Provision of supplemental feed to large herbivores is a common management practice that may motivate selective foraging, thereby influencing plant community composition. Our objective was to assess the effect of a high‐quality supplement on diet composition and nutritional quality for white‐tailed deer (Odocoileus virginianus). We permanently released hand‐reared deer into 4 81‐ha enclosures; in 2 enclosures we provided a pelleted supplement. We conducted bite‐count studies seasonally to assess diet composition and quality. Supplemented deer reduced mast (fruits and pods of woody plants and cacti) in their diets (P < 0.019) during spring and autumn compared to unsupplemented deer. Diets of deer in supplemented enclosures had 2 times greater proportion of browse during spring (P = 0.065) and 5 times greater proportion of forbs during autumn (P = 0.007). Quality of the forage portion of the diet did not vary by treatment during winter or summer. Metabolizable energy concentration was 13% greater (P = 0.054) in spring and digestible protein content was 3 times greater (P = 0.006) during autumn in diets of supplemented compared to unsupplemented deer. Our results support the selective foraging hypothesis during autumn but not during winter, spring, or summer. Furthermore, white‐tailed deer did not reduce the proportion of their diet composed of browse, but did reduce consumption of mast. Supplemented deer continued to eat poor‐quality, chemically defended forage, perhaps to alleviate ruminal acidosis induced by the supplement or because nutrients in the supplement increased the deer's ability to detoxify chemically defended browses. A decline in mast consumption by supplemented deer could influence plant communities, depending on the role of deer in seed dispersal and seed predation. Impacts of supplemental feed on selective foraging of white‐tailed deer in shrub‐dominated rangelands are more complex than suggested by previous research. Long‐term studies of vegetation communities are needed before wildlife managers will be able to fully incorporate effects of supplemental feed into management decisions.
Population estimates derived from aerial surveys of ungulates are biased by imperfect detection, where probability of sighting groups is influenced by variables specific to terrain features and vegetation communities. Therefore, methods for bias-correction must be validated for the region to which they will be applied. Our objectives were to quantify factors affecting detection probability of mule deer (Odocoileus hemionus) during helicopter surveys in Texas, USA, rangelands, and develop a detection probability model to reduce bias in deer population estimates. We placed global positioning system (GPS) collars on 215 deer on 6 sites representative of mule deer range in the southern Great Plains and the Chihuahuan Desert during 2008-2010. We collected data during aerial surveys in January-March and fit logistic regression models to predict detection probability of mule deer based on ecological and behavioral covariates. We evaluated the model using independent estimates of population size derived from a markresight procedure. Detection of mule deer was negatively related to distance from the transect, increasing brush cover, sunlight, and increasing terrain ruggedness (P < 0.01). Probability of detection in brush cover was greater if deer were active (P ¼ 0.02). Population estimates corrected for visibility bias using our detection probability model or mark-resight models averaged 2.1 AE 0.49 (SD; n ¼ 50) and 2.2 AE 0.62 times larger, respectively, than uncorrected counts. Estimates of population size derived from the detection probability model averaged 101 AE 26% of mark-resight estimates. However, the detection probability model did not improve precision of population estimates, probably because of unmodeled variation in availability of deer during surveys. Our detection probability model is a simple and effective means to reduce bias in estimates of mule deer population size in southwestern rangelands. Availability bias may be a persistent issue for surveys of mule deer in the Southwest, and appears to be a primary influence of variance of population estimates. Ó 2016 The Wildlife Society.
Abstract. A premise in ungulate foraging theory is that animals become less selective and expand the breadth of their dietary niche as the availability of palatable forage declines with increasing herbivore population density or drought. Increased niche variation resulting from intraspecific competition is thought to create less similar diet composition and decreased diet overlap between individuals within a population at higher densities than between individuals within less dense populations. These ideas were largely developed in relatively mesic environments and their applicability to ungulate foraging in semiarid environments is unclear. We tested the idea that white-tailed deer (Odocoileus virginianus) contract dietary niche breadth; reduce dietary plant species diversity, richness, and evenness; and become more individualistic in forage choices in response to a fourfold difference in population density (12 deer/km 2 versus 50 deer/km 2 ) in semiarid shrubland in Texas, USA. We used the bite count method to determine diet composition of tame female white-tailed deer seasonally during summer 2009 to spring 2011. We were able to determine impacts of drought on foraging dynamics a posteriori because sampling during each season fortuitously occurred under both drought and non-drought conditions. Population density did not affect diet richness, diversity, breadth, evenness, overlap, and similarity. Diet richness, diversity, breadth, and evenness tended to be greater in non-drought conditions. For white-tailed deer, the idea that dietary niches expand in response to increasing population density is not robust across environments. In semiarid environments, variation in precipitation has a much stronger influence on dietary niche breath and intraspecific diet overlap of deer than population density does.
Concentrated food sources are used frequently in white-tailed deer (Odocoileus virginianus) management and research, but because such food sources are easily defended, aggressive interactions among deer may influence their effectiveness. The objectives of this study were to determine if deer population density or season affect 1) the order or degree of social dominance among different age and sex groups of white-tailed deer, 2) the rate at which aggressive interactions occur, 3) the severity of interactions, and 4) the extent to which subordinate groups avoid dominant groups. We conducted our study in South Texas using 2 sets of 3, 81-ha enclosures managed at varying deer population densities. We captured aggressive interactions using digital trail-cameras placed at sites with spatially concentrated food. We found that bucks !2 years of age were dominant over all other age and sex groups in !87% of their interactions regardless of deer density or season. The odds of a buck dominating over a doe increased by 10% (95% CI ¼ 0-21%) for each additional deer/km 2 during summer, but density had little effect in any other season. Yearling bucks were dominant in 81% (95% CI ¼ 51-100%) of their interactions with does during spring, whereas during other seasons we found no clear order to the dominance hierarchy. Social dominance between yearling bucks and does was not affected by population density. The rate of aggressive interactions increased by 2% (95% CI ¼ 1-3%) for each additional deer/km 2 and did not differ by season. Ten percent (95% CI ¼ 6-14%) of interactions involved more violent behaviors that we characterized as severe; this percentage did not change with population density or season. At all population densities, during all seasons, does avoided bucks at sites with concentrated food; however, the degree of avoidance declined with increasing deer density in all seasons except spring. Our results indicate that as population density increases, so do social pressures that may limit access of subordinate age and sex groups to concentrated food sites. Therefore, concentrated food sites are not equally accessible to all age and sex groups of deer and the effectiveness of such sites in deer management and research may become increasingly limited as population density increases. Ó 2013 The Wildlife Society.
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