Photo: Isabelle Groc
Historical records show that otters were once abundant in California’s estuaries. Now they’re back — and bringing surprising benefits.
This is a story about America’s smallest marine mammals and how they are improving ecosystems.
Isabelle Groc reports at the Guardian, “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 [a biologist at Sonoma State University] 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.’ …
“Sea otters may be North America’s smallest marine mammal, but they have a huge appetite. … 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. …
“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. …
“But the discovery that sea otters could also be important players in estuaries came as an ecological surprise. In fact, scientists had not even expected sea otters to survive in an estuary. … Since the otters were first recovering in kelp forests along the open coast, the scientists who studied the animals assumed that this was their primary habitat. When they started turning up in Elkhorn Slough in the 1980s, they thought that was an anomaly, failing to realise that they were in fact reoccupying old habitats. …
“Estuaries were not even considered in the US fish and wildlife service’s plan for the recovery of the sea otter in California, listed as threatened under the US Endangered Species Act. … In the past decade, the expansion of sea otters on the California coast has been curbed unexpectedly by the presence of the great white sharks. …
“Estuaries could provide otters with an important refuge from sharks and other unfavourable coast conditions, such as storms and warming events. …
“ ‘Once fully recovered, between a quarter and one third of the entire population in California could be accounted for by otters living in estuaries,’ [Tim Tinker, a wildlife biologist at the University of California, Santa Cruz,] says. …
“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.’ ”
More.
Hat tip: Earle.
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