AUTHORS: David G. Hewitt, Timothy E. Fulbright, Charles A. Deyoung, Kory R. Gann, Donald J. Folks, Ryan L. Darr, Kent M. Williamson, Lucas W. Garver, David B. Wester – Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville; Don A. Draeger, Comanche Ranch, Carrizo Springs, TX
ABSTRACT: Density dependent responses in cervids are often mediated through changes in diet quality resulting from deer foraging impacts on vegetation. In regions where vegetation communities have high biomass of poor- and moderate-quality forage and in which production of high-quality forage is dependent on unpredictable precipitation and therefore highly variable, classical density dependent responses may be muted. Furthermore, biologists managing deer in regions of highly variable or poor-quality forage may enhance deer nutrition by providing supplemental feed. The effects of enhanced nutrition on deer foraging behavior could cause increased foraging pressure on preferred forages or could help protect preferred forages. We used tractable female white-tailed deer and the bite count technique to explore the effects of deer density and enhanced nutrition on diet composition, diet quality, and intake rate of deer in semi-arid rangelands of southern Texas, USA.
Our study of deer density was conducted in 4 81-ha enclosures, 2 of which had a target deer density of 12 deer/km2 and 2 of which had a target density of 48 deer/km2. These population densities had been maintained for 5 years before the study began. The proportion of shrubs, forbs, mast, cacti, and subshrubs in deer diets did not differ (P > 0.31) between deer-density treatments. Percent grass in deer diets was higher (P < 0.06) at high deer density but composed only 1.3±0.4% of the diet. Digestible protein and metabolizable energy of diets were similar (P > 0.45) between deer-density treatments. Likewise, bite rate, bite size, and dry matter intake did not vary (P > 0.23) with deer density. Unlike deer density, drought had dramatic (P < 0.05) effects on foraging of tractable deer. During drought, the proportion of shrubs and flowers increased in deer diets while forbs declined. Digestible protein was 31%, 53%, and 54% greater (P = 0.06) during non-drought than drought during autumn, winter, and spring, respectively.
We studied the effects of enhanced nutrition on the composition and quality of deer diets in 4 enclosures each of which had a target deer density of 12 deer/km2. We estimated proportion of pelleted feed in diets of tractable deer and non-tractable deer using ratios of stable isotopes of carbon. Averaged across seasons and nutrition treatments, shrubs composed a majority of the vegetation portion of deer diets (44%), followed by mast (26%), and forbs (15%). Enhanced nutrition influenced the proportion of mast, cacti, and flowers in the diet, but the nature and magnitude of the effect varied by season and year. The trend was for deer in natural nutrition enclosures to eat more mast. We did not detect a statistical difference (P = 0.15) in the proportion of shrubs in diets between natural and enhanced nutrition, but deer with enhanced nutrition consumed 7−24% more shrubs in 5 of 8 seasonal study periods. In the enhanced nutrition treatment, supplemental feed averaged 47−80% of the diet of tractable deer. Of non-tractable deer in all density treatments with enhanced nutrition, 97% (n = 128 deer) ate supplemental feed. For non-tractable deer averaged across density treatments, study sites, and years, percent supplemental feed in deer diets exceeded 70% for all sex and age groups.
We found that a 4-fold difference in white-tailed deer density did not have a large effect on deer diets or foraging behavior, a finding consistent with results from other aspects of this project. Deer density had little influence on vegetation communities and, although deer density had some effects on deer morphology and population dynamics, the effects were subtle. Conversely, variable precipitation had large effects on deer diets, vegetation communities, and deer populations. Our results support the contention that in the western portion of southern Texas, abundant browse and sporadic, precipitation-dependent forbs disrupt the linkage between deer density and vegetation documented in other systems. Furthermore, provision of supplemental food by managers improves the quality of deer diets and does not cause deer to concentrate their foraging on high-quality but rare forages such as forbs.
