Over the decades since sea otters returned to their historic habitat in California’s Elkhorn Slough—a coastal estuary dominated by salt marshes—remarkable positive changes have transformed the landscape. A study by Duke University, published in Nature, sheds light on this phenomenon. Despite expectations of increased erosion due to rising sea levels and stronger tidal currents, the erosion of stream banks and marsh edges in areas with significant otter populations has slowed by up to 90%. As erosion rates decrease, marsh and stream vegetation are rebounding, developing robust roots that better withstand floods and waves. Surprisingly, the essential biophysical elements necessary for fostering a resilient coastal ecosystem are now on a path to recovery, largely thanks to sea otters’ voracious appetite for marsh-dwelling crab species.
Brian Silliman, 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 Wetland and Coasts Center, remarked, “The cost of rebuilding these banks and restoring these marshes would amount to millions for humans.” He continued, “Sea otters stabilize these habitats effortlessly, enjoying a feast of crabs in return.” Silliman, a senior author of the study, emphasized the significance of reintroducing top predators into ecosystems, posing the question of how many other ecosystems worldwide could benefit similarly from such interventions.
Historically, estuaries along the western coast provided crucial foraging and nursery grounds for sea otters, offering abundant crab populations and safe havens for otter pups in protective marshes. However, human activities, particularly fur trading, nearly drove local otter populations to extinction. Consequently, crab populations in places like Elkhorn Slough exploded, leading to the degradation of marsh ecosystems. Brent Hughes, associate professor of biology at Sonoma State University and lead author of the study, explained, “Crabs, by consuming marsh roots and destabilizing the soil, contributed to erosion and marsh collapse over decades until the mid-1980s when sea otters began re-inhabiting the estuary.”
Hughes further elaborated, “After several decades, areas repopulated by sea otters exhibited increased stability, despite challenges such as rising sea levels, intensified water flow, and pollution.” To verify the role of sea otters, researchers conducted extensive surveys and field experiments over nearly a decade, confirming a significant reduction in erosion rates in areas with thriving otter populations. This finding suggests that while the return of sea otters hasn’t fully reversed previous losses, it has sufficiently slowed down degradation, allowing these ecosystems to regain stability amid various external pressures.
Silliman added, “This study challenges the conventional wisdom that coastal geomorphology is solely governed by physical forces and vegetation structure. Our results unequivocally demonstrate the vital role predators play in shaping tidal ecosystems.” The study underscores the potential of reintroducing apex predators as a cost-effective conservation strategy, offering significant theoretical insights into ecosystem dynamics and management practices.