Too many deer are bad for the forest
- January 30, 2017
- American Ornithological Society Publications Office
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. In a new study published in Landscape and Urban Planning, researchers show that areas in the eastern U.S. with high deer numbers tend to have fewer birds that need forest shrubs. These species use low-lying foliage to hide their nests from predators and to hunt for insect prey. Unfortunately, these plants are also on the menu of the white-tailed deer.
And there are a lot of deer. In the past, deer numbers were relatively low, held in check by native predators such as black bears, mountain lions, and red wolves. Now, bears are largely restricted to the Appalachians, mountain lion range has retreated far west, and the red wolf is at the brink of extinction. Deer have fared much better under heavy human settlement in the east -- aside from predator removal, roads and housing slice the forest into pieces, providing forage at sunlit edges and cover in the woods. Virginia, where the study was conducted, may contain as many as one million deer. That's a 36-fold increase in the last 80 years.
While hunters partly filled the role of the departed predators, urban areas are largely off limits to deer harvest. Folks with high-power rifles will usually not prowl the local city park, and so deer mow down nearby gardens relatively undisturbed.
But overabundant deer don't stop at daffodils. As more deer pack suburban forest fragments, they denude understory greenery and with it songbird habitat. The authors found that study sites with many deer were virtually devoid of species like the Hooded Warbler, White-eyed Vireo, and the Prairie Warbler -- species that rely on the forest undergrowth. Most of these birds are already in trouble due to habitat loss. The Prairie Warbler, for instance, a perky yellow bird with streaked sides, has been designated a species of conservation concern following range-wide population declines.
Counting deer and woodland birds was tricky. Adequate assessment of bird numbers required knowledge of their vocalizations, repeated visits to study areas early in the morning when birds were most active, and lots of statistical modeling. Deer are skittish and active at night, but they do leave tangible evidence of their presence -- poop (this evidence is especially copious after a bellyful of warbler habitat). As though counting droppings wasn't fun enough, estimating deer numbers also required lots of statistical work.
Pumas and Wolves---Nature's Deer eaters
Pumas and Wolves---Nature's Deer eaters
Researchers estimated bird and deer numbers at two regions of Virginia -- one on the coast and one inland. Coastal Virginia is more urbanized than the rural inland, and correspondingly the study found more than twice as much forest fragmentation there. The coastal region -- with as many as twenty-eight thousand droppings per hectare -- is where the study found significant correlations between deer and birds. "There were a lot of deer" says Vitek Jirinec, a coauthor of the study. "One of our vehicles was hit by a deer while going out for surveys -- it ran into the side of the car and dented the door. Maybe the deer were onto us."
The fact that humans affect wildlife populations is not new, but sometimes the story is more complicated. This study suggests our land use practices that promote high deer numbers might be changing local ecosystems -- with implications for declining songbirds and the people who enjoy them.
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Materials provided by American Ornithological Society Publications Office. Note: Content may be edited for style and length.
Journal Reference:
- Vitek Jirinec, Daniel A. Cristol, Matthias Leu. Songbird community varies with deer use in a fragmented landscape. Landscape and Urban Planning, 2017; 161: 1 DOI: 10.1016/j.landurbplan.2017.01.0
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Deer proliferation disrupts a forest's natural growth
March 8, 2014 Cornell University
- Antonio DiTommaso, Scott H. Morris, John D. Parker, Caitlin L. Cone, Anurag A. Agrawal. Deer Browsing Delays Succession by Altering Aboveground Vegetation and Belowground Seed Banks. PLoS ONE, 2014; 9 (3): e91155 DOI: 10.1371/journal.pone.0091155
- March 20, 2014
- University of Miami
- S. Kalisz, R. B. Spigler, C. C. Horvitz. In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives. Proceedings of the National Academy of Sciences, 2014; DOI: 10.1073/pnas.1310121111
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 in the soil and disrupting the soil's natural seed banks.
The study, "Deer Browsing Delays Succession by Altering Aboveground Vegetation and Belowground Seed Banks," was published online March 7 in PLOS ONE.
"Deer are slowing down forest succession or natural establishment. In fact, the deer are preventing forests from establishing," says Anurag Agrawal, Cornell professor of ecology and evolutionary biology, a co-author on the paper.
Deer typically prefer to eat native, woody plants and rebuff invasive species. The study showed that when deer consume native plants, the non-native species are left to flourish, dropping seed in the soil.
