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When deer disappear off the menu, hungry wolves turn to sea otters

Although wolves in Alaska will eat just about anything that moves, their typical main course consists of deer, deer, and more deer. But when wolves on one island off the state’s coast finished off nearly all the deer around them, they turned to a surprising substitution: sea otters.

That’s the conclusion of a new study that records a rare instance of a wolf population persisting without large terrestrial prey such as moose or deer. It also highlights the unpredictability of species restoration efforts on local food webs, the authors note, as conservationists have worked to protect and reintroduce wolves and sea otters to the glacier-carved coastline of southeastern Alaska.

“Forever, we’ve just thought that wolves are largely tied to ungulates [hoofed mammals],” says Layne Adams, a wildlife biologist at the U.S. Geological Survey who was not involved in the study. “This is pretty phenomenal that one predator is basically living off of another predator in a different system.”

Small populations of wolves have long been considered doomed without large herbivores to eat. For example, in 1960, a pack of gray wolves (Canis lupus) became established on Coronation Island, a tiny isle off the Alaskan coastline south of Glacier Bay National Park. The wolves quickly decimated the island’s black-tailed deer population. They pivoted to harbor seals before resorting to cannibalism. After 8 years, only a single wolf remained on the island, dooming its pack.

To scientists’ surprise, though, wolves on nearby Pleasant Island have avoided that fate. The wolves swam ashore from the mainland in 2013 and found a buffet of Sitka black-tailed deer. Within a few years, the ravenous wolves devoured nearly every deer on the island. But unlike the Coronation Island wolves, the Pleasant Island wolves have persisted.

To find out how, a team of researchers collected nearly 700 samples of wolf scat and hair between 2015 and 2020. They also outfitted 13 wolves on the island and nearby mainland with GPS collars to monitor their movements and determine where they were feeding.

In the wolves’ scat, the researchers found DNA from nearly 40 different species including snowy owls, porcupines, halibut, and sperm whales. However, the most common prey items were Sitka black-tailed deer (Odocoileus hemionus sitkensis, a stocky subspecies of mule deer) and, unexpectedly, sea otters (Enhydra lutris).

Centuries of hunting by humans had eradicated sea otters from most of the Alaskan coastline. But reintroduction efforts and bolstered legal protection brought these marine mammals back from the brink—and onto the wolves’ menus.

Samples collected in 2015 revealed that deer comprised about 75% of the wolves’ diet. But by 2017, sea otters had become the wolves’ primary prey, the researchers report today in the Proceedings of the National Academy of Sciences. The otters made up nearly 60% of their diet, whereas deer comprised only 7%.

“The addition of sea otters has changed the game and allowed them to be sea wolves, living on primarily marine resources,” says Taal Levi, an ecologist at Oregon State University, Corvallis, and a co-author on the new study.

Swapping out venison for otter meat required the cunning canines to develop a different hunting strategy. Despite their cuddly appearance, sea otters make for formidable creatures in the water with powerful paws and crushing teeth. However, when the animals haul out onto the island’s rocky shores to rest, they become easy grub for wolves. According to Gretchen Roffler, a wildlife biologist at the Alaska Department of Fish and Game and lead author of the study, the wolves drag the sea otters above the high tide line to consume them. While tracking the wolves’ GPS collars, Roffler encountered several grisly otter carcasses with crushed skulls and mangled spines.

Pleasant Island isn’t the only place where wolves dine on sea otters. The team found evidence that mainland wolves also consume large numbers of otters. It appears to be a growing trend: As local glaciers melt, sea otters have taken advantage of the opening coastal real estate, bringing them in closer contact with wolves.

Although the phenomenon has come as a surprise to scientists, Levi says the revival of the two species in the region may in fact be reconnecting an ancient food web. “The reintroduction of sea otters is restoring this interaction between the sea and the land that has existed forever.”

Source: Science Mag