The short version
The purple marsh crab is a native Cape Cod crab that eats salt marsh grass. In recent decades its numbers have exploded in many Cape marshes, and it has stripped the grass from long stretches of creekbank. This study compared aerial photos of 18 Cape Cod marshes, half damaged by the crabs and half not. Tidal creeks widened significantly faster where the crabs were at work. Rising seas threaten every marsh, but crab damage may speed up the losses.
What the researchers did
Stephen Smith, a plant ecologist with the National Park Service at Cape Cod National Seashore, took advantage of the fact that the crabs are common in some Cape marshes and absent from others. That allowed him to separate creek widening caused by the crabs from the background widening that comes with sea-level rise.
- He used GIS tools to trace tidal creeks in 18 marshes on Cape Cod Bay, Nantucket Sound and the Atlantic side. Nine had substantial creekbank losses from crab grazing and nine served as references.
- He traced creeks on georeferenced aerial photos from 2005 to 2021 and measured creek widths at fixed points in each photo year.
- He also measured narrow mosquito ditches less than 2 meters wide, and took a closer look at one Provincetown marsh where crab damage first appeared around 2014.
What they found
- The damage is widespread. High densities of purple marsh crabs have caused major vegetation loss in salt marshes across Cape Cod. Noticeable losses appeared in at least several marshes by the late 1980s and early 1990s.
- Creekbanks take the hit. Creekbanks are the crab’s preferred habitat. The crabs eat the grass above and below ground until it can’t resprout, and riddle the peat with large burrows. That leaves banks prone to erosion and slumping.
- Crab marshes lose ground faster. Between 2010 and 2021, creeks of all sizes, from natural channels to mosquito ditches, expanded significantly faster in crab-damaged marshes than in the reference marshes.
- A shorter life for damaged marshes. All marshes are under threat from sea-level rise. But the author warns that marshes with intense crab grazing
may have a greatly shortened lifespan
(Smith, 2024).
What it means for Waquoit Bay today
Salt marshes ring Waquoit Bay and its rivers and ponds. The paper notes that marshes are critical habitat for many plants and animals, buffer coastlines from erosion and filter groundwater pollutants. Anything that speeds up their loss is a concern for the bay. The study covers marshes across Cape Cod, so local monitoring is the way to know how much crab damage Waquoit Bay’s marshes have.
Scientists have proposed several reasons the crabs have boomed, including fewer predatory fish, warmer air and water, mosquito ditches that mimic creekbank habitat, and softer marsh peat. A Rhode Island study found a different mix of crabs and causes there (see why marsh crabs are multiplying). Our summary is based on the article’s published abstract, highlights and introduction.
Read the paper
Smith, S. (2024). The effects of Sesarma reticulatum (L.) herbivory and sea level rise on creek expansion in Cape Cod salt marshes. Continental Shelf Research, 272, 105146. https://doi.org/10.1016/j.csr.2023.105146
License: Published by Elsevier. The article is not openly licensed, so we link to the publisher’s page rather than hosting a copy. The author reports that the National Park Service provided financial support and paid the publishing charges.
Drafted with AI assistance from the linked source.
