Derksen-Hooijberg M, Angelini C, Lamers LPM, et al. Mutualistic interactions amplify saltmarsh restoration success. J Appl Ecol. 2017;00:1–10. https://doi.org/10.1111/1365-2664.12960
If you spent time at this beach this summer, you may have been lucky enough to pass by a salt marsh. Despite this privilege, you may not have paid the marsh much attention because it’s full of grasses and smells like rotten eggs. But this is insulting to the salt marsh: the positive aspects of such a wetland habitat make it invaluable.
Salt marshes provide many benefits, including storm and flood protection, cycling of nutrients, storage of greenhouse gases, and a habitat for a biologically diverse population. Unfortunately, due to pressures such as climate change and overfishing, a large portion of coastal wetlands and reefs have been degraded or destroyed. As the number of healthy wetlands decreases, we face increased pressures of climate change, a loss of biodiversity, and the declination of fish populations. Fortunately, restoration efforts are underway across the globe to mitigate these potential disasters. In a recent article, Derksen-Hooijberg and his coworkers investigate a new approach to saving wetlands.
Smooth cordgrass, or Spartina alterniflora, is a salt marsh grass found throughout the eastern coast of the US. It is often the front line of vegetation prevailing above the ocean. The grass is flooded at every high tide and subject to the hot and dry conditions of low tide. The cordgrass must be able to grow and survive in a vast spectrum of conditions in a matter of a few hours.
The Ribbed Mussel, or Geukensia demissa, finds itself embedded in the upper soils of the salt marsh. It filter-feeds on the nutrient rich water and often attaches itself near the roots of cordgrass.
Previous studies have shown that these two species interact to their shared benefit. This type of interaction is called “mutualism.” The mussels may filter out toxins in the water and create a more resilient substrate for the grass to grow. While the grass creates a more hospitable environment for the mussels by creating shade that keeps them cool and avoid severe desiccation. By pairing these two species together they make a more hospital place to live for one another.
As mentioned previously, degradation of salt marshes is a global problem that many people in many fields are working to mend. Most restoration practices focus on reestablishing a single species that a habitat is deficient in. The replanting of multiple species, especially a combination which is synergistic in nature, has not yet been extensively studied. In this study, scientists investigated how restorations aimed at mutualistic relationships may impact the overall success of the restoration.
The experiment occurred on the National Estuarine Research facility on Sapelo Island, Georgia. Four different treatments were studied with different species being restored: Grass only, Mussel only, Grass + Mussels and a control (no restoration). Within these sites, researchers measured the quantity of vegetation, nutrient levels in the soil and the development of mussels over a 16-month period.
What they found was quite striking. Growth of the grass increased by 45% when mussels were present. This may have occurred because the mussels reduced the concentration of sulfur surrounding the cordgrass. Sulfate, a form of the element sulfur, is the third most abundant ion found in seawater. As the high tide recedes this ion sits and dries on the marsh soil, it is then broken down by the microbial community. This broken-down sulfur is a toxin to the plants, inhibiting the absorption of essential nutrients and disrupting activity in the roots. In the presence of mussels, sulfide concentrations were reduced by 73%. The presence of mussels also increased nitrogen and phosphorus levels in the cordgrass, both of which are essential to any living organism, suggesting a more healthy and productive cordgrass.
To test how this interaction works in severe conditions, the researchers also employed the same strategy to bare soil on the marsh in order to investigate how this interaction could help with complete restoration. An initial disturbance event was created, and the growth of cordgrass shoots was compared amongst groups with (grass + mussels treatment) and without mussels (grass-only treatment). Ten days after the initial disturbance event, only 50% of the shoots of the cordgrass from the grass-only treatment existed, while 100% of the shoots were present in the grass + mussels treatment. After 13 months, this pattern still held true: 25% of shoots were present in grass-only treatments and 75% of shoots were present in grass + mussel treatments. The cordgrass was not the only one to benefit from this interaction. Mussels who were associated with the grass grew 22% larger than those mussels who were isolated.
Based on the results of this experiment, we can reshape restoration strategies. Restorations typically focus on only one type of species or abiotic feature, such as by planting one species or altering tidal height. In order to improve overall health and productivity of marshes, it is necessary to incorporate mutualistic interactions into the plans of restoration. Mutualistic relationships, such as the one within this article, benefit not only the two involved species, but also other flora and fauna. By creating a more suitable habitat for the cordgrass and mussels, we may be able to regrow and generate more marshland. This would ultimately provide habitats for creatures like crabs, fish, and birds. A simple mutualistic push can radically improve redevelopment of a degraded habitat. So next time you’re on your way to the beach, don’t think of the marsh simply as something that precedes the beach, but as a hub of biodiversity and tremendous activity.