ANIMALS
Sea Otters’ Hunger For Crabs Slowed Erosion And Restored Vegetation In California Estuary
Nature
Sea otters’ insatiable appetite for crabs has helped slow erosion and restore vegetation in a central California coastal habitat where they were reintroduced in the 1980s, according to a new study.
Sea otters were reintroduced to Elkhorn Slough, a salt marsh-dominated coastal estuary in Monterey Bay, in the 1980s, after nearly disappearing from the area. Fur traders hunted the local population nearly to extinction, and the few that remained were driven out by human development and agriculture.
Now the otter population is flourishing, and the erosion of creekbanks and marsh edges in areas they inhabit has slowed by up to 90%, while marsh and streamside vegetation is rebounding, developing densely matted root systems that can stand up to flooding or surging waves.
The key, according to researchers, is their appetite for the plant-eating crabs that boomed in their absence.
“It would cost millions of dollars for humans to rebuild these creekbanks and restore these marshes,” said Brian Silliman, Rachel Carson Distinguished Professor of Marine Conservation Biology at Duke Universitys Nicholas School of the Environment, and Director of Duke RESTORE and Duke Wetland and Coasts Center. “The sea otters are stabilizing them for free in exchange for an all-you-can-eat crab feast.”
The new study was published Wednesday in the journal [Nature](https://www.nature.com/articles/s41586-023-06959-9).
“Crabs eat salt marsh roots, dig into salt marsh soil, and over time can cause a salt marsh to erode and collapse. This had been happening at Elkhorn Slough for decades until sea otters recolonized the estuary,” said lead author Brent Hughes, associate professor of biology at Sonoma State University and a former postdoctoral scholar in Sillimans lab at Duke.
“After a few decades, in areas the sea otters had recolonized, salt marshes and creekbanks were becoming more stable again, despite rising sea levels, increased water flow from inland sources, and greater pollution,” Hughes said.
Hughes and Silliman conducted the study with colleagues from Sonoma State, the University of California Santa Cruz, the University of California Santa Barbara, Nhydra Ecological Research, Moss Landing Marine Labs, U.S. Geological Survey, Fisheries and Oceans Canada, Simon Fraser University, California Department of Water Resources, Monterey Bay Aquarium, and the University of Florida.
Researchers conducted surveys across 13 tidal creeks and small-scale field experiments at five locations around the estuary for nearly a decade. Sea otters were excluded from some test sites while they recolonized others. By the end of the study, erosion had slowed by 80% to 90% in areas with otter populations, and some marshes were even expanding.
“(Remodeling a coastline) is usually something only large-scale physical forces, like hurricanes or extreme tidal flow changes, can do,” said Silliman. “It begs the question: In how many other ecosystems worldwide could the reintroduction of a former top predator yield similar benefits?”
Although the sea otters haven’t completely reversed the former losses, their ability to restabilize their habitats despite the environmental pressures they continue to face gives researchers and conservationists hope.
“In this instance, restoring the otter population was achievable without significant effort, and as a result, we are now unlocking several decades of benefits from that one act of conservation, said co-author Christine Angelini, Ph.D., director of the Center for Coastal Solutions at the University of Florida. She hopes to apply similar lessons to degraded coastal habitats around the Sunshine State.
“All these challenges can feel unsurmountable,” Angelini said. “This study indicates to us that, if we truly understand the ecosystem and know what levers to pull, we can see significant benefits to the health and stability of these systems.”
TMX contributed to this article.
