Starving, starving otters could help marshes face climate change


Watching sea otters sleep and cuddle while they float can be cute.

Watching them eat – their jaws crunching and tearing apart the shells, sucking out the insides – is another story.

A new study published in the journal Nature indicates that the voracious appetite of these predators may help make a California salt marsh more climate resilient.

“They eat a lot. They eat about a quarter to a third of their body weight every day,” said Tim Tinker, a research ecologist at the University of California, Santa Cruz and one of the study’s Canadian co-authors. . “And so whatever they eat, they’re going to have big impacts.”

The findings echo previous evidence in Canada, where hungry otters have helped kelp thrive since its reintroduction to British Columbia.

In Elkhorn Slough, a salt marsh estuary south of San Francisco, a food web is at work. The dominant plant is the gherkin. Its roots are eaten by shore crabs, which also burrow into stream banks, further weakening the soil. Otters eat shore crabs.

But otters were hunted to extinction in the 18th and 19th centuries, so there was less control over the shore crab population. Conservation efforts over several decades have since led otters to repopulate the area.

Initially, Tinker said, researchers thought the otters were simply using the marshes as a resting place. Over time, they realized that otters were “actually eating quite a few shore crabs in these areas.”

It turns out there are thousands of them.

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But to be sure of the impact, the researchers combined past and present analyzes and visual surveys with a very carefully conducted real experiment: they kept the otters away for three years.

“They basically take nets and build little fences around pickle patches,” said Jane Watson, a marine ecologist and professor emeritus at Vancouver Island University, who was not involved in the study. “But they leave a little space under the net so the crabs can come and go.”

The result of the otters returning to the vegetation has been spectacular: healthier gherkins, firmer soil and less erosion.

The image on the left shows soil erosion when otters were kept away from marsh vegetation.  The image on the right shows lush vegetation when they were allowed inside.
The result of an experience of exclusion. On the left, otters were not allowed into this vegetation, leading to soil erosion and plant weakness. On the right, with the otters entering, the vegetation thrives. (Brent B. Hughes)

“They now have an experimental result that shows sea otters reduce the abundance of erosion-causing crabs,” said Watson, who has studied otter ecology for nearly 40 years.

“And they also help promote the growth of (pickleweed), which helps stabilize the bank.”

Helping the kelp

It is similar to dramatic effect that otters have had on the British Columbia coast over the past half century. The food web is at stake here: otters feed on sea urchins, which are kelp “lawnmowers.”

“As the sea otters approach, they remove the urchins. And once they remove the urchins, you get these kelp beds,” Watson told CBC News from Nanaimo, Columbia -British.

This has had a negative effect on economies and communities that rely on shellfish harvesting. But it had climatic advantages.

“In terms of climate change, kelps are photosynthetic organisms and therefore take up carbon. They sequester carbon,” Watson said, adding that research shows that healthier seagrass beds are also more genetically diverse and more resilient.

WATCH | How British Columbia otters help create lush kelp forests:

Sea otters make a comeback off the coast of British Columbia

Sea otters are making a comeback off the coast of British Columbia, after being wiped out by the fur trade in the early 1900s. But as they thrive, they’re also changing the entire ecosystem submarine.

More than a postcard

Decreasing bank erosion in Elkhorn Slough, which flows into the Pacific Ocean, will also make it more climate resilient. Especially as glaciers melt and warmer oceans increase in volume, due to the warming effects of anthropogenic greenhouse gas emissions.

“They’re kind of the first line of defense,” said Tinker, who also works with Dalhousie and Simon Fraser University. “They kind of protect inland areas and inland habitats from sea level rise and massive storms.”

Watson and Tinker say the new research highlights how top predators like sea otters are doing more than previously thought.

“It’s not just big, charismatic, cool objects that are perfect for a postcard,” Tinker said. “They’re doing really important things within these ecosystems, within these food webs.”

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