California sea otters may be reducing erosion as they recolonize historic habitats

The return of sea otters to their former habitat in a Central California estuary has reduced erosion of the region’s creekbanks and marsh edges by up to 90 percent, a new study has found.

These carnivorous, semiaquatic mammals have been able to temper the rate of erosion due to their hunger for plant-eating marsh crabs, according to the study published Wednesday in Nature.

“It would cost millions of dollars for humans to rebuild these creekbanks and restore these marshes,” corresponding author Brian Silliman, a professor of marine conservation biology at Duke University, said in a statement. 

“The sea otters are stabilizing them for free in exchange for an all-you-can-eat crab feast,” added Silliman, who also directs Duke’s Wetland and Coasts Center and the Duke Restore program for ecosystem recovery.

Marsh crab populations exploded after fur traders hunted the local otter population nearly to extinction and the few remaining animals were driven out by agriculture and other human activities, the authors noted.

“Crabs eat salt marsh roots, dig into salt marsh soil, and over time can cause a salt marsh to erode and collapse,” lead author Brent Hughes, an associate professor of biology at Sonoma State University, said in a statement.

The scientists decided to explore the situation in the estuarine water of Monterey Bay’s Elkhorn Slough, where sea otter recolonization efforts began in the mid-1980s.

“After a few decades, in areas the sea otters had recolonized, salt marshes and creekbanks were becoming more stable again,” said Hughes, who was previously a postdoctoral scholar in Silliman’s lab at Duke.

This shift occurred “despite rising sea levels, increased water flow from inland sources, and greater pollution,” Hughes added.

To ascertain precisely what role sea otters played in the transition, the researchers performed comprehensive surveys across 13 tidal creeks and smaller field experiments at five spots around the estuary for about a decade.

The scientists excluded sea otters from some sites and allowed them to recolonize others.

By collecting measurements and observations on the ground and via aerial photography, the authors saw that erosion slowed by as much as 80 to 90 percent by the study’s conclusion. Some marshes, they noted, were even expanding.

“The return of the sea otters didn’t reverse the losses, but it did slow them to a point that these systems could restabilize despite all the other pressures they are subject to,” Hughes said.

Co-author Christine Angelini, director of the University of Florida’s Center for Coastal Solutions, noted in a separate statement how the otter population’s restoration “was achievable without significant effort.”

“As a result, we are now unlocking several decades of benefits from that one act of conservation,” Angelini added.

Silliman also emphasized the crucial theoretical implications derived from their work, which he said “overturns the well-established bottom-up paradigm” that coastal shifts are controlled by plant life and physical forces.

“Our results unequivocally show that predators also play a keystone role in controlling the course of these tidal creeks,” Silliman added.