Isabelle Groc
Fri 14 Aug 2020 07.00 EDT
Last modified on Fri 14 Aug 2020 12.31 EDT
When
Brent Hughes started studying the seagrass beds of Elkhorn Slough, an
estuary in Monterey Bay on California’s central coast, he was surprised
by what he found. In this highly polluted estuary, excessive nutrients
from agricultural runoff spur the growth of algae on seagrass leaves,
which kills the plants. Yet in 2010, Hughes noticed that the seagrass
beds were thriving. It did not make sense.
•••••
Hughes set out to solve the mystery. He examined every possible factor, including water quality, temperature and changes in seagrass coverage over time, going back 50 years. He was not making any progress until he was approached by a boat captain named Yohn Gideon who had been running wildlife tours in the slough since 1995. Over the years, the captain had handed clickers to his passengers, asking them to count the sea otters they saw.
Hughes overlaid the captain’s sea otter counts with historical seagrass coverage data and realised the two graphs were almost perfectly in sync. When sea otter numbers went up, seagrass went up, too. “You don’t see that very often in ecology. That was a eureka moment,” he says.
•••••
When the otters first moved into the slough in the 1980s, they put their big appetites to work eating crabs. With fewer crabs to prey on them, the California sea hares – a sea slug – grew larger and became more abundant. The slugs fed on the algae growing on the seagrass, leaving the leaves healthy and clean.
Hughes had discovered a trophic cascade that had made the seagrass beds the healthiest of any estuary he had seen on the west coast. Since the otters arrived in the slough, the seagrass has recovered and increased by more than 600% in the past three decades.
Sea otters had already shown that they were capable of a large influence on the ecosystem. In the 1970s, biologist James Estes was conducting research in the Aleutian Islands in Alaska and noticed some areas where the seafloor was covered with sea urchins. As herbivores, urchins feed on kelp, and when their numbers are not kept in check by predators, no kelp remains. In contrast, in places where sea otters were present, kelp forests were thriving. Estes demonstrated that by eating urchins, sea otters created the opportunity for kelp to flourish. Sea otters gained their official title as a keystone species.
•••••
Hunted to near extinction by fur traders in the 18th and 19th centuries, sea otters had only survived in a few small, isolated populations in the North Pacific. Then, after the Fur Seal Treaty was signed in 1911, conservation efforts led to an increase in otter numbers. In California, the population grew from 50 in 1914 to about 3,000. On the coast of British Columbia, they were reintroduced between 1969 and 1972 and there are now about 8,100 of them.
•••••
Hughes’ discovery of the positive impact that sea otters had on Elkhorn Slough’s seagrass made him wonder whether they could benefit other places. Still working in the slough, he observed that sea otters moved into the salt marsh portion of the estuary in 2012 and started having a positive influence on this fragile habitat. Before the otters’ arrival, numerous striped shore crabs were burrowing into the muddy banks and fed on the marsh roots, accelerating the erosion of the marsh. By eating the crabs, the otters are now helping make the salt marsh healthier, slowing down erosion, a discovery that could be significant for salt marshes elsewhere. “Salt marshes are one of those ecosystems that globally are in a state of decline. In California, we lost 90% of them,” says Hughes.
•••••
The return of this top predator also comes at a cost to humans, since sea otters compete directly with shellfish fisheries which had developed in their absence. A recent study that analysed the costs and benefits linked to the reintroduction of sea otters to the west coast of Vancouver Island in British Columbia found the benefits – including increasing fish populations, carbon sequestration and ecotourism – outweighed the losses to invertebrate fisheries by seven times.
However, the benefits are not felt equally, especially among indigenous communities who rely on shellfish harvesting for food security.
“Sea otter recovery is different from the recovery of any other species because they have such disproportionally big effects on the ecosystem,” Tinker says. “For most depleted species you are just worried about the conservation of the species but with sea otters, you are thinking how the entire ecosystem is going to change when they recover.”
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