After eating all the deer, wolves in Southeast Alaska developed a taste for sea otters

Sitka black-tailed deer used to be abundant on Pleasant Island, a small island accessible only by boat in the icy waters of Alexander archipelago in southeastern Alaska. From 2013 to 2018, however, their numbers crashed when they became the favorite meal for a pack of wolves which swam to the island from the mainland. But rather than leave, the apex predators shifted their diet to a new food source: sea otters, which were reintroduced to Alaska after being nearly wiped out in the fur trade. Biologists at the Alaska Department of Fish and Game partnered with scientists at Oregon State University to make the discovery, which is thought to be the first instance of sea otters becoming a primary food source for a land-based predator. Gretchen Roffler is a wildlife research biologist at the Alaska Department of Fish and Game. Taal Levi is an associate professor in the department of fisheries, wildlife and conservation sciences at Oregon State University. They join us to discuss the results of the study which was published last month in the Proceedings of the National Academy of Sciences.

Note: The following transcript was created by a computer and edited by a volunteer.

Geoff Norcross: This is Think Out loud on OPB. I’m Geoff Norcross, in for Dave Miller. About 10 years ago, a pack of wolves figured out how to swim from the southeast Alaska mainland to a little island about a mile away. It’s called Pleasant Island and for the next five years they basically ate all the deer on the island. In their search for food, they started going after the sea otters on the shore. That’s a big deal to science. It’s believed to be the first time sea otters have become the primary target for any land based predator. We’re going to talk about this major shift in the predator-prey relationship that was discovered by researchers in Alaska and at Oregon State. Gretchen Roffler is a wildlife research biologist at the Alaska Department of Fish and Game and Taal Levi is an associate professor in the Department of Fisheries Wildlife and Conservation Sciences at Oregon State University. Gretchen, Taal, welcome to Think Out Loud.

Gretchen Roffler: Thanks for having us.

Taal Levi: Thanks very much.

Norcross: Gretchen, why did this wolf population move to the island in the first place?

Roffler: Well, we don’t know why they went there, hard to say what is on a wolf’s mind, but we know based on observations from people who live in this area and who do spend a lot of time on Pleasant Island hunting deer that wolves were first seen over there in 2013. So a pair of wolves was observed on Pleasant Island and we assume that they went over there to find a new home range and they found an island that was uninhabited by other wolves and had an abundant deer population.

Norcross: And then those two wolves became many more wolves. Is that what we’re thinking?

Roffler: Yes. So they reached a high of 13 wolves in 2017-2018.

Norcross: Were they bred or did just other wolves make the swim?

Roffler: Well, as far as we know, what happened is they started reproducing and they had many litters of pups over the years.

Norcross: Okay, can you describe them? How are the wolves different from the wolves on the mainland, Alaska and Canada?

Roffler: These wolves are part of the same genetic grouping as wolves throughout Southeast Alaska. So they’re commonly referred to as Alexander Archipelago wolves or coastal wolves. We know that the wolves, interestingly on Pleasant Island, they’re a bit smaller than wolves that have been captured in Wade and other parts of Southeast Alaska. So on average, these wolves are about 60 pounds, whereas wolves in other parts of Southeast Alaska are around 65 to 75 pounds.

Norcross: And they’re good swimmers?

Roffler: They are, they’re very good at swimming and it’s not uncommon for wolves to swim to islands.

Norcross: And we’ll talk about the otters in a minute. But let’s touch on the deer that were there before the wolves showed up. What can you tell me about them? How healthy was the population and what happened?

Roffler: Well, the population was pretty abundant. It ranged between about 120 to 200 deer approximately. And we measure deer abundance by going to the island and walking in lines and transects to count deer pellets. And then we based our estimates on this. So these surveys have been completed since the early 1990s and the deer population stayed pretty abundant up until about 2015. So two years after the wolves reached Pleasant Island, and after that they basically crashed. In 2018, we couldn’t find any deer pellets and the same story in 2021. So the island is now pretty much devoid of any sign of deer.

Norcross: So after eating all the deer, the wolves could have swum back to the mainland and find more deer there. But they didn’t. They stayed. Why?

