The report below was produced by the Center for Animal Protection
White-Tailed Deer - An Ecological Perspective
Do hunters replace predators?
"A key point made by authors in all parts is that even if agreement could be reached on a correct density of deer, maintaining such a level would be both difficult and of questionable value. Stability in either a deer population or an ecosystem is an elusive state. . . In addition, the idea that predators can serve to maintain prey populations at stable levels may be incorrect. Therefore, attempts to recreate a mythical stable population density through hunting may not be a sound strategy, if the goal is to maintain ecosystem health." (pgs. 1-7).
"The easier, but not necessarily correct, approach is to say that deer are abundant in comparison with historical densities, then their numbers should be reduced." (pgs. 1-7)
"This approach has been taken by the U.S. Forest Service when arguing that high deer populations can have negative effects on forest resources. . . However, care must be taken to assure that subjective criteria about what the natural world should look like are not confused with objective management goals. Caughley felt only when high-density populations disrupt ecosystem function should they be considered truly overabundant." (pgs. 1-7)
Game management and clear-cutting for deer detrimental to other species
"At the forest-stand level, deer densities sufficient to satisfy hunter demands may be too high to permit sapling recruitment for a preferred browse species. Unfortunately, the old adage that good forestry management is good wildlife management may be appropriate for encouraging species such as white-tailed deer or ruffed grouse, that thrive under early successional conditions that may not hold for other organisms, such as forest-interior birds, salamanders and wildflowers." (1-7)
How deer affect ecosystems
The Smithsonian's McShea: "Clearly, deer browsing affects some species, but is this effect sufficient to affect ecosystem function?" And, "Does a forest composed of browse-resistant species have any less biological worth than one with a significant complement of browse-sensitive species?" (pgs. 1-7)
Increasingly, independent scientists are pursuing those answers.
"Concerns about deer density may be misplaced in the presence of more severe impacts on understory composition." (pgs. 298-309) "Not only do large herbivores respond to plant heterogeneity, but they are important agents in creating it." (pgs. 298-309) "Density of woody stems did not vary with distance from woodlot, near vs. far away from fences or between deer exclosures and control plots." "Diversity showed no effect." (pgs. 298-309)
"Presence vs. absence of woody species was related to six different variables: fences, woodlot, soil, moisture, diversity, mean coverage, variation in coverage"; "Black walnut was the only species to show significant effect of deer exclusion occurring more frequently when deer were present." (pgs. 346-365)
"Results presented here suggest a relatively weak effect of deer on the establishment of woody plants during the first 8 years of plant succession. (pgs. 310-326)
"Species richness averaged only one species higher where deer were excluded than where they were present." (pgs. 310-321)
"Distance from forest edge, presence vs. absence of fences, soil moisture and features of colonizing plant community all accounted for more of the variation in the presence or absence of individual woody plant species than did exclusion of deer. Whereas black walnut showed the strongest response to deer exclusion, it occurred more frequently outside than inside deer exclosures -- a result that might reflect squirrel accessibility and caching rather than deer effects per se." (pgs. 310-326)
At later successional stages, deer have greater impact.
Not so simple: early "negative" plant response can become positive in the long term. "Some herb species showed strong response to deer exclusion early in the study. However, the relationship between short-and-longer-term responses of the vegetation to herbivory appeared to be buffered by community-level interactions involving other consumers and plants. For example, Bowers and Sacchi reported that red clover occurred in deer exclosures 3 years into the study at abundances 150% that in controls. However, a subsequent density-dependent infection of red clover by a rust-formed fungus more strongly affected red clover abundance (especially in deer exclosures) than did herbivory on forage where deer were present than where they were excluded. So with the direct effect of herbivory on forage plants was negative, by keeping plant abundances below that at which fungal infections became lethal, the net effect of herbivores was positive for some plant species. Cases like this stress that simple cause-and-effect relationships between herbivore and pressure and plant responses may be difficult to infer from abundance data alone." (pgs. 310-326)
The fence effect
"The fence effect is clearly related to the dispersal of propagules by birds, who used fence tops for perching, and hence, defecation, thereby contributing to the local seed rain . . . it is clear that the fence effect is partially an artifact of using exclosures to limit accessibility by deer." (pgs. 310-326)
The author goes on to note that studies showing more deer impact in later successional stages were performed in clear-cuts surrounded by forests -- "prime foraging sites of open areas of high net productivity are localized, limited and surrounded by forests." (pgs. 310-326) [These sites would show more damage than in less absolute landscapes.]
