Snail herbivory affects seedling establishment in a temperate forest in the Ozarks

Species-specific herbivores are hypothesized to maintain plant diversity by preventing the dominance of any one plant species. However, a large proportion of herbivores have wide host ranges, and these generalists could have similar effects on plant community composition if they exhibit differences in their host preference. Here, we coupled laboratory and field experiments to test whether a common forest-understorey snail (Neohelix alleni), a generalist herbivore, has the potential to influence forest composition through differential preference of their plant hosts. We first performed a cafeteria-style experiment to test whether N. alleni showed feeding preferences among leaves of five tree species and one shrub common to temperate forests in Missouri, USA. We then conducted a factorial snail and deer exclusion experiment to decouple the effects of snail herbivory from those of white-tailed deer on seedling establishment of 1-month-old newly germinated seedlings of these six woody species in the field. Finally, we examined whether variation in both snail feeding preference and experimentally measured effects of snails on seedling establishment across plant species were related to their relative abundance measured in a 12-ha forest plot. In the laboratory, we found that snails preferred leaves of woody species that were less abundant in the forests relative to those species that were more common. In the forest, we found that experimental exclusion of snails had a stronger positive effect on seedling above-ground biomass and survival over a 1-year period than did exclusion of deer. Plant species found to be more preferred in the laboratory were also those that had lower seedling establishment in the forest due to the negative effects of snails. Synthesis. Collectively, our results suggest that greater susceptibility to snail herbivory limits seedling establishment, perhaps contributing to differences in tree species relative abundance. Although less appreciated than their insect and mammal counterparts, herbivory by snails may be significant drivers to the assembly of forest tree communities.