There is increasing evidence that predators can exert a certain level of control on lower trophic levels through both consumptive and non-consumptive effects on prey. Alternatively, absence of predators may affect plant communities due to increases in herbivory and changes in herbivore behavior. Plant morphology, physiology, and/or life history strategies may also be altered as a result of this selective pressure. Our objective was to determine if predator exclusion induced increased physical defenses in two heavily-browsed herbaceous plant species. We conducted our study in a mature longleaf pine woodland in southwestern Georgia, USA. We measured stem thorn density in two plant species, sand blackberry (Rubus cuneifolius Pursh) and saw greenbrier (Smilax bona-nox L.), in four long-term, 40-ha experimental mesocarnivore exclosures. Coyotes (Canis latrans), the major predator of white-tailed deer in the region, have been excluded from these sites for ~15 years. As a result, white-tailed deer (Odocoileus virginianus) foraging effort, usage, and recruitment are significantly higher than in nearby control sites. We compared thorn counts from plants collected in mesocarnivore exclosures to control sites with background predator and herbivore populations, and to smaller (1600 m2) herbivore exclosures within each treatment and control site.
Results/Conclusions
We found no significant effect of predator treatment on thorn density for either plant species. There were significant differences in thorn density for both species between herbivore exclosures and control. For R. cuneifolius and S. bona-nox, thorn density was 13% and 30% lower respectively in herbivore exclosures. This suggests that a complete lack of deer herbivory can alter plant investment in physical defenses for these heavily-browsed species. On-going analyses is testing for differences in chemical defense constituents in stems and leaves from the same samples. These results build on previous studies that demonstrate declines in abundance of preferred browse species and increased oak recruitment with predator exclusion in these sites. Future work will focus on induced changes in other plant defense strategies including other morphological traits, such as hairiness, and differences in C allocation strategies.