"In response, deer undoubtedly concentrate foraging within clear-cuts where net productivity and food availability would be high. One result would be strong, localized deer effects on the vegetation." (pgs. 310-326)
By contrast, the author's study site was "embedded within an agricultural landscape mosaic composed of an overabundance of fields but relatively little forest habitat." (pgs. 310-326)
"Evaluating the full effect of deer overabundance requires an approach that both stresses within-community responses of vegetation to herbivory as well as considers the importance of landscape structure that may function to increase herbivore intensity in some areas and decrease it in others." (pgs. 310-326)
Forest composition and diversity -- Eastern Hemlocks
"Alverson et al (1988) suggested that deer browsing is central to decimated eastern hemlock regeneration in Wisconsin and the region. The temporal and spatial extrapolation of Alverson et al have been questioned (Mlagdenoff and Stearns, 1993) on the basis that alternative hypotheses concerning species life history, ecosystem dynamics, land-use, and climate have not been adequately evaluated. In fact, Mlagdenoff and Stearns (1993) found deer browsing to be only a secondary factor in eastern hemlock decline. Without interpretation in longterm contexts we hypothesize that the impact of deer browsing can be counterintuitive simply because of the inertia inherent in current vegetative composition or successional status. That is, time lags between browsing effects and understory growth to forest dominance must be considered, and tree species' life histories can exert greater control on forest successional trend than can even extreme browsing effects. (pgs. 346-365)
Browsing effects on seedling and sapling diversity -- Bottomland Hardwood Forest
Under control conditions of no browsing (absence of deer): "Diversity showed a downward trend to a low value of approximately 7 (then exhibited recovery to a value of 10, then a final value of 8)." "The low in diversity resulted from plot domination by large American beech trees that reduce many seedlings and sprouts and exclude seedlings of many less shade-tolerant species. At the death if these canopy dominants, the recovery of seedling diversity results from a mix of tree species seedlings with a broader range of shade tolerances." (pgs. 346-365)
"Although also displaying significant temporal variation, both high and moderate browsing scenarios generally resulted in greater understory diversity than did the control simulation. . . Browsing increased the seedling death rate of American beech regeneration by slowing seedling growth rate. Decreased survival of American beech seedlings is apparently sufficient to allow establishment of other species, primarily the relatively shade-tolerant red maple, oak species and green ash. This high turnover is the mechanism for increased seedling diversity. Low browsing intensity, on the other hand, displays only a moderate effect on seedling diversity and actually may lower diversity by having less of an effect on American beech seedlings relative to other species." (pgs. 346-365).
". . . heavy browsing in mixed species forests can interact with species life histories to increase diversity of the seedling and sapling stratum" -- but does not eliminate the potential for browsing to eliminate preferred species. (pgs. 346-365)
"This rapid departure followed by a decline in displacement is common to all three browsing intensities and results from browsing interaction with tree life histories. The initial displacement is caused by selective browsing [fewer deer] of early successional species (yellow poplar and sweet gum). Even under control conditions these early successional species are eventually replaced with more shade-tolerant American Beech and red maple, and thus the amount of displacement declines over time. (pgs. 346-2365)
Hemlocks
"However, when dealing with long-lived organisms such as trees, temporal predictions of browsing impacts on forest composition are inherently longterm. The difficulties of making such predictions were highlighted by Mlagdenoff and Stearns (1993), who found that alleviation of intense deer browsing may have little direct effect on local and regional eastern hemlock population recovery. Their analysis implicated climate variation, land use, natural disturbance and natural regeneration cycles as key factors in eastern hemlock decline and decreased potential for population recovery. " (pgs. 346-365)
"It is clear that deer browsing cannot be isolated as the single direct impact on longterm forest succession. Likewise, indirect effects of browsing cannot be overlooked as a feedback to forest ecosystem processes land succession. Moose browsing has been demonstrated to influence soil nutrient dynamics significantly by changing boreal forest stand composition and thus the quality and quantity of litter returned to the soil." (pgs. 346-365)
A Vacuum
The bulk of the Smithsonian review, which also included game agency tracts negative toward deer, did not expand upon single-species management techniques that lead, directly and indirectly, to artificially high numbers of deer. In fact, even chapters containing prescriptions for remedial killing purposely left enough surplus animals to allow for continued recreational killing, underscoring that even as deer are being killed as "pests," numbers are reduced by means that continue to stimulate breeding.
On a collision course for forty years, federally-funded game propagation and management --until 1986 fully described in annual Pittman-Robertson reports -- and demographic change have collided.
Other studies show a correlation between overabundance of deer and lower numbers of ground-nesting birds in interior forests. The more candid actually refer to "artificial" enhancement of deer numbers for game purposes as causing high numbers, a nuance that rarely makes it into the headlines. McShea states that only when rarer species are affected, some kind of action should be taken. But action is not synonymous with lethal means that increase doe fertility. Action could and should include addressing game programs that directly and indirectly cause or maintain high fertility; regional, not local, management; buffering edge areas and monitoring development and landscaping and can include, in the unusual event that rare species are effected, fencing or enclosing target areas. Significantly, McShea adds "This is not the same as saying that any reduction in plant density or diversity by herbivores must be stopped. Herbivores are a part of forest ecosystems; their abundance, and the abundance of their food, should be expected to fluctuate over time." In the absence of stimulation caused by hunting, that fluctuation, highs and lows, are natural.
That relationship is not simplistic and may depend on a series of biological and sociological variables including what human activity. Both sprawl and related fragmentation and decades of aggressive management for surplus are the man-made culprits. Concern for ground-nesting birds can't be selective, ignoring deliberate management, including clear-cutting, mowing, prescriptive burning, bulldozing and hunting quotas -- so clearly impacting ground-nesting birds. Nor is the criticism biologically defensible whilst ongoing white-tailed deer management, to the tune of hundreds of millions of dollars and hundreds of thousands of acres, is politically off limits.
Most of what the public hears about deer and ecology is unceasingly negative, failing to inform the public that disturbances including hurricanes, wildfires, ice storms, blight, drought and erosion impact forest health and succession, alone and in conjunction with deer, and that deer browsing, the feeding of a native species, does not occur in a vacuum. If there is high impact but low disturbance, canopy changes only very slowly. Deer browsing creates browse lines, but that impact is cyclical and impermanent -- and natural.
"Depending on the disturbance regime, a large impact of deer on sapling establishment did not always translate into a similar impact on the resulting canopy composition. In the absence of large-scale disturbances the forest canopy is resistant to change despite large amounts of understory herbivory. . . deer affect only the smaller stage classes of trees likely to die due to limiting factors" (Cross, Paul C.; McShea, William, et al. The Combined Effects of Browsing and Disturbance on Forest Succession: A Case Study of White-tailed Deer Impacts in Virginia).
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Endangered and Nongame Species Program, Advisory Committee. November 21, 2001. Meeting Minutes.
Susan Russell
Policy Director
Center for Animal Protection
