Physical factors mediate effects of grazing by a non-indigenous snail species on saltmarsh cordgrass (Spartina alterniflora) in New England marshes

In the southeastern US, grazing by a common indigenous littorinid snail has caused large declines in the biomass of saltmarsh cord- grass (Spartina alterniflora). In northeastern marshes, a closely related but non-indigenous snail may also negatively affect production of this key marsh-building plant. We manipulated densities of the gastropod Littorina littorea at two sites to investigate the effect of its grazing on plant production and sediment accumulation. The effects of the manipulation differed between sites. The site with longer inundation periods, lower elevation, and poorer drainage attributable to smaller sediment grain size had more stressful conditions for S. alterniflora. At that site, protection from snail grazing resulted in higher end-of-season plant biomass than all the other treatments and controls. Rates of sediment accumulation were also lower at that site, and the difference between sites increased as the season progressed. At the site where physical conditions were benign, snail manipulation had no effect on S. alterniflora biomass. The nature of the physical conditions at a site may influence the susceptibility of S. alterniflora to grazing pressure by this ubiquitous snail species. Accelerating anthropogenic impacts, such as sea-level rise, could further stress saltmarsh plants, leaving them increasingly susceptible to herbivory.