Understanding the Impact of Predators on White-tailed Deer

When considering the effects of predators on their environment, the first thing that comes to mind is that predators kill prey through hunting. However, reality is more complex. Predators often have preferences for specific prey, and this can change the population numbers and demographics (sex, age, etc.) of prey species and the species composition of an ecosystem. While these are often the “direct effects” of predation, a growing interest in predator-prey research are the “indirect effects,” which can be much more subtle.

Predators can indirectly impact prey by causing them to attempt to avoid attacks by moving to safer areas that might have less food, aggregating into herds and groups where they need to share resources, and increasing their vigilance, among other behaviors. These behaviors reduce the time spent eating, or the food available, and can sometimes simply stress prey enough to impact their health. In some contexts, it has been shown that the impact predators have on prey through changing their behavior and increasing stress alone might have a larger impact on prey than directly hunting.

Bobcats and coyotes are two of the main predators in Illinois and are considered mesocarnivores. Mesocarnivores are smaller predators, located lower in the food chain than apex predators such as mountain lions and wolves. As a result, they generally spend their lives balancing between being predators and being prey. They hunt to survive, but generally hunt smaller prey and avoid the risks associated with competing with apex predators. Mesocarnivores are more cautious than apex predators, but their ability to be generalists and use a variety of resources and habitats makes them adaptable and more able to thrive in human-created environments than many other wildlife species. In the past 200 years, apex predators have been eliminated from much of the country, allowing mesocarnivores to take over as the dominant predators throughout the Midwest.

Coyotes specifically have taken advantage of the lack of apex predators and changes in habitat. Historically, these animals were primarily located in prairies and deserts in the western U.S. but are now found across the continental U.S. This has led to an immense boom in coyote populations which can be seen throughout Illinois. Bobcats, however, have had a more tumultuous existence. Nearly extirpated in the mid-1900s, bobcats became protected as a threatened species from 1977 to 1999. Bobcat abundance has significantly increased over the past decades, with the highest numbers found in the southern half of the state. This increase led to 2015 legislation allowing a regulated bobcat season to be implemented by the Illinois Department of Natural Resources (IDNR) in 2016.

For decades, the Cooperative Wildlife Research Laboratory (CWRL) at Southern Illinois University-Carbondale has partnered with IDNR to investigate white-tailed deer ecology and population dynamics in Illinois. This long-term project has addressed various questions about deer reproduction and recruitment, survival, social behavior, distribution, and dispersal rates, along with exploring non-hunting human interactions. A predator aspect was incorporated in early 2018 to examine how predators may be impacting white-tailed deer populations, specifically by predating on fawns.

The current goal of this project is to understand more about coyote and bobcat behavior and their effects on white-tailed deer. Bobcats and coyotes have been shown to prey more on fawns than adult deer, so another goal of the project is to quantify the direct effects of predation on fawns in Illinois. For the deer portion of the study, Michael Egan, a Ph.D. candidate, traps white-tailed deer in central and southern Illinois during the winter. His team catches both bucks and does and fits each individual with a GPS collar that transmits locations for approximately 2 to 3 years. M.S. students Nicole Gorman and Abby Weber are focusing on predator behavior and actively trapping bobcats and coyotes at these same field sites, fitting these individuals with GPS collars that drop off after a year. Once late spring arrives, the priority switches to capturing and collaring white-tailed deer fawns. Technicians actively track these fawns using radio telemetry from their early life until they are out of peak vulnerability to determine the fate of each individual.

With these efforts in the field, we are able to compare predator and deer movement to see if predators are tracking deer and their fawns, and if deer are altering their behaviors to avoid predators. In addition, the differences in the central and southern Illinois study sites allow us to compare how habitat differences may affect predator behavior and their relationship with deer. We started by studying bobcat and coyote habitat requirements and the extent of their movements. This means investigating how these predators rank forest, agriculture, human settlements, and other habitat types.

We found that individuals in agricultural central Illinois wandered farther for resources than in forested southern Illinois, and that both bobcats and coyotes preferred forested habitat over agriculture. This information tells us that the habitat available to these predators makes a large impact on where they choose to spend their time, which leads into our work involving predation on white-tailed deer fawns.

Some of the questions we are starting to tackle include whether bobcats and coyotes are specifically targeting fawns or are simply encountering them as they wander. We are also interested in understanding when fawns are most vulnerable to predation from bobcats and coyotes and if some areas are safer for the fawns. Lastly, we hope to explore the overarching impacts of this predation on deer populations. This includes looking into how predation on fawns might directly affect deer population numbers, as well as if deer are indirectly affected by predators by changing their movements in response to predators in the area.

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Nicole Gorman and Abby Weber are Master’s students in the Cooperative Wildlife Research Lab at Southern Illinois University advised by Dr. Guillaume Bastille-Rousseau. They are researching mesocarnivore spatial ecology and response to white-tailed deer in southern and central Illinois.

Dr. Guillaume Bastille-Rousseau is an assistant professor with the Cooperative Wildlife Research Laboratory at SIU. His research focuses on the spatial wildlife ecology of terrestrial species, mostly focusing on animal movement.