Hydrology, Herbivory, and the Decline of Spartina patens (Aiton) Muhl. in Outer Cape Cod Salt Marshes (Massachusetts, U.S.A.)

Abstract

SMITH, S.M.; MEDEIROS, K.C., and TYRRELL, M.C., 2012. Hydrology, herbivory, and the decline of Spartina patens (Aiton) Muhl. in outer Cape Cod salt marshes (Massachusetts, U.S.A.). Salt marsh dieback in different regions of the United States exhibits considerable variability in symptoms, processes, and theoretical or proven causes. On Cape Cod (Massachusetts), where losses within the low-marsh zone (elevations below mean high tide, dominated by smooth cordgrass [Spartina alterniflora Loisel.]) have been particularly severe, recent studies suggest that intense grazing pressure from increased abundances of a native, herbivorous, purple marsh crab (Sesarma reticulatum) is to blame. Low-marsh dieback is spatially heterogeneous because it is closely related to the distribution of the crabs' preferred substrate (peat vs. sand or mud). However, vegetation losses have also occurred in the high marsh, which is comprised of mainly saltmeadow cordgrass (Spartina patens [Aiton] Muhl). In contrast to the low marsh, high-marsh losses consistently occur along the seaward-most edge of this zone, suggesting a link with hydrology (flooding frequency). In this study, we attempted to determine the relative contribution of environmental factors and crab herbivory to high-marsh dieback. To do this, we (1) characterized tidal regimes in dieback vs. healthy areas, (2) assessed the extent of herbivory on S. patens using crab-exclosure cages, (3) documented the ability of S. patens to recover from simulated grazing (clipping) in different marshes and in different areas of individual marshes, and (4) estimated densities of S. reticulatum in two high-marsh dieback areas. The results indicate that S. patens losses are likely the result of a combination of stressors. Flooding frequency and salinities are higher in dieback areas, which impart a higher level of physiological stress. Plants growing there also seem to have a much-reduced capacity to recover from both simulated and actual grazing by the herbivorous crab, S. reticulatum. Continued losses of high-marsh vegetation could eliminate this community from coastal wetlands on Cape Cod, Massachusetts.