vegetation, for instance, by consuming tree saplings and inhibiting forest succession. Despite the strong influence small mammals have on the ecosystem, little is known about how they persist in urban areas.

Through the health and stress of urban wildlife initiative, the Davee Center for Epidemiology and the Urban Wildlife Institute have begun monitoring these small, elusive species along an urban to exurban gradient starting along the downtown Chicago lakeshore and moving straight west to the suburbs of DuPage County.

With this data, zoo researchers can determine the community composition of small mammal species and pinpoint which factors influence the presence or absence of different groups of animals. Such factors could include the vegetative diversity at a site, connectivity between suitable habitat patches, and the amount of impervious surface in the surrounding neighborhood.

Measuring Small Mammal Stress Hormones

Hair and fecal samples were collected from each individual small mammal to look at long-term (hair) and short-term (feces) stress. High cortisol levels, which correlate to high stress levels, can deteriorate an animal’s health, lowering their reproductive success and making them more susceptible to disease. The goal was to evaluate this physiological stress along an urban to exurban gradient to identify causes and trends, which can lead to land–management recommendations that increase the health of a native small mammal community and thus create a healthier and more diverse ecosystem as a whole.

The Small Mammal Role in Lyme Disease

Certain small mammal species serve as key vectors for sideases, such as Lyme disease (Lyme borreliosisis), that can be transmitted to humans. In Cook County, human cases of Lyme disease have increased dramatically in the last decade. Ticks contract Lyme disease by feeding on animals that carry the disease, like mice, and can then transmit the disease when they feed on another animal that is susceptible, such as humans. Scientists have hypothesized that more diverse ecosystems may reduce the transmission of Lyme disease.

This “dilution effect” theory suggests that the presence of more species that do not act as carriers for Lyme disease would reduce the chance of ticks feeding on carriers like mice. If fewer ticks are infected, then the risk of transmission decreases. Lincoln Park Zoo researchers explore this theory by performing tick drags at study sites and removing ticks from small mammals to be analyzed in the lab for potential pathogens.

Findings could influence human health, especially in densely populated urban centers of Chicago, by identifying which small mammal communities have Lyme disease and testing the importance of this “dilution effect.”

Small Mammal, Large Impact

This study is advancing our understanding of small mammal population dynamics in urban and exurban areas. Zoo researchers don’t only identify small mammal community composition; they also examine the relationship to regional carnivore persistence by pairing survey data with existing motion-activated field camera images from a long-term biodiversity monitoring program.

Studies of multiple species are extremely rare, and researchers have a strong need for these studies in urban regions. This information could be critical to conserving and creating functional ecosystems in city environments. Observing how these relationships can influence not only small mammal presence, but also stress levels through cortisol levels from hair and feces will further define the health of these species.

Finally, investigating Lyme disease to determine if the “dilution effect” is supported in Chicago will give researchers a better understanding of ecosystem health and Lyme disease presence/absence from exurban to urban regions, which could have implications for human transmission of the disease. These results can assist in future land-management strategies to build wildlife-friendly cities around the world.