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

Abstract

I www.JCRonlinc.org 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.). Joumal of Coastal Research, 28(3) 602-612 West Palm Beach (Florida), ISSN 0749-0208.Salt marsh diehack 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 helow mean high tide, dominated hy smooth cordgrass [Spartina alterniflora Loisel.]) have been particularly severe, recent studies suggest that intense grazing pressure from increased ahundances of a native, herhivorous, purple marsh crab (Sesarma reticulatum) is to blame. Low-marsh dieback is spatially heterogeneous because it is closely related to the distrihution of the crahs' preferred suhstrate (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 diehack. 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 (chpping) in different marshes and in different areas of individual marshes, and (4) estimated densities of S. reticulatum in two high-marsh diehack 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 hy the herbivorous crab, S. reticulatum. Continued losses of high-marsh vegetation could eliminate this community from coastal wetlands on Cape Cod, Massachusetts. ADDITIONAL INDEX WORDS: Salt marsh, Spartina patens, vegetation loss. Cape Cod.Mean dying = 48% (SE = 5%); mean healthy = 18% (SE = 4%).

Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

“…Our study shows that this vegetation shift is ongoing in Narragansett Bay salt marshes and confirms that S. patens salt meadow is declining in multiple marshes across the southern New England region (Smith 2009;Smith et al 2012;Warren and Niering 1993). Our long-term data from Coggeshall Marsh also demonstrate that these vegetation changes in Rhode Island salt marshes have accelerated in recent years coincident with extreme increases in sea levels during 2009-2010 (Goddard et al 2015).…”

“…We did not observe any evidence of crab herbivory (Smith et al 2012) or wrack formation prior to the onset of dieback on the mid-marsh platform. We documented dieback that was likely caused by crab herbivory, but this was along creek edges, while wrack was generally found at higher elevations on the marsh near the I. frutescens shrub zone.…”

“…Our long-term data from Coggeshall Marsh also demonstrate that these vegetation changes in Rhode Island salt marshes have accelerated in recent years coincident with extreme increases in sea levels during 2009-2010 (Goddard et al 2015). However, whereas the S. patens decline in our study was primarily due to replacement by S. alterniflora, there was less evidence of impacts from encroaching S. alterniflora on outer Cape Cod where flooding stress and crab herbivory both contributed to high marsh dieback (Smith et al 2012). This indicates that multiple stressors may be acting synergistically to drive the loss of S. patens salt meadow from southern New England salt marshes.…”

Spatial Ecology of Fiddler Crabs,Uca pugnax, in Southern New England Salt Marsh Landscapes: Potential Habitat Expansion in Relation to Salt Marsh Change

“…However, over the last several decades it has become increasingly evident that marshes in New England, along the US coast, and indeed globally, are exhibiting a suite of physical and ecosystem alterations. These include marsh erosion and loss, changing plant community structure, and marsh plant dieback, and they appear to be shifting these ecosystems into different Pearson 2007, Castillo et al 2000, Donnelly and Bertness 2001, Gedan et al 2011, Hartig et al 2002, Holdredge et al 2009, Miller et al 2001, Smith 2009, Smith et al 2012, Tiner et al 2006, Warren and Niering, 1993. A major question that arises is how are resident and non-resident fauna that inhabit and/or utilize salt marsh ecosystems responding to changes occurring in many of these systems?…”

Mimosa pigra dieback in the Northern Territory, Australia: Investigation into possible causes

“…An understanding of the primary causal mechanisms of dieback events is complicated by the complex interactions of biotic and abiotic factors, and many of the dieback symptoms are not unique to those caused by single agents (Elmer 2014). Thus, there is a need to consider the "multiple stress hypothesis" (Smith et al 2012;Mitchell et al 2013;Bergstrom et al 2015) in evaluating and describing this phenomenon. M. pigra has also been the subject of classical biological research control programs for more than 30 years and still remains a serious problem.…”