Terrestrial molluscs can alter plant production and species composition both directly, as a result of herbivory, or indirectly, as a result of detritivory.  The banded wood snail (Cepaea nemoralis) is a non-native species in North America but it has become widespread throughout the northeastern United States and southern Ontario over the last century.  Populations thrive in a variety of habitats including woodlands, roadsides, disturbed, grasslands, and old fields.  While these snails prefer feeding on dead plant material, they also exhibit a preference for certain live plants.  We used snail exclosures to simultaneously investigate the effects of snail herbivory on transplants of a highly palatable forb species (Cirsium arvense) and the effects of snail detritivory on grass litter mass in a temperate old field.  We also performed snail fecal analyses at frequent intervals to determine the ratio of dead to live plant material in their diet over summer. 

Results/Conclusions

Despite visual evidence of snail feeding on plants in unexclosed plots, snail exclosures had no effect on the aboveground biomass of C. arvense over summer.  Likewise, although snails maintained at artificially high densities removed small but significant quantities of grass leaf litter, the exclosure effect on litter mass relative to unexclosed plots was not significant.  Fecal analyses revealed that green plant material comprised approximately 10% of the overall snail diet, with the remainder consisting of plant litter and soil.  Juvenile and adult snails did not differ in their consumption of live material and there was no clear seasonal trend in the consumption of green material.  Overall, our results reveal that presence of C. nemoralis had no direct effect on either live plant biomass or the litter layer, despite this species being highly conspicuous at our field site.