A study within the National Estuarine Research Reserve System to determine the destructiveness of crabs to salt marshes found that, while crabs can be a problem in some areas, a far greater threat facing marshes is sea level rise.
Crabs play an important role in maintaining a balanced ecosystem in salt marshes, but too many crabs can upset that balance and wreak havoc on a marsh.
If you’ve walked along the river banks of a salt marsh at low tide and seen holes in the mud that make it appear like Swiss cheese, then you’ve seen the burrows that are a hallmark of the crab community. Those burrows are actually good. Among the benefits, they are a way for oxygen to get into the sediment and can also increase a marsh’s ability to drain water.
The crabs also help keep vegetation and the population of other marsh inhabitants in check, as well as serve as a food source for predators, like the Clapper Rail.
Too many crabs, however, can upset the balance of the ecosystem, and crab populations have been blamed for having a negative impact on salt marshes.
To find out how much of a threat crabs are to marshes, the Elkhorn Slough National Estuarine Research Reserve in California collaborated with scientists from 14 other sites in 13 coastal states, including Delaware’s National Estuarine Research Reserve on a study looking at their impact.
The results of the study showed that, in most of the marshes, sea level rise was a bigger contributor to marsh degradation than the crabs.
Dr. Kerstin Wasson, research coordinator at the Elkhorn Slough Reserve in California, said their reserve was one of only four where the study showed crab burrow density having a significant negative relationship with marsh vegetation. Even so, Wasson said, the effect of elevation was stronger than that of crabs across the entire marsh.
“Our marshes are drowning faster than they’re getting attacked by crabs, which is why we’re investing in major marsh restoration initiatives to raise elevation,” she said.
At the St. Jones Reserve in Dover, Dr. Kari St. Laurent, research coordinator at the DNERR, said the crab community isn’t a problem here either.
“We’re not seeing it because our ecosystem is functioning as it should be,” she said.
Sea level rise, however, is an issue. Of the areas included in the study, 97 percent had unvegetated bare ground, she said.
“Low-lying areas in the marsh could be seeing more prolonged and intense inundation periods,” she said. “We’re more vulnerable to sea level rise.”
The study sampled salt marshes along transect lines – straight lines along which samples are taken at fixed, predetermined intervals – extending from uplands down to the edge of tidal creeks. In contrast, most recent studies showing crabs caused dramatic die-off of marsh vegetation occurred in areas where researchers had identified lots of crab burrows, which tend to be at lower elevations and near tidal creek banks.
Wasson said patterns in nature differ across scales. “What is true at one scale may not be true at another,” she said. “We found that crabs can cause a lot of harm at a local scale, in some parts of the marshes, but they don’t seem to be a main cause of marsh dieback at a national scale.”
Dr. Kenny Raposa of the Narragansett Bay Reserve in Rhode Island, another lead scientist in the study, said the crab population will increase in marshes as waters rise.
“The health of our marshes will be challenged by rising seas and increasing crab burrows at the same time.”
St. Laurent said various marsh restoration techniques, such as adding sediment to build them up so they don’t drown, and allowing for natural marsh migration are ways to ensure the continued health of marshes being degraded by sea level rise.
She said the study was also valuable for the Reserve because no invasive crabs were found in the study in the St. Jones. Red jointed fiddler crabs were documented along with sand fiddler crabs, as well as purple marsh crabs. Purple marsh crabs were responsible for salt marsh losses in New England, but St. Laurent said the native crab most likely is not a problem in the small numbers found here.
The results of the study — Pattern and scale: evaluating generalities in crab distributions and marsh dynamics from small plots to a national scale — were published in the journal Ecology.