Gastropod Herbivore Preference for Seedlings of Two Native and Two Exotic Grass Species

Herbivores can affect the composition of grassland communities. We examined seedling herbivory by the introduced snail, Helix aspersa, on four grass species common in California grasslands. To assess preference, standardized tests using equal amounts of seedlings Of the same age were conducted in addition to tests in which live seedlings were exposed to snail herbivory. Helix aspersa exhibited the greatest preference for native Bromus carinatus, while native Nassella pulchra and exotic B. hordeaceus were moderately preferred. Exotic Avena fatua was least preferred. Assessment Of preference using both standardized and live Seedling trials yielded consistent results, suggesting that preference is due to variation in seedling quality rather than differences in per-seedling biomass that are likely to exist under field conditions. Avena fatua is very common in California grasslands while B. carinatus is relatively rare. Our results Suggest that the high level of snail preference for B. carinatus and aversion to A. fatua, coupled with severe effects on future growth due to complete defoliation of live seedlings, may reinforce patterns of establishment of these grasses observed in natural communities.