Deer change the landscape indirectly

February 7, 2017 by Terry Devitt, University of Wisconsin-Madison
Forest understory differences inside and outside of a fence designed to exclude deer at UW–Madison’s Kemp Research Station in Vilas County. The forest on the left is outside the exclosure and shows the dramatic effects of deer on forest understory. Credit: Katie Frerker

It is widely known that the white-tailed deer is a nonstop eater. Unless it is sleeping or fleeing from a predator, the keystone North American herbivore is nearly always nibbling.

Ecologically, deer herbivory is a fairly well understood phenomenon. The presence, abundance and reproductive success of many plant species are directly affected by deer, whose populations are orders of magnitude greater in some regions than they were before European settlement.

Now, scientists are looking beyond herbivory to better understand the indirect effects of deer on eastern North American landscapes. In particular, scientists are interested in how the animal's presence and behaviors affect the composition and overall health of the wildflowers and other herbs—what scientists call understory communities—that blanket the .

"Deer are affecting understory communities in many different ways," explains Autumn Sabo, a University of Wisconsin–Madison plant ecologist and the lead author of a new study that teases out some of the secondary impacts of on forest ecosystems. "It is only in recent years that scientists have started to look at factors beyond herbivory."

Writing this week (Feb. 6, 2017) in the Journal of Ecology, Sabo and her colleagues detail how deer affect forest plant composition by altering facets of the forest environment, including light availability, soil compaction, and the thickness of a particular layer of soil.

A sugar maple sapling nipped off by white-tailed deer, one of North America’s emblematic keystone herbivores. Credit: Autumn Sabo

The study focused on 17 deer exclosures, patches of forest ranging in size from a hundred square meters to eight hectares, where 2- to 3-meter high fences have been installed to keep deer out. The exclosures, some of which have been in place for decades, are located in the temperate hardwood forests of northern Wisconsin and Michigan's Upper Peninsula.

Archaeological evidence suggests that deer were once far less abundant in eastern North America, perhaps as few as two to four deer per square kilometer. Today, on average, there are about seven deer per square kilometer in the areas studied by Sabo and her colleagues.

"In northern Wisconsin and the Upper Peninsula of Michigan, we found little herbivory damage on forest herbs," says Sabo, who conducted the study with Katie Frerker of the U.S. Forest Service, and Don Waller and Eric Kruger of UW–Madison. "Deer seem to be eating primarily young trees."

Findings from the new study suggest scientists need to revisit their thinking about the effects of deer as, previously, most ecologists believed the biggest impact of deer on understory species was herbivory. That idea may be true in places like the northeastern U.S. where a lot of direct browsing damage has been observed on wildflowers, says Sabo. But for northern forests in the Upper Great Lakes region, the indirect effects of deer nipping off saplings appear to be more important.

Because young trees have been nibbled down, light levels on the forest floor increase and a whitish layer of soil known as the E horizon thickens as colored nutrients are lost. Deer also increase soil compaction, likely through hoof action.

UW-Madison plant ecologist Autumn Sabo (left) and U.S. Forest Service ecologist Katie Frerker examine foliage near a deer exclosure in Door County’s Peninsula State Park. Credit: Andy Jandl

"Shifts in these environmental factors may result in forest composition changes," notes Sabo, a graduate student and instructor in UW–Madison's Department of Forest and Wildlife Ecology. "For example, more tree cover is correlated with more herbs that are spread by animals eating their fruit, including trillium. Higher light levels favor raspberries and ferns, and the thicker soil E horizons correlate with fewer lillies and violets."

Soils that have been compacted grow more grasses and sedges and also are more favorable to non-native plant species such as dandelions, she adds.

The results of the new study may provide an additional explanation for why forest understory plant communities are slow to recover after pressure from deer is eased.

"With exclosures, sometimes you see almost immediate effects," according to Sabo. "Other times you see very little change, which makes us suspect legacy effects" from having had a lot of on the landscape.

The findings of the new study can help scientists design experiments to show the effects of changes in light and soil resources on plant communities. Critically, the results can also inform efforts to conserve forest biodiversity and improve forest restoration techniques.

Explore further: Deer account for almost half of long-term forest change, study finds

More information: Autumn E. Sabo et al. Deer-mediated changes in environment compound the direct impacts of herbivory on understorey plant communities, Journal of Ecology (2017). DOI: 10.1111/1365-2745.12748

Related Stories

Too many Bambi are bad for the forest

January 30, 2017

Overabundant deer can spell trouble for people, including frequent car collisions and the spread of zoonotic diseases. But deer can also disrupt wildlife communities—such as forest songbirds—by eating away their habitat. ...

Deer proliferation disrupts a forest's natural growth

March 8, 2014

By literally looking below the surface and digging up the dirt, Cornell researchers have discovered that a burgeoning deer population forever alters the progression of a forest's natural future by creating environmental havoc ...

Trees recognize roe deer by saliva

September 12, 2016

Trees are able to distinguish whether one of their buds or shoots has been randomly torn off or has been eaten by a roe deer. In the case of roe deer browsing, they activate corresponding defence mechanisms. This is the result ...

Biologists support Ann Arbor deer cull

January 15, 2016

A University of Michigan evolutionary biologist says he and many of his U-M colleagues support the city of Ann Arbor's plans to kill up to 100 deer this winter, calling the cull "a positive step toward ecological sustainability."

Recommended for you

Researchers redefine the origin of the cellular powerhouse

April 25, 2018

In a new study published by Nature, an international team of researchers led by Uppsala University in Sweden proposes a new evolutionary origin for mitochondria—also known as the "powerhouse of the cell." Mitochondria are ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.