Why ecologists support Ann Arbor’s deer cull

Christopher Dick is an associate professor of ecology and evolutionary biology at the University of Michigan and director of the E.S. George Reserve and U-M Herbarium.

In her recent guest column, Tanya Hilgendorf of the Humane Society of Huron Valley wrote that Ann Arbor’s deer cull was part of a “‘Green Scare’ put forward mostly by those who, under an ‘ecological’ guise seek to eradicate any plant or animal not currently in popular favor.”

I have discussed the urban deer issue with academic biologists at the University of Michigan, including ecologists, botanists, zoologists, restoration ecologists and landscape architects. We are all in support of city council’s decision to conduct a cull.

Ann Arbor’s high deer abundance is part of a much broader phenomenon. In the past 100 years whitetail deer numbers have swelled to historic highs across their range. Deer overabundance poses a threat to many North American ecosystems.

Consider that a single whitetail selectively consumes roughly 3,000 lbs. (1.5 tons) of plant material each year. Herds remove swaths of forest wildflowers and damage the woody understory. This impacts native butterflies, bees, small mammals, amphibians and certain birds. Whitetails alter forest composition by browsing oak and other hardwood seedlings. Their food preferences allow unpalatable species to proliferate, including invasive garlic mustard and Japanese barberry, which inhibit the next generation of forest trees and native wildflowers.

Deer are present in even higher abundance in urban settings. City parks and suburban gardens are rich in preferred food. The deer are safe from hunting and natural predators. They can wander through lawns and along roads in broad daylight. They habituate to humans and become a nuisance to many residents.

Critics of the cull demand more details. How many deer are too many? What about non-lethal methods? Do culls even work? These are important questions.

Opposing view: Are Ann Arbor’s deer enough of a threat to warrant firepower?

From ecological and conservation perspectives, an ideal deer herd will coexist with a full range of native species. By several measures, Ann Arbor’s herd size has surpassed this threshold. Botanists at the U-M have long noted declines in native plants that deer favor, through decades of observation, and by comparison with landscapes where deer are excluded or managed. In a 2015 study, an ecological team surveyed browsing impacts in Ann Arbor’s Bird Hills Nature Area. They found browsing damage in 80 percent of the tree saplings.

For more than a decade, the Humane Society of the United States has inserted itself into urban deer conflicts and lobbied for non-lethal methods of deer control, including contraceptive drugs and surgical sterilizations (note, the Humane Society of Huron Valley is not funded by nor affiliated with the national group). City council has agreed to consult with the Humane Society of the United States on non-lethal methods in coming years.

Contraceptive drugs for deer are illegal in Michigan and difficult to administer. Surgical sterilization is an expensive ($1000 per deer) and invasive procedure. Both approaches are ineffective for controlling free-ranging herds.

For evidence that culls help to restore ecological balance and biodiversity, we can look to the University of Michigan’s ES George Reserve. In 1928, four does and two bucks were released into the 1300-acre research tract. The population rose from six to more than 160 animals in six years, creating heavy browsing pressure and suppressing plant succession. Since 1942, reserve stewards have periodically culled the herd, with a steady recovery of oaks, hickories, maple and other native species.

Apart from its ecological value, deer culling is an important tool for combatting emergent diseases. These include deer Chronic Wasting Disease and Lyme disease. Lyme disease causes life-debilitating symptoms in people (and pets) and is associated with high deer density.

The Ann Arbor deer cull may not reverse decades of ecological degradation or prevent all diseases. But with around 150 tons of buds, leaves and flowers that will be spared this year alone, it is a positive step toward ecological sustainability.