Sean J Sharp (@seancologie, Linkedin), from University of Maryland, discusses his article: Large grazers suppress a foundational plant and reduce soil carbon concentration in eastern US saltmarshes
Grazing has been a common practice in temperate salt marshes for millennia. In European saltmarshes, the grazing activity of large animals like sheep and cows can boost plant diversity with no clear influence on ‘blue-carbon’ – the carbon sequestered in the soils of coastal ecosystems like salt marshes, mangrove forests, and seagrass beds. Grazers are also included in calls for trophic rewilding strategies to restore the top-down control once provided by large animals that have been extirpated or whose numbers have greatly declined during the Anthropocene. However, the impact of grazing in salt marshes outside of Europe, including the extensive salt marshes found along the eastern seaboard of the US, is not well studied. A meta-analysis led by co-author Kate Davidson suggested that grazing in American salt marshes could lead to declines in ‘blue-carbon’. To test this, Kate forged a cross-Atlantic collaboration with her colleagues at the University of Florida and the University of Houston to examine the effect of large grazers on US salt marshes imposed by a particularly charismatic species: wild horses. Our study examines for the first time how the grazing of these large-bodied equids affects the structure and function of salt marshes along the eastern US Atlantic coast.
Cumberland Island, Georgia, USA is a picturesque subtropical sea island home to a large population of wild horses – relics of a colonial past. On this island, we began a 4 year-long exclusion experiment where we found that these grazers impose a cascade of negative effects on the health of eastern US saltmarshes. These effects include removing sediment-trapping aboveground plants, reducing marsh-building root biomass, and compromising ecosystem resilience to drought. Lower marsh productivity, lack of carbon inputs from plants, and reduced sediment trapping may lead to less ‘blue-carbon’, a characteristic of other grazed marshes we surveyed across the region as part of our study. By analyzing the cascading interactions triggered by grazing we were able to further link the negative effects grazing imposes on salt marshes to the reduced soil carbon in grazed marshes elsewhere in the region.
Why does this diverge from observations in European marshes? Compared to floristically diverse European salt marshes, marshes of the eastern US, including those on Cumberland Island, are often dominated by a single species – smooth cordgrass (Spartina alternifora). When this grass is grazed, there is no similar species to take its place. Instead, grazed patches are often colonized by low-stature succulent plants with much lower productivity and biomass generation than S. alterniflora. Thus, grazing activity maintains these marshes in a suppressed state, with the grass present only as stubble, just millimeters tall, failing to accumulate greater biomass. This divergence from the positive grazing effects in European marshes highlights the context-dependent nature of top-down control in ecosystems globally. Differences in the ecological community structure across seemingly similar biomes can respond quite differently to identical top-down pressures.
As rewilding efforts accelerate, we advocate a better understanding of the impact of large grazers on regional ecosystem health and function before fully embracing this practice.
