Effects of nutritional enhancement and white-tailed deer density on vegetation dynamics in semi-arid thornscru

Increasing herbivore densities and providing nutritional enhancement, via supplemental feed, may alter structure of shrub communities. Our objectives were (1) to determine if published literature supports the hypothesis that enhancing the nutritional plane of deer (Cervidae) leads to overuse of palatable plants, (2) determine if increasing white-tailed deer (Odocoileus virginianus) densities and providing nutritional enhancement reduces growth of 2 palatable shrub species and if nutritional quality of browse increased with deer density, and to (3) determine if increasing deer densities in combination with relief from nutritional constraints results in overuse of palatable plant species, reduction in species richness, and an increase in less preferred plant species. Recent studies have not provided conclusive evidence that enhancing the nutritional plane, via supplemental feeding, enables herbivores to concentrate feeding on the most palatable forage species in their environment or that it results in degradation of vegetation. However providing supplemental feed does promote localized vegetation degradation by congregating animals near feeders. We constructed 6, 81-ha enclosures on each of 2 ranches in South Texas containing a goal of 13, 31, or 50 deer/km2. Pelleted feed was provided ad libitum to one of each pair of enclosures with similar densities on each ranch. We measured Texas kidneywood (Eysenhardtia texana) spiny hackberry (Celtis ehrenbergiana) canopy and height and also thorn length of between spiny hackberry plants that were protected from browsing and exposed June 2005 and 2007-2012. We also measured density, percent cover, and standing crop of forbs, half shrubs, and woody plants split into palatability groups during 2004--2012. Width and height of protected kidneywoods, at medium deer density, except for 2007, averaged 36% taller (P ≤ 0.01) and 26% wider (P ≤ 0.01) than browsed plants. Height and width of spiny hackberry did not differ between treatments (P > 0.23). A four--fold difference in white-tailed deer density and addition of supplement resulted in little change in the plant community. Concerns that providing nutritional enhancement results in excessive use of these palatable species at the deer densities in our study appear unfounded. The environmental stochasticity characteristic of the semiarid region dominated by Tamaulipan thornscrub has a much greater effect on vegetation dynamics than herbivory by white-tailed deer.