Herbivores have been suggested to facilitate shrub seedling recruitment by preferentially consuming herbaceous vegetation in California sage scrub (CSS) plant communities; however, herbivory has received limited study in this community. Furthermore, understanding the differences in herbivore pressure over time and in space, among microsites, is needed to understand processes of community assembly. At two CSS sites, we investigated the effect of vertebrate herbivory over time on the survival of caged and uncaged Artemisia californica seedlings. At one of the sites, we investigated the effects of herbivory on shrub seedling growth and survival among microsites by planting A. californica and Salvia mellifera seedlings in three microsite treatments (under live shrubs, under dead shrubs, and in exposed areas), with a nested cage treatment (no cage and cage) in each microsite treatment. We used cameras in both studies to record the most abundant herbivore species.
Results/Conclusions
We found that mortality for uncaged A. californica seedlings was twice that of caged seedlings, and occurred in the summer and early fall, when previous studies of herbivory in CSS were not conducted. Planted shrub seedlings grew largest in exposed areas and had slower growth under shrubs. Cages significantly increased A. californica growth by nearly 60%. In exposed areas, A. californica seedlings were 1.7 times the size of uncaged seedlings. The three main herbivores observed were Sylvilagus audubonii (desert cottontail), Zonotrichia leucophrys (white-crowned sparrow), and Otospermophilus beecheyi (California gray squirrel). Seedling growth and herbivore abundance both peaked at the same time of year, during wetter, favorable growing conditions. We found large and significant effects of herbivory during shrub recruitment, demonstrating that vertebrate herbivores may be a major factor in the assembly of CSS plant communities.