“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.
Legacy of top-down herbivore pressure ricochets back up multiple trophic levels in forest canopies over 30 years, Ecosphere, Jan 2011Removal of top-down control on herbivores can result in a trophic cascade where herbivore pressure on plants results in changes in plant communities. These altered plant communities are hypothesized to exert bottom-up control on subsequent herbivory via changes in plant quality or productivity. But it remains untested whether top-down perturbation causes long term changes in plants that ricochet back up the new food chain that depends on them. In a large-scale, 30-yr controlled field experiment, we show that 10 yr of top-down control of an ungulate herbivore (white-tailed deer, Odocoileus virginianus) created contrasting forest tree communities exerting bottom-up effects that ricochet back up 3 trophic levels 20–30 yr later. Higher ungulate densities during stand initiation caused significant reductions in tree species diversity, canopy foliage density, canopy insect density, and bird density in young (ca. 30 yr old) forests. Because recruitment of trees from seedlings to the canopy occurs over a relatively brief period (ca. 10 yr), with membership in the canopy lasting an order of magnitude longer, our results show that even short-term perturbations in ungulate density may cause centuries-long disruptions to forest ecosystem structure and function. In documenting this five-step trophic ricochet, we unite key concepts of trophic theory with the extensive literature on effects of ungulate overabundance. As predators decline and ungulate herbivores increase worldwide, similar impacts may result that persist long after herbivore density becomes effectively managed.
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.
The impact of deer on relationships between tree growth and mortality in an old-growth beech-maple forest
Zachary T. Long, Thomas H. Pendergast IV, Walter P. Carson, Department of Biological Sciences, University of Pittsburgh
Forest Ecology and Management 252 (2007)
White-tailed deer have been at high levels in the northeastern forests of the United States for decades and have strongly influenced forest dynamics. In this long-term study, we found that the composition of the overstory and understory assemblages of an old-growth beech-maple forest differed significantly. We used exclosures to test the hypothesis that deer contributed to these differences by differentially influencing the relationship between growth and mortality among seedlings of the six most abundant tree species. In the absence of deer, we found that the mortality of the six species decreased with increased growth and that interspecific differences in the relationships between growth and mortality coincided with previously observed shade-tolerance rankings. In the presence of deer, mortality decreased with growth only for the browse tolerant species (American beech, black cherry, and sugar maple). Mortality did not decrease with growth for preferred browse species (oak species, ash species, and red maple), rather, this relationship was eliminated in the presence of deer. The changes in growth and mortality relationships in the presence of browsing generally corresponded to observed changes in seedling density following the removal of deer. Sugar maple, ash, black cherry, and total stem density increased in the absence of deer. Our results suggest that the relationship between survival and growth in the understory, a metric of shade tolerance, is a fairly plastic response that varies depending upon the presence and absence of herbivores. Our results indicate that deer have contributed to the differences between understory and overstory vegetation, with browse tolerant species increasing in abundance at the expense of preferred browse species.
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.
Relative deer density and sustainability: a conceptual framework for integrating deer management with ecosystem management
Wildlife Society Bulletin, 1997
This framework replaces the variety of old carrying capacity concepts: sustained yield of maximum numbers of deer for harvest and sustained yield of timber. All of these can be expressed in the common currency of RDD, which would help clarify apparent differences when data are collected on landscapes with differing carrying capacities.
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.
The terrestrial vegetation and flora of North and South Manitou Islands, Sleeping Bear Dunes National Lakeshore
University of Michigan, 1983
The vegetation and flora of North and South Manitou Islands in northern Lake Michigan were surveyed during the summer of 1982 and the spring of 1983, the first comprehensive study of both islands. The vegetation associations were mapped and described, and a catalogue of 490 vascular plant species was compiled. A noticeable difference in forest structure and floristic composition between the islands was observed, largely due to an introduced deer herd on North Manitou. Twenty permanent plots were established on the islands to observe any future changes at selected sites. The island distributions of eight native species listed as threatened or of special concern by the State of Michigan were mapped and fragile habitats were identified.