Wolves on an Alaskan Island have eaten so many deer they have resorted to hunting sea otters – the first time the sea mammals have become the prey for a land predator, scientists said.
The wild animals devoured so many of the deer in just two years, the population on Pleasant Island plummeted and so they switched to primarily eating the sea otters that live nearby.
By tracking the wolves with GPS collars and analyzing their feces, researchers found that in 2015 deer were the main food of the wolves, representing 75 percent of their diet, while sea otters only made up a quarter.
But by 2017, the wolves moved to primarily munching on sea otters, with more than half of their diet being marine mammals (57 percent).
That same year, the deer population declined to seven percent – a pattern that continued until 2021, when scientists stopped studying the animals.
Researchers at Oregon State University and the Alaska Department of Fish and Game studied the wolf pack on Pleasant Island and the adjacent mainland from 2015 to 2021.
Taal Levi, an associate professor at Oregon State, said: “Sea otters are this famous predator in the near-shore ecosystem and wolves are one of the most famous apex predators in terrestrial systems.
“So, it’s pretty surprising that sea otters have become the most important resource feeding wolves. You have top predators feeding on a top predator.”
Historically wolves and sea otters likely lived in the area of the island researchers were investigating – next to Glacier Bay, about 40 miles west of Juneau, the capital city of the state of Alaska.
The island is about 20 square miles, uninhabited and accessible only by boat or float plane.
During the 1800s and much of the 1900s, populations of sea otters in this region were wiped out from fur trade hunting. Unlike wolves in the continental U.S., Southeast Alaskan wolves were not hunted to local extinction.
Only in recent decades, particularly with the reintroduction and legal protection of sea otters, have the populations of both species recovered and once again overlapped, scientists said.
This has given new opportunities for changing predator-prey relationships between the two species, experts found.
Dr. Gretchen Roffler, a wildlife research biologist with the Alaska Department of Fish and Game, and others from the department collected 689 wolf droppings, many along the island’s shoreline.
Once the dung was collected, members of Levi’s lab in Oregon used molecular tools, such as DNA metabarcoding and genotyping of the scat, to identify individual wolves and determine their diets.
Dr. Roffler also captured and placed GPS collars on four wolves on the island and nine on the mainland.
The researchers were curious whether wolves were traveling between the mainland and island, considering other scientists have found they are capable of swimming up to eight miles between land masses.
Both the GPS collar data and genotypes of the droppings confirmed they were not, indicating that the island wolf pack is stable and that the island is not a hunting ground for mainland wolves, scientists said.
Locations from the GPS-collared wolves also showed evidence that the wolves are killing sea otters when they are in shallow water or are resting on rocks near shore exposed at low tide, researchers found.
Dr. Roffler and her crew have investigated wolf GPS clusters on Pleasant Island for three, 30-day field seasons since 2021 and found evidence of 28 sea otters killed by wolves.
She said: “The thing that really surprised me is that sea otters became the main prey of wolves on this island.
“Occasionally eating a sea otter that has washed up on the beach because it died, that is not unusual.
“But the fact that wolves are eating so many of them indicates it has become a widespread behavior pattern throughout this pack and something that they learned how to do very quickly.
“And from the work we are doing investigating kill sites, we are learning that wolves are actively killing the sea otters.
“So, they aren’t just scavenging sea otters that are dead or dying, they are stalking them and hunting them and killing them and dragging them up onto the land above the high tide line to consume them.”
Shortly after wolves colonized Pleasant Island in 2013, the deer population on the island plummeted. With the wolves having consumed most of the deer, which was their main food source, Prof Levi said he would have expected the wolves to leave the island or die off.
Instead, the wolves stayed, and the pack grew to a density not previously seen with wolf populations. He believes the main reason is the availability of sea otters as a food source.
In another paper published in 2021, the same researchers documented a similar pattern throughout the Alexander Archipelago, a group of Southeastern Alaskan islands that includes Pleasant Island.
The research has now expanded to study wolves and sea otters in Katmai National Park and Preserve, which is in southwest Alaska, about 700 miles from Pleasant Island, the researchers said.
These latest findings were published in PNAS.
Produced in association with SWNS Talker.
