Pleasant Island in Alaska doesn’t exactly live up to its name. The frigid, 20-square-mile island is uninhabited by humans but is home to a remarkably large and rich ecosystem, including deer, otters, red squirrels, and even brown bears. But in 2013, the island got a new addition: wolves.
When wolves colonized the island in 2013, they set up a natural experiment.
“This provided a great opportunity to study predator-prey dynamics in wolves and deer,” says Roffler Gretchen. “We were interested to see how the newly colonizing wolf population would affect the deer population and predicted that the wolves might eat all of the deer and then leave the island as it is only 1.5km from the mainland.”
The first part of the prediction came true. The deer population of around 120-200 deer collapsed. But instead of migrating to greener pastures, the wolves stayed on an island and shifted their diet to unexpected prey: sea otters.
Sea otters are themselves a top predator in the coastal ecosystem, while wolves are a top predator in the terrestrial range. — so it’s pretty surprising that you end up with an eat-eat-your-other dynamic, says Taal Levi, an associate professor at Oregon State. “You have top predators feeding on a top predator,” says Levi.
Gretchen, Levi and colleagues studied the diets of wolves throughout southeastern Alaska as these wolves were being petitioned for inclusion under the Endangered Species Act—so it was important to know more about their dietary ecology.
They tracked some of the wolves with GPS collars and analyzed their droppings. They found that in 2015, deer were the main food of wolves, making up 75% of their diet. By 2017, wolves transitioned to eating primarily sea otters (57% of their diet), while deer made up just 7% of their diet. The pattern continued through 2020, when the study ended.
Otters themselves have had a rough history in the area. In the 19th and 20th centuries, sea otters were hunted in the region by fur traders and were virtually eradicated from the region. Local wolves, unlike wolves in other parts of the US, have not been hunted to extinction. But only in the last few decades, thanks to legal protections for sea otters, have the two populations overlapped.
But the researchers didn’t expect wolves, a terrestrial species, to get so good at eating sea otters – which, as the name suggests, spend most of their time at sea.
“They both eat otters and hunt them when the sea otters come ashore. Sea otters are very unlikely to be vulnerable to wolves in the ocean,” Levi notes.
Wolves have often been seen patrolling the Pleasant Island shoreline and examining rocky outcrops. The GPS data confirmed that they spend a lot of time in the intertidal zone, as if looking for something – and they did: looking for otters to ambush them.
Sea otters retreat to rocks to conserve energy, Roffler says. But that makes them more vulnerable to predators, as they’re slow and clumsy on land – and wolves are quick to take their chance. “We have gathered evidence that wolves ambush sea otters when they are moving onto land or in shallow water,” adds Gretchen.
This new twist on the ecosystem is a very interesting case study, Gretchen continues.
“Previously, there has been research on the effects of marine predators on sea otter populations, but very little attention has been paid to the effects of terrestrial predators on sea otters or how sea otters might be productive marine prey for terrestrial predators. This interaction was unexpected but had a profound impact, at least on Pleasant Island.”
At the moment it is not clear how the otters are adapting to this (or if they are adapting at all). The biggest effect could be a change in behavior, forcing them to spend more time at sea, even if conserving energy on land would be beneficial for them – the effects of this could be stressful in the long run, but it’s something that’s warranted is futurology, says Levi.
But overall, researchers don’t expect wolves to have a major impact on sea otter populations. The more important impact on the ecosystem is that wolf population dynamics can be decoupled from the large mammals that make up their typical prey.
“This allows wolves to remain plentiful even as they cause the decline of large herbivore populations. That is, sea otters may allow wolves to hold large herbivores at lower densities, affecting vegetation and the animals that depend on it (eg, bees, birds, bears for flower and berry resources) across a vast coastline eventually be occupied by sea otters as their recovery continues,” adds Levi.
This surprising finding of wolf nutrition definitely warrants further studies to better understand the interactions in this ecosystem — and Levi says they’re working on exactly that.
“We are now increasingly following the wolf-sea otter story with additional field studies, including one by graduate student Ellen Dymit, comparing land survey areas with and without sea otters along the colonizing front of sea otter population expansion.”
The study was published in Proceedings of the National Academy of Sciences.