AUTHORS: Timothy E. Fulbright, Charles A. Deyoung, David G. Hewitt, Whitney J. Gann, Eric D. Grahmann, David B. Wester, Brandi L. Felts, Lindsey M. Phillips – Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville; Don A. Draeger, Comanche Ranch, Carrizo Springs, TX
ABSTRACT: Density-dependent behavior underpins white-tailed deer (Odocoileus virginianus) theory and management application in North America. Researchers have not focused on linking vegetation dynamics, nutrition, and deer dynamics, which has left a gap in our knowledge of the mechanisms underlying density dependence. We conducted a series of designed experiments during 2004-2012 to determine how strongly white-tailed deer density, vegetation composition, and deer nutrition (natural and supplemented) are linked in a semiarid environment where the coefficient of variation (CV) of annual precipitation exceeds 30%. Our study was replicated on 2 sites with thornshrub vegetation in Dimmit County, Texas, USA. During late 2003, 6 81-ha enclosures surrounded by 2.4-m tall woven wire fence were constructed on each study site. The experimental design included 2 nutrition treatments and 3 deer densities in a factorial array, with study sites as blocks. Abundance targets for low, medium, and high deer densities in enclosures were 10 deer (equivalent to 13 deer/km2), 25 deer (31 deer/km2), and 40 deer (50 deer/km2). Each study site had 2 enclosures with each deer density. Deer in 1 enclosure at each density were provided with a high-quality pelleted supplement ad libitum, which we termed enhanced nutrition; and the other enclosure at each density provided natural nutrition from the vegetation. We determined if increasing deer density and enhanced nutrition resulted in a decline in preferred forbs and shrubs and an increase in plants less preferred by deer as predicted by traditional theories on succession and retrogression. We sampled all 12 enclosures via 20, 50-m permanent transects in each enclosure. Percent canopy cover of preferred forbs was similar (P = 0.13) among deer densities averaged across nutrition treatments and sampling dates (8 ± 2 [mean ± SE], 5 ± 1, and 4 ± 1 in low, medium, and high deer density enclosures, respectively). Averaged across deer densities, preferred forb canopy cover was similar between nutrition treatments in 2004; but by 2012 averaged 20 ± 3% in enhanced nutrition enclosures compared to 8–13% in natural nutrition enclosures. Percent canopy cover of other forbs, preferred shrubs, other shrubs, and grasses, as well as Shannon’s index, evenness, and species richness were similar (P > 0.01) among deer densities, averaged across nutrition treatments and sampling dates. The density dependent response by the deer population was not directly reflected by vegetation responses. Vegetation in our study area was more strongly linked to variation in precipitation than to deer density. In addition, the ephemeral nature of annual forbs, asynchrony in phenology of the vegetation, and plant attributes that act as herbivore defenses may have constrained vegetation responses. Foraging by deer with enhanced nutrition did not result in a reduction in preferred plants in the vegetation community and enhanced nutrition may have had a protective effect on preferred forbs because ≤50% of deer diets consisted of vegetation. Comparison of vegetation responses to different combinations of deer density and enhanced nutrition revealed an indirect response to deer density. Preferred forbs increased over time in enclosures with enhanced nutrition compared to enclosures with natural nutrition. This can be interpreted as a density effect because deer consumed less vegetation with enhanced nutrition. Preferred forbs increased more in enhanced nutrition enclosures with low densities than in enhanced nutrition enclosures with high deer densities, provided further evidence of an effect of deer density. Linkages between vegetation and herbivores may be partial in highly stochastic environments, making detection of density-dependent effects more difficult.