As forests normally mature, their grasses give way to herbs and shrubs, and then new trees eventually take root. Expanding deer populations in the Northeast, however, stall forest development and promote the growth of thorny thickets of buckthorn, viburnum and multiflora rose bushes. If deer leave the forests alone, such trees as cottonwood, locust and sumac can sprout and grow unimpeded.
The researchers found that the impacts of deer grazing on vegetation were severe and resulted in bare soil and reduced plant biomass, less recruitment of woody species and relatively fewer native species. And the deer's negative impact on seed banks resulted in significantly decreased overall species richness and relatively more short-lived species of both annual and biennial plants.
Co-author Antonio DiTommaso, Cornell associate professor of weed ecology and management, and research technician Scott Morris gathered soil cores -- from both within and outside of fenced "deer exclosures" -- and germinated the seed. They found the soil cores from outside of the exclosures contained many more seeds from non-native species.
Deer select forests for their trees but in doing so disrupt forest system growth trajectories, concludes the study.
"It's obvious that the deer are affecting the above-ground species, but it's like an iceberg. There are major effects below the soil surface. We are seeing a divergence of seeds contained within the soil from what should be there," says DiTommaso. "We are not seeing the seeds of woody plants. Instead, we're seeing an escalation of non-native seed and the virtual elimination of woody plant seeds."
The multiyear study was conducted on Cornell land near Freese Road in Ithaca, where the deer density is about 39 animals per square kilometer -- about 10 times greater than it was before European settlement in the late 1700s.
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Materials provided by Cornell University. Original written by Joe Schwartz. Note: Content may be edited for style and length.
Journal Reference:
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Excessive deer populations hurt native plant biodiversity
Too much garlic mustard growing in the forests of Pennsylvania? Actually, the problem may be too many deer.
A new study published online in the Proceedings of the National Academy of Sciences concludes that an overpopulation of deer is the primary reason garlic mustard is crowding out native plants, such as trillium, which are preferred food for wild deer.
"Our findings show that there is a link between disruption of the native animal community and invasion by non-native plant species," says Carol Horvitz, professor of ecology in the University of Miami College of Arts and Sciences and co-author of the study. "Similar links maybe found in other ecosystems between disrupted fauna and declining diversity of flora."
Deer density in the U.S. is about four to 10 times what it was prior to European settlement of North America. "Our findings imply that management of overabundant grazing animals would be beneficial for conservation of plant biodiversity," says Horvitz, who is also a founding member of UM's Institute of Theoretical and Mathematical Ecology.
The study was initiated in 2003 at the Trillium Trail Nature Reserve in Fox Chapel, Pa., by a team of researchers from the University of Miami and University of Pittsburgh. The project takes a long view on why invasive garlic mustard plants thrive to the detriment of native species.
Garlic mustard (Alliaria petiolata) is a plant native to Europe and Asia, and is inedible by deer standards. It was brought to the United States -- Long Island, N.Y., specifically -- in the 1860s for use as a kitchen herb.
Instead, it became a menace, colonizing forest floors in the Eastern U.S. and Canada, and has been found in Washington, Utah, and British Columbia, achieving the dubious distinction of being one of very few non-native plants to successfully invade forest understories. The persistence of garlic mustard greatly reduces forest biodiversity.
To study the effect of rampant deer on trillium and garlic mustard populations, the researchers established multiple 196-square-meter plots in the forest. Half were fenced to exclude deer. Years of observation and hours of statistical analysis later, the team found that in plots where deer were excluded, the trillium population is increasing, and the garlic mustard population is trending toward zero.
"This demonstrates that the high population growth rate of the invader is caused by the high abundance of deer," says Susan Kalisz, professor of evolutionary ecology in the University of Pittsburgh's Department of Biological Sciences and principal investigator of the study. This effect is reversible with deer exclusion. The team's results support "an ecological theory that native species in communities can exert biotic resistance." This means that native plants as a group can successfully compete against invaders. If the native plants are allowed to thrive rather than being consumed by deer, the combined natural competitive advantages of those plants -- including trillium -- allow them to repel the outsiders.
"When people walk in the woods where deer are overabundant, they don't realize what's missing," Kalisz says. "They don't know what used to be growing there. They don't know that species are being lost and replaced by invaders."
The solution seems simple, then: Reduce deer populations, restore natives and prevent invasion. "It's not simple," Kalisz says. "Deer management policies vary from state to state and deer don't respect political boundaries." Some states keep deer populations low, while others prefer to maintain higher populations to appeal to groups such as hunters. "Yet, deer exact a toll not only on forest species, but also farms, orchards, and even your car and your car insurance rate," Kalisz says.
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Materials provided by University of Miami. Note: Content may be edited for style and length.
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