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Close-up of a young sea otter (Enhydra lutris) floating in the ocean.

Sea otters in California prey on crabs, thereby inadvertently protecting the vegetation that holds creek banks together.Credit: David Hayes/Alamy

Sea otters are helping to keep the shores of a central Californian estuary from crumbling into the ocean. They act as erosion control by feasting on shore crabs — crustaceans whose burrowing and vegetation-munching habits contribute to unstable salt-marsh banks.

By the twentieth century, humans had hunted sea otters (Enhydra lutris) nearly to extinction for their fur. But conservation efforts have helped population sizes to increase, and otters are re-establishing themselves in their historical haunts, including in the salt marshes of Monterey Bay’s Elkhorn Slough. Moreover, in marsh creeks with high numbers of sea otters, erosion rates are lower than when there were fewer sea otters, researchers report today in Nature1.

“It’s remarkable when you think about it,” says Jane Watson, a community ecologist at Vancouver Island University in Nanaimo, Canada. “You can have a single animal, the sea otter, come in and through predation actually mitigate the effects of erosion.”

Natural vegetation protectors

Salt marshes provide crucial habitats for wildlife but are threatened globally. Several factors, such as increased water flow and sea-level rise, contribute to the amount of erosion in Elkhorn Slough — estimated at around 30 centimetres per year. Striped shore crabs (Pachygrapsus crassipes) also play a part by eating the roots of pickleweed (Salicornia pacifica), an abundant plant that helps to hold the slough’s sandy banks together. Because sea otters eat the crabs, study co-author Brent Hughes, a marine ecologist at Sonoma State University in Rohnert Park, California and colleagues wanted to know whether the predator’s recovery would change the levels of erosion in the area.

To dig into the correlation between erosion and sea otters, the team compared several lines of evidence, such as historical erosion rates in Elkhorn Slough and sea otter population trends. They also did a predator-exclusion experiment, in which otters could eat burrowing crabs in some creeks but not in others. Then, the team compared how much vegetation grew in each of those areas.

Turning the tide on erosion

On banks where sea otters could prey on crabs, the vegetation was denser than on those from which they were excluded. Hughes says that sea otters have had a similarly positive impact on other vegetation elsewhere, such as seagrass and giant kelp. “It’s almost like, wherever they go, they’re protecting vegetation,” he says. In areas where otters had returned, erosion slowed from 30 cm a year to 10 cm a year.

As their populations increase and they reclaim their historical range, sea otters could turn the tide of erosion in other salt marsh habitats — particularly the marshes of the nearby San Francisco Bay, Watson says.

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