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

Top-down forces, such as grazing and predation, have long been thought to be 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 periwinkle, Littoraria irrorata, grazing on live saltmarsh cordgrass, Spartina alterniflora. To examine the relative importance of periwinkle grazing and nutrient availability in controlling marsh grass growth, we manipulated snail and nitrogen levels in a season-long field experiment in a Virginia salt marsh. Snails differentially affected plant growth at varying nitrogen levels. III unfertilized plots, snail removal increased S. alterniflora aboveground production by 38%, whereas in nitrogen addition plots, snail removal increased cordgrass growth by 78%. Snail addition decreased aboveground production by 51% in unfertilized stands, while in fertilized stands, snail addition led to even greater reductions (66%). By comparison, nitrogen addition increased S. alterniflora production by 443% in snail removal treatments, while in controls and snail addition treatments, fertilization effects were dampened significantly, as nitrogen enrichment increased marsh grass growth by 322% and 189%, respectively. Feeding assays examining the rate at which snails ingested live S. alterniflora revealed that snail consumption alone could not be responsible for these large biomass reductions. Experimental results suggest that grazer-induced defoliation and grazer control of plant demand for nitrogen fertilizer were the likely mechanisms involved. A survey of salt marshes from Maryland to Georgia showed that periwinkle grazing on live S. alterniflora is widespread. Our results show that L. irrorata can exert strong top-down control of S. alterniflora production, and that this effect increases with increasing nitrogen avaliability. Together, these findings question the widely accepted notion that grazers play a relatively unimportant role in the salt marsh community.