Superabundant crabs threaten the resilience of salt marshes

The resilience of salt marshes can be compromised by an overabundance of crabs. A long-term study by researchers at UC Santa Cruz in California’s Elkhorn Slough revealed that an excessive number of crabs causes tidal creek banks to become more vulnerable to erosion.

The experts report that coastal salt marshes are threatened by erosion that is associated with rising seas, pounding waves, and tidal flows. These vulnerabilities are made worse by superabundant crabs, which are found at their highest densities along the estuary’s tidal creeks.

The striped shore crab can be found all along the West Coast of North America. The study highlights the dual role of these crabs as both consumers of salt marsh vegetation and as ecosystem engineers.

“Their burrowing weakens the creekbank edges, so that whole chunks of marsh will sometimes calve off, and by lowering biomass they are reducing the ability of marsh plants to prevent erosion,” said study lead author Kathryn Beheshti.

The researchers analyzed the effects of crabs on the vegetation and sediments along eroding creekbank edges over the course of five years. They found that reducing crab abundance led to increased growth of salt marsh vegetation and enhanced sediment density.

“Field experiments that span multiple seasons and years are rare,” said study co-author Kerstin Wasson. “This work demonstrates the value of long-term studies by showing that burrows, which weaken the stability of tidal creek banks, persist despite the near absence of the crabs that build them.”

According to study co-author Professor Brent Hughes, the crabs were most abundant in spring and summer, when the pickleweed marshes are at peak production. “This synchrony suggests that the effect of crabs as consumers is more punctuated than their more chronic effect as engineers.”

Elkhorn Slough is one of the largest estuaries in California. It has been substantially altered by human activities, and erosion is steadily eating away at the marsh. “It’s a big issue, because when the marsh erodes away along the tidal creeks it’s a permanent loss,” explained Beheshti.

Study co-author Professor Christine Angelini of the University of Florida said that salt marshes in the southeastern United States appear to be a harbinger of what’s to come for marshes along the Pacific coast, with sea-level rise amplifying the effects of what would otherwise be considered an innocuous crab.

The researchers are calling for similar long-term studies to be conducted in other West Coast marsh systems to investigate whether the impacts of crabs are widespread. “It’d be great for contextualizing our findings,” said Beheshti.

“We’d like to know if Elkhorn Slough is the canary in the coal mine, signaling yet another pathway for accelerated marsh edge loss for one of California’s rarest coastal habitats.”

The study is published in the journal Ecosphere.

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By Chrissy Sexton, Earth.com Staff Writer