Deer herds regularly thinned by hunting had little impact on riverbottom plant life, a University of Wisconsin-Madison study has shown. “Quality deer management” and traditional deer hunts seemed equally effective in preserving plant populations along the Wisconsin River, according to Rebecca Christoffel, a wildlife ecology graduate student at the College of Agricultural and Life Sciences. On the other hand, a deer herd that flourished under a strict no-hunting policy took its toll on plant and animal life in the area, she found.
“This implies that trophy management, quality deer management or other harvesting strategies are not likely to affect vegetation in significantly different ways,” Christoffel says. “Strategy per se is not as important as ultimate overwinter deer density – that is, the size of the deer herd. Thus, private landowners can choose from a variety of deer harvesting options and still maintain the integrity of the native plant communities found on their properties.”
Christoffel studied the relationship between plant community dynamics and deer abundance under three deer-harvest strategies in the Wisconsin River floodplain forest. She chose three study areas along the river between Wisconsin Dells and Portage. The sites had similar plant communities, and each site has had a different deer harvest strategy for at least the past five years.
The 1,400-acre Leopold Memorial Reserve includes floodplain forest, marshes, prairies, oak woods and agricultural crops. Hunter access is by invitation only, and deer have been hunted under a “Quality Hunting Ecology” program established in 1991, in which a hunter must kill two antlerless deer before killing a buck.
The 6,000-acre Pine Island Wildlife Area is about a mile downriver from the Leopold reserve. Plant communities include floodplain forest, prairie, marsh, savanna, oak woods, and retired agricultural fields. Pine Island is heavily hunted, with traditional deer management (any licensed hunter could shoot a buck; with bonus tags or hunter”s choice permits, hunters could shoot antlerless deer). Deer numbers at Pine Island and the Leopold reserve were similar.
The third site is a 2,400-acre private tract on the north side of the river, across from the Leopold reserve. Much of the land is farmed, but there are also some set-aside fields and more than 300 acres of floodplain forest mixed with savanna remnants. No deer hunting is allowed and deer are actively protected during the deer hunting season. This site had many more deer per square mile than the two hunted sites.
To measure the effects of deer browsing, Christoffel built fenced exclosure plots, which kept out deer but let in rabbits and other plant eaters. Over two years, she monitored browsing rates and changes in plant density and reproductive rates in the exclosures and nearby unfenced plots. She sampled the vegetation to document species richness and diversity and determine browse preference ratings for selected plants.
On the hunted properties, most browse rates were less than 25 percent On the unhunted property, browse rates varied but could be greater than 80 percent on preferred plant species, Christoffel found. Because deer numbers were so high on the unhunted property, deer trails were abundant in the floodplain forest, which was the only cover around. That many deer, just feeding along the trails, had a substantial impact. Farmlands can buffer deer damage to natural plant communities, but only to a certain point. When deer numbers get too high, farmlands can no longer buffer those plant communities, she explains.
The unhunted site had more than twice as many deer per square mile as the Leopold reserve or Pine Island. Plant species richness (number of species present) was related to deer abundance, and deer abundance appears related to harvest strategy. Species richness was consistently higher at the Leopold reserve than at Pine Island or the unhunted site. Plant reproductive rates, an indirect measure of deer browsing intensity, were lower at the unhunted privately-owned tract than either of the other properties, in almost all cases. “This supports the conclusion that at high deer densities, reproductive output by plants is inhibited. Stem densities of preferentially browsed plants were lower at the privately-owned tract in all but a few cases. Other environmental factors such as soil elevation and decreased competition may explain these exceptions,” Christoffel says.
Whitetail populations can skyrocket under complete protection. At the densities seen on the unhunted site, deer can significantly alter the vegetation, reducing species richness and structural diversity. The hunting strategies employed at the Leopold reserve and Pine Island had the same, or similar, vegetative impacts, and deer herbivory did not seem to reduce plant species richness or diversity, Christoffel concluded.
“Given the results of long-term studies in parks and other non-hunting areas, I would advise landowners to actively participate in deer management. If deer populations exceed DNR goals, the potential for plant damage increases,” Christoffel notes.
If deer numbers increased at the Leopold reserve and Pine Island, or on any forested site in southern Wisconsin, you would end up with results similar to the unhunted site in this study, she points out. “You would have reduced species richness, lower plant reproductive rates, and less vegetative structural diversity. We also found a reduction in the number and species richness of small mammals at the unhunted site. We didn”t measure songbird abundance and diversity on our sites, but a Pennsylvania study found reduced abundance and diversity of the songbird community at high deer densities in forests.”