The landscape of the Elkhorn Trough, a saltmarsh-dominated coastal estuary in central California, has changed dramatically in the decades since sea otters began recolonizing the area. In areas with large populations of otters, erosion of stream banks and marsh edges was slowed by up to 90%, at a time when rising sea levels and stronger tidal currents should have had the opposite effect.

Sea otters are an apex predator and their return to California estuaries could help slow erosion and restore the estuary's degraded geology. A sea otter in the mouth of the Elkhorn River estuary in Monterey Bay, California, USA. Image source: KilliiiYuyan

As erosion slows, vegetation along marshes and streams is rebounding, setting down dense root systems that are increasingly able to withstand flooding or rough surf.

The return of sea otters to California's Elkhorn Everglades has significantly reduced erosion, helping to restore this important coastal ecosystem. A sea otter eats crabs in the estuary of the Elkhorn River Channel in Monterey Bay, California, USA. Image source: KilliiiYuyan

Sea otters' role in ecosystem restoration

Published today (January 31) in "A study in the journal Nature shows that sea otters' preference for plant-based marsh crabs is largely responsible for the recovery of biophysical characteristics necessary for estuaries to become a resilient coastal ecosystem.

"It costs millions of dollars for humans to rebuild these creek banks and restore these marshes," said Brian Silliman, the Rachel Carson Distinguished Professor of Marine Conservation Biology at Duke University's Nicholas School of the Environment and director of Duke RESTORE and the Duke Wetlands and Coastal Center. "Sea otters provide them with a stable environment for free in exchange for an all-you-can-eat crab meal."

Despite the challenges of rising sea levels and increasing pollution, the recolonization of sea otters in a central California estuary has unexpectedly slowed erosion and enhanced marsh vegetation. A sea otter in the Elkhorn Slough Estuary of Monterey Bay, California, USA. Image source: KilliiiYuyan

"(Reshaping coastlines) is usually only possible with large-scale physical forces, such as hurricanes or extreme tidal current changes," said Silliman, the paper's senior author.

"Our study draws on field experiments, modeling and before-and-after measurements to highlight the profound benefits that can accrue to ecosystems when apex predators are reintroduced," Silliman said. "This begs the question: In how many other ecosystems around the world would the reintroduction of a former apex predator produce similar benefits?

Historical background and experimental results

The estuaries on the West Coast were once important feeding and nursery habitats for sea otters, who found plentiful crabs in the estuary’s protective marshes and a safe haven for their newborn sea otter pups. To stay warm in the cold Pacific waters, adult sea otters need to eat 25% of their body weight every day, or about 20 to 25 pounds, and crabs are one of their favorite foods.

In central California's Elkhorn Swamp, sea otters have played an important role in stabilizing stream banks and marshes, reversing decades of ecological degradation.

"The crabs eat the roots of the salt marsh and chew the soil of the salt marsh, which over time causes the salt marsh to erode and collapse," said Brent Hughes, lead author of the new study and an associate professor of biology at Sonoma State University. "This situation continued in the Elkhorn Marsh for decades until sea otters reoccupied the estuary in the mid-1980s."

"Decades later, in areas where sea otters had recolonized, salt marshes and stream banks became more stable again despite rising sea levels, increased water flow from inland, and increased pollution," Hughes said.

The re-emergence of sea otters in Elkhorn Slough has caused erosion rates to plummet and the marsh to rebound, providing a cost-effective, natural solution to coastal erosion problems. A sea otter eats crabs in the estuary of the Elkhorn River in Monterey Bay, California, USA. Image source: KilliiiYuyan

To test what role sea otters played in all this, the researchers conducted large-scale surveys of 13 tidal creeks and conducted small-scale field experiments at five sites around the estuaries for nearly a decade. Sea otters were excluded from some trial sites but allowed to recolonize others. Measurements and observations collected through ground and aerial photography confirmed that at sites with large populations of sea otters, erosion slowed by 80 to 90 percent by the end of the study, and some marshlands were even expanding. Model simulations also yielded similar results.

"The return of the sea otters didn't reverse the losses, but it did slow them down so that these systems could re-stabilize despite all the other stresses they were experiencing," Hughes said. "That suggests this could be a very effective and cost-effective new tool in our conservation toolkit."

Silliman added: "This study also has important theoretical implications. This work overturns the established bottom-up model that coastal landscapes are governed by interactions between physical forces and plant structure. Our results clearly show that predators also play a key role in controlling these tidal streams."

Compiled source: ScitechDaily