Effect of Terrestrial Mollusc Herbivory on Holocarpha macradenia (Asteraceae) Seedlings in California Coastal Prairie under Different Clipping Regimes

Abstract

Herbivory is generally overlooked as a factor influencing the viability of endangered plant populations in western North American grasslands. I conducted experiments to determine the effects of terrestrial mollusc herbivory for survival of a federally endangered, annual forb, Holocarpha macradenia (Santa Cruz tarplant), in California's coastal prairie. In addition, I utilized litter removal and clipping regimes from a continuing study of site-specific responses to evaluate the relationship between herbivory rates from terrestrial molluscs and disturbance regimes. I planted seedlings of H. macradenia into copper bordered quadrats that excluded molluscs and into non-copper bordered controls at two sites in California's coastal prairie. Three treatment regimes of vegetation clipping and litter removal were utilized. Herbivory damage was assessed using an ordinal scale every two weeks throughout the rainy season. Molluscs were also trapped and identified. Herbivory amounts were significantly less when molluscs were excluded from plots for all disturbance regimes; however, there was no significant difference in herbivory levels between the disturbance regimes. A significant amount of invertebrate herbivory on H. macradenia was by the non-native slug, Deroceras reticulatum (grey garden slug), and this herbivory significantly affected the survivability of H. macradenia seedlings in California coastal prairie.