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October 23, 2013

Are Deer Responsible for Biodiversity Loss and Forest-Growth Failures?

Deer are scapegoats for larger ecological problems

Deer and fawn, white-tailed

iStockphoto

The concept of overpopulation, or how many deer are too many for a given area, is a subjective one. While it may be true that deer densities are at historic highs, this statement by itself has little meaning.

To begin with, the forests of today in no way represent historical conditions in this region, nor will they ever again, thanks to human influences. Urbanization has created an abundance of edge habitat, which is ideal habitat for browsing deer. Deer have simply adjusted their populations to available habitat and resources.

It is easy to point the finger at deer and blame them for our forest growth woes, yet the reality is that forests are affected by many things: acid rain, insect damage, disease, forest fragmentation, pollutants, loss of soil fertility, animal browsing, invasive and other competing plant species, parasitic organisms, climatic and weather extremes, over-development … and deer. It is vital in addressing deer-human conflicts that we not use deer as scapegoats for larger and more systemic ecological problems.

Fighting nature

We may want to see more biodiversity in certain areas because we saw it there in the past. Yet nature is not static; a forest floor carpeted with wild flowers can rapidly transition to another state as a result of forest succession. Certain plant species are shaded out as trees mature and the forest canopy closes. Later successional stages are, by their very nature, less diverse.

While we may want to see a certain flower grow somewhere, this doesn't mean it "should" be there. There is no forest blueprint for what should grow. What we want to see in the natural world is influenced by our aesthetic preferences—which may not be grounded in any biological reality. 

It is easy to point the finger at deer and blame them for our forest growth woes, yet the reality is that forests are impacted by many things.

Some surprising scientific research findings

  • Trillium is often used as an indicator of high deer abundance. Yet research points to soil acidity having a largely invisible yet possibly strong influence on where purple trillium and other wildflowers will grow.
  • A Yale Forestry School study assessed 120 study plots in six Connecticut towns to determine deer impact on forests. The conclusion was that deer density is not a leading factor in tree regeneration decline or loss of plant diversity across western Connecticut.
  • Some research indicates that deer hunting can have unintended negative consequences for snakes, salamanders, amphibians, and many invertebrates.
  • Deer "exclosures" (small fenced-in areas) are repeatedly pointed to as proof of deer impacts on forests, yet we must be mindful that these protected areas show us what a patch of forest might look like without any deer. Such a state is no more "natural" than deer presence in great numbers. Allowing some deer hunting will not necessarily bring forth what is seen in the deer exclosures.

Deer impact on songbirds

Deer browsing can alter bird-nesting habitat. But the sad reality is that many factors are directly affecting native songbirds. The destruction of over-wintering habitat in South America, the broad-spectrum use of pesticides, pollution, the destruction and development of deciduous forests,  plate-glass windows, cat-induced mortality, weather extremes, and diseases like West Nile Virus are just a few of those factors. Deer impact on forest communities is only one of these factors.

Surprisingly, new evidence suggests that some songbirds, like robins and other thrushes, actually prefer to nest in invasive shrubs like honeysuckle and buckthorn. The shrubs' growth habits may leave the birds less vulnerable to predation. This type of finding underscores how complex the web of nature is, and how it's important to scrutinize some of our general cause-and-effect assumptions.

A final thought

There's no denying that deer can have a significant influence on our forests. Deer impacts can be very visible and deer browsing can unquestionably alter forest structure.

However, the bottom line is that single-species management has never been a viable way to manage a multi-faceted problem. By intensively managing one component of a forest, the result may be unforeseen effects on other components such as the spread of certain invasive plant species. Since the return of both deer and forests to greater abundance and health, there simply has not been enough time for us to study and understand the complex ecological associations involved and how to properly manage them.

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