Browsing by overabundant white-tailed deer has altered ecological relationships in forest communities across eastern North America. Recent but limited work suggests that deer browsing also selects for particular plant defensive traits.
“Dramatic” response by flora & fauna to climate change, USGS WUWT, Jan 11, 2012
The U.S. Geological Survey and University of Montana study not only showed that the abundance of deciduous trees and their associated songbirds in mountainous Arizona have declined over the last 22 years as snowpack has declined, but it also experimentally demonstrated that declining snowfall indirectly affects plants and birds by enabling more winter browsing by elk. Increased winter browsing by elk results in trickle-down ecological effects such as lowering the quality of habitat for songbirds.
Climate impacts on bird and plant communities from altered animal–plant interactions
Thomas E. Martin & John L. Maron
Nature Climate Change, Jan 10, 2012
Effects of climate on plant communities may provide an alternative, but particularly powerful, influence on animal populations because plants provide their habitats. Here, we show that abundances of deciduous trees and associated songbirds have declined with decreasing snowfall over 22 years of study in montane Arizona, USA. We experimentally tested the hypothesis that declining snowfall indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk (Cervus canadensis). We excluded elk from one of two paired snowmelt drainages (10 ha per drainage), and replicated this paired experiment across three distant canyons. Over six years, we reversed multi-decade declines in plant and bird populations by experimentally inhibiting heavy winter herbivory associated with declining snowfall. Moreover, predation rates on songbird nests decreased in exclosures, despite higher abundances of nest predators, demonstrating the over-riding importance of habitat quality to avian recruitment.
Growing Deer Population Hurts Survival Of Forests
NPR, June 15, 2011
The deer side of the fence has a carpet of grass, a shrubby looking thing, and some large trees – things that are either too big for deer to eat, or are among the very few plants they don’t like to eat. Inside it is practically a jungle. Dozens of different almost exotic looking plants are tumbling over one another. Many of them are young trees.
You have the old trees. And when the old trees go, there is something here [in the enclosure] to take its place. Out there, I don’t see anything out there that’s a small tree.
One of the surprising things they’ve found with this experiment is that deer allow invasive species to flourish. And with fewer native plants, there are fewer birds who depend on them for nests and food. There are fewer mice and fewer chipmunks here when they have to compete with deer.
Legacy of top-down herbivore pressure ricochets back up multiple trophic levels in forest canopies over 30 years, Ecosphere, Jan 2011
Deer Damage After Foliage Falls
Fairfield County, 2010
Dr. Georgina Scholl, Vice Chairman of the Alliance, stated “Residents have forgotten that it is not normal to look through the forest and see the rise and fall of our topography hundreds of feet out. The shrubs and saplings that once comprised the leafy understory used to block your view. Today, such lower areas look more like manicured parklands”.
With each deer consuming approximately 10 pounds of vegetation each day, the forest cannot regenerate its vegetation fast enough to support 60+ deer per square mile.
Appetite for trouble
Wisconsin Natural Resources Magazine, Oct 2007
Even small numbers of deer can have dramatic consequences where the land can’t support a larger herd. At a deer density of 12 to 15 animals per square mile of range, herbaceous plants like trillium, Indian cucumber, showy lady’s slipper and white fringed orchid decline. When deer densities reach 20 to 25 animals per square, species like pines, white cedar, hemlock, oaks and Canada yew can stop regenerating and small mammals like red-backed voles, an important prey species, starve out without the forest floor vegetation they need. At 25 to 35 animals per square mile of range, birds like hooded warblers decline from lack of needed ground, shrub and tree layers.
Overabundant White-tailed Deer and the Alteration of Forested Communities
Department of Ecology and Evolution, Rutgers University-School of Environmental & Biological Sciences, 2006?
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.
A demographic study of deer browsing impacts on Trillium grandiflorum
Plant Ecology, May 2003
Thomas P. Rooney and Kevin Gross
A moderate drought during the study could account for the negative population growth rate, but deer browsing accelerates the rate of decline. Population growth is most sensitive to the proportion of plants remaining in the nonflowering stage, and deer browsing reduces this proportion. Browsing damage was relatively low in this study (5.4% of stems in 1998, 11.5% in 1999) compared to another study of browsing impacts on T. grandiflorum, indicating deer could have far more severe demographic consequences in populations subject to higher levels of browsing.
Height of White-Flowered Trillium (Trillium Grandiflorum) as an Index of Deer Browsing Intensity
Roger C. Anderson
Feb 1, 1994
Trillium stem height was positively correlated with reproductive output by perennial herbaceous plants and negatively correlated with the percent of the herbaceous understory that is browsed. This indicates change in stem height is as indication of the general status of the herbaceous flora as influenced by deer browsing. Based on deer population densities associated with study sites supporting Trillium populations with stable stem heights and flowering plants, maintenance of deer densities of 4—6 individuals/km2 is recommended for deciduous forests in northeastern Illinois.