Sea otters eat more than just urchins. They also eat crabs which, in California’s Elkhorn Slough estuary, were shown to contribute to the slowing of bank erosion by 69%.
Researchers at the Nicholas School of the Environment published this discovery in Nature on Jan. 31, presenting “revolutionary” evidence of predator control over marsh landscapes. The study offers a novel interpretation of salt marsh decline and opens the door for new approaches to shoreline remediation.
According to Brian Silliman, Rachel Carson distinguished professor of marine conservation biology and director of the Duke Wetland and Coasts Center, the study was envisioned after observing a strong species interaction between sea otters and burrowing crabs.
“The otters that are in these creeks eat 25% of their body weight a day to keep warm,” Silliman said. “And 80% of what they’re eating in the marshes are these crabs.”
The team then hypothesized that the density of crabs in the marsh would increase without sea otters, beginning their experiment by placing a cage over a plot of the marsh that would keep out sea otters.
“The number of crabs doubled over a two-year period when we excluded otters,” Silliman said. “The doubling of crabs cascaded down to suppress production of the plants. It changed the structure of the sediment. It became weaker.”
The caged sections saw a 48% decrease in above-ground plant biomass and a 15% decrease in below-ground plant biomass. The sediment bulk density decreased by 8%.
According to the Nature article, crabs contribute to erosion by excavating sediment that plants depend on for nutrient growth and stability. Crabs also eat plant roots, which help maintain the integrity of creekbanks.
After these findings, the team scaled up their hypothesis to see if sea otters changed the soil structure of the entire estuary. Fortunately, long-term data was already available from groups who tracked sea otter populations and oceanographers who monitored erosion.
The 45 years of data revealed that during periods of otter expansion, erosion rates went down. The study claims that “around 100 sea otters were associated with a reduced rate of widening from 0.38 m per year to 0.10 m per year, with a decreasing effect on creekbank stabilization per sea otter introduced.”
“We didn’t believe our results. It was really amazing,” Silliman said.
The researchers decided to conduct another test, going to 13 tidal creeks in the Elkhorn Slough estuary to perform a before-after comparison. The study determined that erosion rates were 69% lower in creeks with high sea otter densities than low densities, confirming the team's predictions.
Together, these findings demonstrate what is known as a trophic cascade — greater sea otters lead to fewer crabs, fewer crabs enable more robust plant growth and these plants help slow down erosion.
“The idea that you could have the entire landscape controlled by food webs is very contrarian and controversial,” Silliman said.
The Nature article details that while most studies on marsh edge erosion focus on physio-chemical explanations like climate change, Silliman and his team’s findings point to control by “top-down forces,” like the recolonization of a predator in an ecosystem.
“Predators are a powerful conservation intervention that not only helps themselves, they can help the ecosystem,” Silliman said. “The benefit they provide to these marshes is worth tens of millions of dollars.”
“To slow the erosion with what we have — bulldozers, adding sediment [and] changing the watershed — would cost that much or more, and the otters are doing it for essentially free,” he said.
These findings build on decades of research, most recently following a November 2023 Science article also co-authored by Silliman.
The Science study “did a big synthesis of plant restoration studies around the world,” Silliman said. “What we found is that the most powerful thing you can do to speed up restoration of restored sites is to have predators’ presence.”
Duke scientists were the main drivers of the Nature study, according to Silliman. In fact, the study’s lead author, Brent Hughes, was a postdoctoral scholar in the Silliman Lab until beginning his biology professorship at Sonoma State University.
Silliman cited the Nicholas School Board of Trustees and funding from the Foundation of the Carolinas as significant sources of support for the publication.
Looking to the future, Silliman and his colleagues are investigating how their new understanding of trophic cascade control “can be used to stabilize a system that’s under stress.”
Get The Chronicle straight to your inbox
Signup for our weekly newsletter. Cancel at any time.
Michael Austin is a Trinity sophomore and an associate news editor for the news department.