Featured image: An oyster reef restoration project in progress. From Virginia State Parks, 2015.
The negative and the positive
When we see nature documentaries or picture an ecosystem, we usually see negative species interactions, where one species benefits and the other is harmed: predation in which a lion eats a zebra; seedlings competing for light on the forest floor; or even the “zombie ant” fungus that parasitizes ants. However, ecological communities are not solely shaped by these negative interactions such as competition or biological or physical stress. Positive species interactions, in which one organism reduces stress from negative interactions for another organism, can play an important and underrated role in ecosystems and ecosystem restoration. These positive interactions, called ecological facilitation, harm neither species and help at least one. For example, the marsh grass Spartina alterniflora, can stabilize the sediment intertidal zones and buffer the waves coming over it, allowing plants that are less tolerant to large wave action to colonize. Facilitation (or lack of) can alter the outcomes of ecological restoration, and so newer research is focusing on how facilitation works to improve restoration projects. In a review published this year, a research team led by Simon E. Reeves from The Nature Conservancy examined the state of the science on facilitation in oyster reefs to better understand the interactions taking place, with the goal of improving oyster reef restoration.
Oyster reefs are formed by as oysters cluster on hard, submerged substrates and fuse together, forming 2D expanses or high-relief reefs. They occur in salty or brackish waters and provide habitat for countless marine species, including the commercially important blue crabs, shrimp, anchovies, striped bass and more. They also provide several other ecosystems services, including improving water quality through filter feeding, being a source of food themselves, and providing coastal protection from storms. However, a variety of disturbances such as development, nutrient pollution, wetland loss, dredging, and overfishing have reduced oyster populations to historical lows. According to a study published in 2011 by a team led by Michael Beck, over 85% of oyster reefs globally are gone. Efforts are now underway to restore these reefs and all the ecosystem services they provide, and these projects can benefit by understanding and designing for the positive species interactions occurring in these reefs.
Facilitating a reef rebound
The review led by Reeves identified several forms of facilitation, including oysters helping other oysters, as well as oysters helping or being helped by other marine species. Oysters are gregarious settlers. This means they like to settle and grow in areas where other oysters already exist, since aggregations of oysters perform better than lone oysters. To do so, they rely on several settlement cues, including their senses of smell and hearing. Oyster larvae are attracted to the smell of same-species adults, settling onto substrate once they receive this olfactory cue. They also use “oyster reef soundscapes” as auditory cues to settle and join an oyster reef. These settlement cues can be used in restoration projects by encouraging oyster larvae to settle in desired areas by using recordings of healthy oyster reef soundscapes or adding some live, adult oysters to the area to release olfactory settlement cues.
Oysters also make life for other species a little easier. On top of increasing oyster populations, restoring oyster reefs increases habitat area available for colonization by other species. The actual structure of an oyster reef can alter the environmental conditions of the area, providing refuge for other organisms from biotic or abiotic stressors such as predation, strong water flow, or unfavorable water chemistry and temperatures. This inter-specific facilitation depends on the region and context. For example, in warmer coastal zones the oysters provide shade for other organisms, while in cooler estuaries the oysters play a more important role in providing refuge from predators. Oyster reefs can also be involved in long-distance facilitation by helping other ecosystems. For example, the water filtration and wave attenuation ecosystem services provided by oyster reefs can benefit nearby seagrass beds, and yet many reef reconstruction processes explicitly steer clear from seagrass areas in order to avoid impacting them. A more holistic approach of habitat reconstruction that incorporates the effects of various surrounding ecosystems can maximize restoration success and produce benefits for adjacent areas.
Oysters may seem like the busy helpers of the reef ecosystem, while every other species enjoys the benefits; however, fish and other species help oysters survive through trophic facilitation, a form of indirect positive species interaction. Fish such as the oyster toadfish eat and scare away the mud crab that like to feed on juvenile oysters, indirectly enhancing oyster survival. This type of facilitation has been documented for a variety of species, helping oysters survive to adulthood and keeping the reef healthy.
Just as ecology generally tends to emphasize negative species interactions, ecological restoration usually focuses on a single species, such as a single species of oysters used for rebuilding oyster reefs. However, a more holistic approach including multiple native foundation species and nearby ecosystems can enhance restoration outcomes and make ecosystems such as oyster reefs more resilient to future disturbances.
Beck, M. W., Brumbaugh, R. D., Airoldi, L., Carranza, A., Coen, L. D., Crawford, C., et al. (2011). Oyster reefs at risk and recommendations for conservation, restoration, and management. Bioscience 61, 107–116. doi: 10.1525/bio.2011.61.2.5
Reeves SE, Renzi JJ, Fobert EK, Silliman BR, Hancock B and Gillies CL (2020) Facilitating Better Outcomes: How Positive Species Interactions Can Improve Oyster Reef Restoration. Front. Mar. Sci. 7:656. doi: 10.3389/fmars.2020.00656