Bringing an ecosystem back to life is no easy feat, but sometimes when we give nature a little push, it can bounce back.

Consider the Elkhorn Slough in California's Monterey County. This tidal salt marsh is the second largest in California, but parts of it weren't much of a home for wildlife in the early 2000s. It was, as the San Francisco Chronicle described it, "a muddy scoured-out channel." The reason? A lack of eelgrass in the slough. Mud and erosion kicked into high gear, leaving a habitat that very few organisms were happy to call home.

Thanks to a 15-year rehabilitation program, however, eelgrass is thriving again, and it's all because of sea otters.

Save the kelp, save the sea otter

A sea otter floats in the sunny waters of Moss Landing A sea otter takes a break after doing its part for the environment. (Photo: Don DeBold/flickr)

The southern sea otter (Enhydra lutris nereis) once called long stretches of the West Coast home, stretching from Baja, California, to the Pacific Northwest. Hunting of the charismatic sea creature in the 1700s hit the population hard, to the point that by the 1920s, they were believed to be extinct. But eventually a small population was discovered near Big Sur. Since 1977, the sea otter has been listed as an endangered species and efforts to keep the animal thriving have intensified.

Today, thanks to various conservation efforts, the population in the wild has held steady at 3,000 for well over a decade, but it hasn't grown as much as scientists would like. Not helping matters is that the sea otters are keeping to a very small portion of this historic range, living in waters that stretch from Half Moon Bay to Point Conception, about a 300-mile stretch of California's coast. This means that they're competing for food in a relatively small area.

Sea otter peeks out from the water at Moss Landing The southern sea otter population has struggled to grow much since it reached around 3,000 individuals. (Photo: Don DeBold/flickr)

The environment is not helping matters. A study published in Ecography looked at 725 sea otter strandings between 1984 and 2015. Researchers found that an uptick in strandings occurred due to a substantial increase in shark bites outside of the now-normal ranges. Within the current ranges, "symptoms of energetic stress" accounted for more than 63 percent of strandings.

The study identifies sea kelp, like eelgrass, as one of the primary factors for whether or not strandings even occur. Indeed, when there is at least 10 percent kelp cover, strandings are "virtually absent."

"Our analyses reveal that declining kelp cover may therefore constrain the population's spatial expansion and recovery in two key ways," the researchers wrote. "Absence of kelp intensifies density-independent threats in the range peripheries, and likely limits dispersal of reproductive females, which depend on kelp canopy for nursery habitat."

Home sweet kelp

A group of sea otters float in the waters of Moss Landing Sea otters benefit from the presence of eelgrass and other types of kelp, and the eelgrass benefits from the presence of the sea otters. (Photo: Pacific Southwest Region USFWS/flickr)

So the kelp helps keep sea otters alive, and as the sea otter rehabilitation efforts in the Elkhorn Slough demonstrate, the sea otters keep the kelp alive, too.

The collapse of the eelgrass in the Elkhorn Slough was the result of a breakdown in the ecosystem's balance, as the Chronicle reported. Crabs in the slough would eat sea slugs that in turn ate algae. This algae killed the eelgrass, and without the eelgrass, the slough became a muddy mess incapable of supporting fish and other invertebrates.

Despite all this, a group of about 50 transient male sea otters were living in the slough, likely because they were safe from predators there. So, in the early 2000s, the Monterebay Aquarium, which rescues and rehabilitates sea otters, decided that it might be a suitable spot to release sea otters back into the wild, especially animals that would need extra monitoring.

A sea otter munches on a crab in the Elkhorn Slough When sea otters munch on crabs, they're helping to keep eelgrass alive. (Photo: Pacific Southwest Region USFWS/flickr)

In the 15 years since, both the otter and the eelgrass populations have done well. The otters eat the crabs and that allows the sea slugs to flourish. When the sea slugs are doing well, the eelgrass is algae-free and allowed to flourish. And when the eelgrass is flourishing, the otters are able to use it as a nursery to produce more otters. If sharks were around, it would also mean more ways to hide from them.

'Growing in places where it didn't even exist before'

Karl Mayer, the sea otter program coordinator for the Monterey Bay Aquarium, took the Chronicle around the slough, pointing out patches of eelgrass that were coming in strong.

Two sea otters float in the waters of the Elkhorn Slough The Monterey Bay Aquarium expects the souther sea otter population to increase in 2018. (Photo: Don DeBold/flickr)

"This is the largest bed of eelgrass," he said in reference to a patch of kelp with a group of a half-dozen otters hanging out in and around area. "This was less than half this size a few years ago. It's growing in places where it didn't even exist before."

The aquarium expects the sea otter population in the slough to grow to 145 this year after they release several rescued pups currently in the rehabilitation program. This is just a start, however. With the combination of sea otters and kelp, Mayer and others believe that reintroducing other rescued sea otters to new areas could improve the presence of kelp and allow the sea otters to begin thriving in new waters.

"Cumulatively we have this unprecedented data set off all the otters that were released," Mayer said. "They wound up being a really valuable tool from an ecological standpoint. They are a means of learning about the wild population ... and a mechanism by which sea otters expand."

Sea otters are helping save their own habitat
Eelgrass is thriving again, thanks to the help of a sea otter rehabilitation program.