Deer Impacts on the Environment

Deer Overbrowsing Destroys the Woodland Ecology

The scientific literature on deer overbrowsing clearly shows that overbrowsing deer can damage ecosystems, both directly and indirectly.  Aldo Leopold, the father of wildlife ecology, was the pioneer, writing about deer overbrowsing in his Sand County Almanac in 1949.

Deer populations will continue to grow until they eat all the food available. There is no biological mechanism that limits herd growth other than predators. Even before the biological carrying capacity of an area is reached, however, deer can damage ecosystems by overbrowsing the forest and field. 

Excerpt from Sand County Almanac by Aldo Leopold

Deer can have profound effects on preferred woody and herbaceous browse species. Deer browse of woody vegetation alters the subcanopy and the shrub layer which are made up of small understory trees, young recruits for future canopy openings, and shrubs.  The overbrowsing of the herbaceous level affects one level and can virtually eradicate an entire plant during one browsing episode. 
(Thomas E. Almendinger’s Overabundant White-tailed Deer and the Alteration of Forested Communities)

Deer overabundance and deer-harming overpopulation are not limited to urban areas. The effects of region-wide deer overabundance have been studied extensively. Browsing deer prevent regeneration of trees and often pave the way for invasive plants.  From the US Forest Service, White-tailed Deer in Northeastern Forests Understanding and Assessing Impacts, “The scientific evidence is clear: white-tailed deer overabundance is a threat to millions of acres of forest land in the Northeastern United States. As keystone herbivores, whitetails can have disproportionately large impacts on biodiversity and forest dynamics. Impacts may be obvious or may cascade through the ecosystem in ways not fully understood.” 

Deer browsing can devastate many species, such as these examples from Cool Green Science.

  • High deer populations caused declines of >90% for many orchids in the mid‐Atlantic region in Maryland (Knapp & Wiegand, 2014). Deer browsing also threatens understory herbs like Trillium and American ginseng.
  • Overbrowsing is causing widespread declines of North American songbird populations (Chollet 2012, deCalesta 1994).
  • In Pennsylvania, aboveground insect abundance, richness, and diversity were up to 50% higher where deer were excluded for 60 years (Chips et al., 2015).
Conifer that died after repeated buck rubs.
Conifer that died after repeated buck rubs.

They further state: In our opinion, no other threat to forested habitats is greater [than deer] at this point in time — not lack of fire, not habitat conversion, not climate change. Only invasive exotic insects and disease have been comparable in magnitude. We can argue about which threat is more significant than another, but no one who walks the eastern forests today can deny the impact of deer to forest condition.

The results of an exclosure study in Rose Valley, PA demonstrate the outsized impact deer can have on plant diversity. The pie chart shows the diminished number of species with deer present and the picture below shows the impacts on the vegetation.

From the New York State Community Deer Management Guide, two forested parks in New York City. The photo on the left shows severe deer damage; the photo on the right shows a healthy understory.  

Trillium Damage in the U-M Nichols Arboretum
(click to change the picture)

The first picture shows a group of about 18 trillium and a couple of solomon seals. Only one trillium is going to bloom. It is hard to see what is missing.
The same picture but with all 27 of the browsed trillium stems marked. Deer browsed 60% of the plants.

Deer Damage to Forests Can Accelerate Climate Change

Trees and forests are excellent carbon sinks because trees capture and sequester CO2.  The destruction of the woods and forests by an overabundance of deer accelerates climate change and works against any net zero carbon emissions goal. 

The oak tree is the primary tree cover in Ann Arbor’s natural areas. It’s a hardwood that sequesters lots of carbon. An acre of mature oaks can sequester 5-10 MtC02e per year. And there’s over a thousand acres of treed natural areas in Ann Arbor. But deer browse is preventing the regeneration and replacement of our oak trees, which can significantly reduce the amount of carbon being sequestered.

Deer Overabundance Destroys Private Landscaping and Gardens

In residential neighborhoods, as deer numbers and densities increase, many residents become dismayed at the levels of destruction of valued vegetable and flower gardens and landscapes. Deer need to eat five to ten pounds of vegetation every day. Bucks can kill young trees when they rub the bark with their antlers. 

City ordinances prohibit hunting and prohibit fences sufficiently tall to keep deer out. Even if the expensive fencing and netting protects your gardens and landscaping, fencing merely moves the deer to another yard. Repellent chemicals, dogs, and other deer chasers have the same effect.

Native plants are preferred by deer. “Deer resistant plantings” are therefore often undesirable non-native species that do not support the native pollinators and the native ecosystem.

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