More and more places around the world are facing the challenge of flood risk due to sea-level rise, land subsidence and frequent storm surges. Some are approaching this problem by developing dikes or storm barriers. But construction of these hard engineering solutions can also destroy coastal ecosystems and the services they provide. For example, in addition to being important habitat, salt marshes in front of a dike actually reduce both the chance of a dike breaching and the impact of a dike breach. When we destroy them, we also destroy a valuable ally in our efforts to prevent flooding.
Meanwhile as water levels rise, salt marshes are also at increasing risk from erosion. Finding a way to manage salt marshes for optimal coastal defense became the goal of researchers at the University of Groningen (UG) and the Royal Netherlands Institute for Sea Research (NIOZ). They found that grazing by both cattle and small herbivores such as geese and hare and artificial mowing can reduce salt marsh erosion, and contribute to a nature-based coastal defense system.
The started by to looking at how “salt marsh management (i.e. grazing by large vs. small grazers vs. artificial mowing), marsh elevation and marsh age affect soil stability (i.e. soil collapse) and intrinsic lateral erodibility of salt marshes.”
The collected 78 soil samples at different sites on the salt marsh of Schiermonnikoog, a barrier island in the Dutch Wadden Sea. The salt marshes here include long-term exclosures as examples of soil areas where no grazing by cows or smaller herbivores had occurred.
Samples from grazed areas were compared with samples from the exclosures and artificially mowed sites. Soil cores were also transported to the lab and exposed to artificial waves in controlled tanks. Researchers then measured soil erosion for a total of 38 hours.
Researcher Beatriz Marin-Diaz, describes what they found. “The sandiest cores eroded most heavily, whereas soils with more clay were more erosion resistant. Cow grazing enhanced this erosion resistance by compacting the soil by trampling.” The team was also surprised to learn how the smaller herbivores contributed to erosion prevention. Their grazing actually changed the type of vegetation in the salt marsh to plants with high root densities that were good at binding the soil together. In addition, the researchers found that artificial mowing contributed to erosion resistance by excluding burying animals from the soil that destabilize the sediment.
The scientists summarized their findings, along with recommendations for grazing management in salt marshes:
“Overall, marshes with thinner cohesive and/or fine-grained top layers are more sensitive to lateral erosion than marshes with deep cohesive soils, independently of the management. Grazing and artificial mowing can reduce the erodibility of fine-grained soils, making salt marshes more resilient to lateral erosion.
“However, compaction by large grazers simultaneously leads to thinner fine-grained layers and lower elevation, potentially leading to more inundation under sea-level rise. Hence, to effectively manage salt marshes to enhance their contribution to coastal protection, we recommend (a) moderate/rotational livestock grazing, avoiding high intensity grazing in sediment-poor systems sensitive to sea-level rise and (b) investigating measures to preserve small grazers.”
What Can You Do With This?
If you’re living or grazing in an area experiencing increased flooding risks, consider approaching those managing the response with this information. It could be one more service you, as a grazier, can provide to your community.