Sea otters help kelp forests recover, but there's more to the story

When sea otters returned to islands along Southern California and British Columbia, researchers saw kelp forests revive. These forests had been wiped out by unchecked sea urchin populations. However, the speed of recovery varied by location, leaving scientists searching for answers.

A new study from CU Boulder sheds light on this puzzle. It confirms that sea otters, a keystone species, help kelp forests grow back, but their impact depends on interactions with other marine species. The researchers analyzed decades of data to map changes in ecosystems over time.

“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.

“A more modern view is that they are still very important, but they can have different effects in different places.”

Role of sea otters in forest health

Scientists have been studying keystone species – animals that have a big impact on their environment – for a long time. One of these scientists, Jim Estes, spent his career researching sea otters. He discovered that sea otters play a key role in keeping kelp forests healthy.

Otters eat sea urchins, which are animals that feed on kelp. When there aren’t enough otters, sea urchins multiply and eat too much kelp. This destroys kelp forests, turning them into empty underwater areas with little life.

But when otters are present, they keep the urchin population under control, allowing kelp forests to grow and support many marine species.

To understand this better, Estes and scientist Jane Watson studied what happened when sea otters were brought back to areas where they had disappeared.

They focused on two locations: Nicolas Island in California and Vancouver Island in British Columbia. Over 30 years, they collected detailed data to see how sea otters influenced their surroundings.

Before otters were reintroduced, both locations had too many sea urchins, and the kelp forests had almost disappeared. Their research provided strong evidence that sea otters are crucial for maintaining a balanced ecosystem.

Different rates of recovery

The data showed that while kelp forests recovered in both locations, British Columbia saw much faster regrowth.

In this region, the return of sea otters triggered a clear chain reaction: otters ate sea urchins, allowing kelp forests to flourish. However, in California, recovery was slower, revealing gaps in understanding.

To explore these differences, Langendorf developed a new model that visualized species interactions over time. This model functioned like a movie, capturing how different species influenced one another across decades.

Complex web of interactions

The model provided a clearer picture of what slowed kelp recovery in Southern California. In this region, species interactions were more complex.

Unlike in British Columbia, where otters had a strong effect on urchins, California’s ecosystem had additional competition among different species. This weakened the direct influence of sea otters on the system, slowing the pace of recovery.

“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 Dan Doak, a professor of environmental studies at CU Boulder.

“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.”

Lessons from otters and forests

This study introduces a method that can help researchers understand how ecosystems change when species are reintroduced. By analyzing long-term data, scientists can predict how different species interact in shifting environments.

“The dynamic nature of ecosystems has long kept ecologists from understanding what species need and how best to manage them,” Langendorf 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.”

This approach could provide insights into conservation efforts worldwide, helping scientists protect and restore fragile ecosystems.

The study is published in the journal Proceedings of the National Academy of Sciences.

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