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When the whitetail deer population becomes too high, the surrounding ecosystem experiences a cascade of consequences affecting everything from humans to microorganisms.

High numbers of deer lead to increased tick-borne diseases and vehicle accidents, and they place more pressure on native vegetation from intense browsing. But when it comes to deer management, many municipalities and organizations roll out their plans without the necessary background data to inform which strategies work best in which circumstances.

“There was no good way of assessing deer impacts in a standardized way,” said Bernd Blossey, associate professor in the Department of Natural Resources and co-chair of the Cornell Deer Management Committee.

Ten years ago, Blossey set out to change that. He and his collaborators devised a research project based in and around the woods of Ithaca, and their study, “Red Oak Seedlings as Indicators of Deer Browse Pressure: Gauging the Outcome of Different White‐Tailed Deer Management Approaches,” was published Nov. 8 in Ecology and Evolution.

For the project, the team set up three specific deer-management zones and implemented a different strategy in each: fertility control, using surgical sterilization; recreational hunting; and no management.

The study showed that neither fertility control nor recreational hunting was effective at reducing whitetail deer populations or the pressure they place on native vegetation.

“Despite deer sterilization rates of 90%, our findings demonstrate that there is no hope for using fertility control to reduce deer populations or their impact,” Blossey said. “Similarly, the findings from our study also demonstrate that recreational hunting does not control the deer population, and it does not help in reducing deer impacts.”

This knowledge was a game-changer for the university’s deer management program.

Todd Bittner, director of natural areas at Cornell Botanic Gardens and co-chair of the Cornell Campus Deer Management Committee, said Blossey’s research has driven the university to explore better methods to meet its management goals.

“Our policy has been a science-driven evolution in trying less aggressive methods, leading to more aggressive ones, to lessen the impact of deer populations on people and vegetation,” Bittner said. “We are now employing some of the lessons and tools from Bernd’s work; his research has allowed us to access the efficacy of what we’re doing.”

The university now employs a combination of all the deer management strategies allowed by the New York State Department of Environmental Conservation. Bittner said that using these tools in tandem with the findings from Blossey’s study has enabled the university to better align management strategies with outcomes.

“Most urban deer management efforts don’t work, and that’s an ineffective use of taxpayer money,” Bittner said. “But now we have the results to say here’s what does work. Cornell is a leader in this research arena.”

How Blossey’s team compiled those numbers was a novel strategy, and the secret to their success: red oak seedlings.

In each of the three management zones, the team planted hundreds of young oaks. Thanks to deer’s preference for tasty tree leaves, the team could use the seedlings as an indicator of the intensity of deer browsing.

“Our study is unique; no other study has shown a clear link between the changes in deer population and browsing rates,” Blossey said. “We could assess the size of the deer population because we had individually marked more than 100 deer. Then we paired ecological outcome measurements with the deer population size.”

This model has since been replicated across New York state, and has been successfully implemented in the Midwest.

Jacqueline Corteau, an ecologist and biologist, used Blossey’s model to start a similar deer monitoring project in November 2015 by planting red oak seedlings in the forests around Ann Arbor, Michigan. She said that using the seedlings provided a sound way to gather baseline numbers and to measure the deer’s impact on other vegetation, like native wildflowers.

“Standardized monitoring methods are vital for tracking whether deer management efforts are resulting in decreased deer damage to vegetation,” Corteau said.

This kind of long-term, evidence-based research is critical for creating sustainable, effective deer management strategies. While the university’s most recent approach has made progress, the results are still falling short of conservation goals.

“The time frames for ecosystems, biological invasions or conservation efforts play out over decades or centuries,” Blossey said. “Short-term work may be misleading in assuming that all relevant factors have been captured.”

Said Bittner: “We are using our learning laboratories here at Cornell to solve some of the biggest conservation challenges we face.”

Co-authors on the study include Paul Curtis, extension wildlife specialist in Cornell’s Department of Natural Resources; Jason Boulanger, assistant professor of wildlife ecology at the University of North Dakota; and Andrea Dávalos, assistant professor in the Biological Sciences Department at SUNY Cortland.

More Community-Based Deer Management resources can be found on the Community Deer Advisor website.

This article also appeared in the Cornell Chronicle.

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