When sea otters dig for clams in seagrass meadows, they leave a pockmarked “moonscape” in their wake. It’s “full of craters, just pits everywhere,” says Erin Foster, an ecologist at the Hakai Institute. But this rough treatment may be doing the otherwise lush meadows a reproductive favor: Foster and her colleagues have found the seagrass meadows in these areas are much more genetically diverse—and thus resilient—than they are in places without otters.
It’s a “major advance” says Dalhousie University marine ecologist Boris Worm, who wasn’t involved with the work. The study, he says, shows “large animals may help to actively maintain the quality and resilience of their habitat.”
Seagrass—a terrestrial plant that made its way back into the oceans—has two methods of reproducing. It can clone itself by sending out roots that pop up new shoots, which are genetically identical to the parent plant. Or it can reproduce sexually by flowering and dispersing seeds.
Foster and her colleagues were interested in eelgrass, a type of seagrass found in the cool waters of the North Pacific Ocean. They thought the stress of the otter disturbance might cause the eelgrass to flower more—which it tends to do under stress—and might also clear new spots on the sea floor where seedlings could flourish.
The researchers compared sections of DNA in 462 eelgrass shoots from 15 different meadows off the coast of British Columbia. In six of the meadows, otters had been living comfortably for more than 30 years; in six, the animals had been absent for more than 100 years, having never returned since being wiped out by human hunters; and in three, otter populations had only been re-established in the past 10 years.
The results of the analysis were so striking that Foster repeated it “about 20 times,” she says. The researchers found that otters dug up about 5% of the meadows where they lived. And in the meadows with long-established otter populations, eelgrass genetic diversity was one-third higher than in comparable areas without otters, the team reports today in Science.
That diversity may translate to a greater ability to thrive in difficult or changing conditions. A lack of genetic diversity is a real threat, Worm says, because meadows with little genetic diversity are “inherently vulnerable” to changes in the environment—such as warming waters. Genetic diversity can counteract that vulnerability by ensuring a mix of plants equipped to survive different threats.
Foster is quick to say the result may be a “rediscovery.” Indigenous Seri people in the Gulf of California have long harvested eelgrass, she notes, and expected better harvests from more disturbed meadows.
Still, this is some of the first research to explore the genetic consequences of large animal disturbance, Worm says. It’s “fascinating” that otters, a species at the top of the food chain, can help change the genetic diversity of the plants in their habitat, all the way at the bottom of the food chain, he says.
The link between large animals and genetic diversity is “novel and exciting,” says Douglas McCauley, an ecologist at the University of California, Santa Barbara. But he cautions that the result doesn’t mean disturbing ecosystems will always lead to more genetic diversity; in some circumstances, it could produce the opposite effect. For instance, he says, intense grazing in grasslands could suppress flowering and boost clonal growth instead, which would reduce diversity.
If anything, the finding hints that otters could be a crucial element for seagrass restoration, McCauley says. Some restoration attempts “have seen lots of ups and downs,” he notes. “I can’t help but wonder how such efforts would fare with otters added back in those systems.”