Top-down control of Spartina alterniflora production by periwinkle grazingin a Virginia salt marsh

Top-down forces, such as grazing and predation, have long been thought to b e unimportant in controlling plant growth in salt marshes. Instead, bottom- up forces, such as porewater ammonium and oxygen availability, are thought to be the primary regulating factors. In the field, we observed the periwin kle, Littoraria irrorata, grazing on live saltmarsh cordgrass, Spartina alt erniflora. To examine the relative importance of periwinkle grazing and nut rient availability in controlling marsh grass growth, we manipulated snail and nitrogen levels in a season-long field experiment in a Virginia salt ma rsh. Snails differentially affected plant growth at varying nitrogen levels. III unfertilized plots, snail removal increased S. alterniflora aboveground pr oduction by 38%, whereas in nitrogen addition plots, snail removal increase d cordgrass growth by 78%. Snail addition decreased aboveground production by 51% in unfertilized stands, while in fertilized stands, snail addition l ed to even greater reductions (66%). By comparison, nitrogen addition incre ased S. alterniflora production by 443% in snail removal treatments, while in controls and snail addition treatments, fertilization effects were dampe ned significantly, as nitrogen enrichment increased marsh grass growth by 3 22% and 189%, respectively. Feeding assays examining the rate at which snails ingested live S. alternif lora revealed that snail consumption alone could not be responsible for the se large biomass reductions. Experimental results suggest that grazer-induc ed defoliation and grazer control of plant demand for nitrogen fertilizer w ere the likely mechanisms involved. A survey of salt marshes from Maryland to Georgia showed that periwinkle grazing on live S. alterniflora is widesp read. Our results show that L. irrorata can exert strong top-down control of S. a lterniflora production, and that this effect increases with increasing nitr ogen avaliability. Together, these findings question the widely accepted no tion that grazers play a relatively unimportant role in the salt marsh comm unity.