Sea otter: Comeback protects against erosion

Sea otters eat burrowing crabs in the marshes along the California coast, protecting them from erosion. © Killiii Yuyan

Once almost extinct, sea otters have been able to spread again in recent decades - and with an astonishing side effect, a study shows: the return of the small predators to their former habitats on the California coast protects the salt marshes there from erosion. Because in these ecosystems crossed by river arms, the sea otters now keep the burrowing crabs in check. Similarly, in other cases, the comeback of predators could benefit the stability of their native ecosystems, the scientists say.

They were once widespread along the coasts of the North Pacific - but then almost all sea otters were made into fur coats. Hunting bans in the 20th century were able to save the animals from extinction just in time. This tragic story then developed into a prime example of successful animal protection: from small remaining populations, the sea otters managed to slowly regain some parts of their former range. They are now hunting marine animals again in the coastal areas of California. Until now, they were primarily known for their forays into the local kelp forests. Studies have already highlighted the beneficial effect of the sea otters' comeback on this ecosystem: by keeping the kelp-eating sea urchins in check, they contribute significantly to the preservation of the threatened kelp forests.

Return to another ecosystem

But as researchers led by Brent Hughes from Duke University in Beaufort report, the sea otters were also able to reclaim another former habitat in California - and also stabilize it. These are the estuaries of rivers that are characterized by a landscape of aquifers and salt marshes. The most important of these Californian tidal swamps is Elkhorn Slough in Monterey Bay. However, as the scientists report, it is a threatened ecosystem: rising sea levels and increased river water runoff are eating away at the edges of the salt marshes.

A sea otter swims through an arm of the Elkhorn Slough in California. © Brent Hughes

As the research team reports, the study was based on evidence that the areas of Elkhorn Slough where sea otters have resettled since the 1980s were comparatively less affected by erosion. The team followed up on this lead through a systematic, long-term investigation in the area. The researchers precisely recorded the occurrence of otters and erosion processes on the bank areas and examined possible influencing factors. In some places they also used barriers to keep the animals away from certain areas in order to generate comparison opportunities.

How otters influence coastal development

As the team reports, there was actually a clear connection between the otter populations and the erosion processes: the on-site investigations and aerial photographs showed that the areas populated by the animals were 80 to 90 percent less affected by decay than the control sections . According to the team, this effect is due to sea otters' preference for a particular prey item in the marsh: they eat large quantities of crabs, which are known to promote erosion. Because they burrow in the bank area and eat the roots of the salt marsh plants, which stabilize the material. Where the otters thin out the crab populations, the marsh vegetation can recover and form dense root networks that protect the bank from erosion, the scientists explain.

“The return of sea otters has not reversed the losses, but it has slowed them enough to allow this ecosystem to stabilize despite the stresses it is under,” says Hughes. Senior author Brian Silliman from Duke University adds: “Rebuilding these riparian areas and measures to stabilize the swamps would cost people a lot of money. “The sea otters, on the other hand, stabilize them for free in exchange for a crab feast,” says Silliman.

In addition to the specific results, the researchers also see a fundamental message in their study. “It illustrates the far-reaching positive effects that reintroducing a once-lost predator can have on ecosystems,” says Silliman. “The question therefore arises as to how many other ecosystems around the world the reintroduction of a former member of the community could bring similar benefits,” says the scientist.

Source: Duke University, University of Florida, specialist article: Nature, doi: 10.1038/s41586-023-06959-9

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