Pleasant Island in Alaska is not exactly befitting of its name. The frigid, 20-square-mile island is uninhabited by humans, but it hosts a remarkably large and rich ecosystem that features 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, it set up a natural experiment.
“This provided a great opportunity to study predator-prey dynamics of wolves and deer,” says Roffler Gretchen. “We were interested in seeing how the newly colonizing wolf population would impact the deer population and predicted that the wolves might eat all the deer, and then leave the island as it is only separated from the mainland by 1.5 km.”
The first part of the prediction came true. The deer population of around 120-200 deer plummeted. But instead of moving to greener pastures, the wolves remained on an island and shifted their diet to unexpected prey: sea otters.
Sea otters are themselves a top predator in the near-shore ecosystem, while wolves are an apex predator in the terrestrial area. — so it’s pretty surprising that you end up with a dynamic where one eats the other, says Taal Levi, an associate professor at Oregon State. “You have top predators feeding on a top predator,” Levi says.
Gretchen, Levi, and colleagues were studying the wolf diets throughout southeast Alaska, as these wolves are petitioned for listing under the endangered species act — so knowing more about their feeding ecology was important.
They tracked some of the wolves with GPS collars and analyzed their scat. They found that in 2015, deer were the primary food of wolves, representing 75% of their diet. By 2017, wolves transitioned to eating primarily sea otters (57% of their diet), while deer only made up 7% of their diet. The pattern held through to 2020, when the study ended.
Otters themselves have had a rough history in the area. During the 19th and 20th century, sea otters in the region were hunted by fur traders and basically wiped out from the region. Local wolves were not hunted to extinction, unlike wolves in other parts of the US. But it was only in recent decades, thanks to the legal protection granted to sea otters, that the two populations overlapped.
But researchers weren’t expecting wolves, a terrestrial species, to become so proficient at eating sea otters — which, as the name implies, spend most of their time at sea.
“They are both scavenging otters and hunting them when the sea otters haul on land. Sea otters are very unlikely to be vulnerable to wolves in the ocean,” notes Levi.
Wolves were often seen patrolling the shoreline of Pleasant Island and investigating rocky outcrops. The GPS data confirmed that they spend a lot of time in the intertidal zone, as if looking for something — and indeed they were: they were looking for otters to ambush.
Sea otters haul out on rocks to conserve energy, says Roffler. But this makes them more vulnerable to predation as they are slow and awkward on land — and wolves are quick to take their chance. “We have collected evidence of wolves killing sea otters by ambush when they haul out on land or are in shallow water,” Gretchen adds.
This new twist to the ecosystem makes for a very interesting case study, Gretchen continues.
“Previously there have been investigations into the effects of marine predators on sea otter populations, but until now very little attention has been paid to the impact of terrestrial predators on sea otters, or how sea otters may be an abundant marine prey to terrestrial predators. This interaction was unexpected, but has had profound effects, at least on Pleasant Island.”
For now, it’s not clear how the otters are adapting to this (or if they are adapting at all). The biggest effect might be a behavioral change that forces them to spend more time at sea, even when it would be beneficial to them to conserve energy on land — the effects of this could be stressful in the long term, but this is something that warrants future research, Levi says.
But overall, the researchers don’t expect that wolves will have a big effect on the sea otter population. The more important ecosystem implication is that wolf population dynamics can be decoupled from the large mammals that make up their typical prey.
“This allows wolves to remain abundant even as they cause large herbivore populations to decline. That is, sea otters may allow wolves to maintain large herbivores at lower densities, which has implications for vegetation and the animals that depend on it (bees, birds, bears for floral and berry resources, for example), across a huge coastline that will be eventually occupied by sea otters as their recovery continues,” Levi adds.
This surprising finding of wolf diets definitely warrants more studies to better understand the interactions in this ecosystem — and Levi says they’re working exactly on that.
“We are now increasingly following up on the wolf-sea otter story with additional field studies, including one by PhD student Ellen Dymit, comparing mainland study 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.