Sea otters help kelp forests recover — but how fast depends on where they are

(Press-News.org) When sea otters were reintroduced along the coastlines of islands in Southern California and British Columbia, researchers saw kelp forests return to areas that were destroyed by sea urchins. But how slow or fast they grew back depended on the location — and until now, scientists didn’t understand why.

New CU Boulder research found sea otters, an important keystone species, play a vital role in kelp forest recovery, but their level of influence depends on what other species they interact with in salty Pacific Ocean waters. The study, published today in PNAS, used decades of observations to create a time series of interactions, like a movie that shows changes in the numbers of the local species, and crucially, the patterns of how they interact through time, to understand how the reintroduction of sea otters helped Pacific Ocean kelp forests recover.

“We always thought keystone species control their ecosystem the same way, regardless of where they are or what else is in the ecosystem,” said Ryan Langendorf, lead author of the paper, Environmental Studies researcher, and former postdoctoral researcher at CIRES. “A more modern view is that they are still very important, but they can have different effects in different places.” 

Researchers' fascination with keystone species spans decades. Jim Estes, a retired USGS scientist and the paper’s co-author, spent his career researching sea otters and how their presence along rocky shorelines shaped kelp forests. While working on remote Alaskan islands, he found where sea otters were absent, sea urchin populations mushroomed, overtaking kelp forests. He concluded the small mammal was integral to maintaining harmony in coastal reef ecosystems: by feeding on sea urchins they in turn maintained the health of kelp forests —  dense groups of brown algae that are rich in biodiversity and provide shelter to many species. 

Estes, University of British Columbia researcher Jane Watson, and other co-authors of the study led two 30-year data community collection studies documenting what happened after reintroducing sea otters to Nicolas Island in California in the 1980s and Vancouver Island in British Columbia in the 1970s. The data sets represent two of the most complete studies looking at the effect of keystone species on local ecosystems. Both research areas were mostly  “urchin barrens” — sites where sea urchins had overgrazed in the absence of sea otters — when the studies began. 

Thirty years of data revealed that while kelp forests grew back in both locations, forests in British Columbia regenerated much faster than in Southern California. British Columbia was a classic example of the domino effect, which ecologists call a trophic cascade, that comes with reintroducing a keystone species: otters eat urchins, so kelp can regrow. But the slower return in the south revealed a gap in understanding. 

To understand these differences, Langendorf developed a novel community model that created a movie of species interactions, to understand the changes in the ecosystem over the 30 years at both sites. 

Creating the movie provided researchers with answers. The model highlighted how all living things — sea otters, urchins, kelp — interacted over time in both regions. This revealed more competition between the different urchin, kelp, and other species in California, which slowed down the influence sea otters had on the entire system. In short: sea otters in California didn’t have as strong of an effect on sea urchins as they did up north because of the complex web interactions that took place in the Canadian ecosystem.

“Almost all studies of ecological communities assume that these interaction strengths are static – that “the rules of the game” don’t change even as the abundances of species do,” said CU Boulder Environmental Studies professor and co-author of the paper, Dan Doak. “By developing a way to estimate these changing rules, Ryan has added to our appreciation of this particular system, as well as pioneering a more powerful way to understand other ecological systems.”

The new model can help researchers better understand how ecosystems change when species are reintroduced to places that are constantly changing and evolving. 

“The dynamic nature of ecosystems has long kept ecologists from understanding what species need and how best to manage them,” Lagendorf said. “Being able to turn common survey data into a movie of species reacting to changes in their environment and each other feels like renewed hope for a field that more than ever needs to offer useful advice about how to help the many complex living systems we live with and cherish.”
 

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