Roffler: Yeah. And we assumed that’s what was going to happen, that the wolves would just simply leave, because just a mile away, there is the mainland with a very abundant moose population, in addition to other prey available. Instead, based on multiple lines of evidence, the wolves remained on Pleasant Island and they shifted their diet towards one that is more based on marine prey, especially sea otters.

Norcross: What alerted you to the fact that this was going on and the research opportunity that it presented?

Roffler: The main thing, the main line of evidence was that we had started picking up wolf scat whenever we could find it. And we would do targeted collection trips to Pleasant Island and walk the periphery of the island and into the interior and look for wolf scat and collect them. And we sent them to Taal and we could see the change in diet over time. So deer made up most of that, that was the most common prey that we found in wolf scats up until about 2017 when it switched to sea otters.

Norcross: So Taal Levi, this is a good opportunity to bring you in. Gretchen and her team, as she said, mailed this wolf scat to you here in Oregon. What did you do with it?

Levi: It’s always fun. We get a care package of all these giant boxes full of wolf scats.

Norcross: Oh, I didn’t get you anything.

[Laughing]

Levi: Gretchen was sending us scats actually from all of Southeast Alaska. One of the interesting things about this is that we just kind of got lucky. We started detecting prey species from across scats from southern Southeast Alaska to northern Southeast Alaska and sea otters started to pop up and we use this molecular tool I can talk about, called DNA meta barcoding. But essentially we’re trying to amplify a small section of the mitochondrial genome, sequence it and compare that sequence to a reference database or those many sequences. So from any one scat, we might get hundreds of thousands of sequences that tell us all the species in that scat. So we started to see sea otters appear in certain parts of Southeast Alaska and that led us to pursue longer term research to try to figure out exactly what was going on.

Norcross: Were you surprised by what you saw?

Levi: We were extremely surprised. I started emailing sea otter biologists, asking people how could wolves be getting sea otters? We thought scavenging was probably going to be a major thing. But when we first started seeing the sea otters, they were not the main component, because there were still a lot of deer. So just in the first year, but then as we kept monitoring over the coming years, seeing that sea otters became the most important prey item was quite a surprise.

Norcross: Yeah, the scat tells a story. Gretchen, you can be as descriptive as you want. But what does wolf scat containing the remains of sea otters look like?

Roffler: Yeah, it has a really distinct characteristic look. Sea otter fur is very dense and soft and so wolves that have eaten sea otter, their scat looks like long silky rope that is usually gray.

Norcross: And how did you know that the wolves were actually hunting the otters instead of just scavenging carcasses that they found along the coastline? How did you know that they were actually going after live otters?

Roffler: We started to piece this together. And beginning in 2020, we started to capture wolves and put GPS collars on them so that we could learn more about their movement patterns and their home range size. Initially, we were interested in seeing if they’re swimming to the mainland, on a regular basis. But we also use these data to try to investigate kill sites. So places where we’ll spend a considerable amount of time, at least a couple hours, they’ll make a cluster of GPS locations. So during our investigation seasons, we go to these sites on the ground and we’re looking for any evidence of prey remains. And that’s when we started to find the freshly killed sea otter carcasses.

Norcross: Taal, why do you think this finding is important?

Levi: Well, predator prey interactions can be tightly linked. So you imagine in some situations, a cycle where prey goes up and then predators reproduce more and their population grows up and then they cause the prey to decline and then the predator population declines. But when you introduce another really abundant source of food, in this case from the marine system, it has the potential to keep the predator at high density even as they suppress their normal terrestrial prey. So this has the potential to influence terrestrial interactions between predators and prey across a pretty large area, as sea otters recolonize.

Based on the results here, we’ve started to do research on the mainland and I should say, one of the study areas in our study was also, or two on the mainland, but one was in Glacier Bay, where sea otters were also a really key prey item for mainland wolves. So that led us to look at these paired mainland sites, one with sea otters and one without sea otters. And what we want to know is if wolf density is much higher in the presence of sea otters? And are wolves able to pack along the coast at much higher density? And if that influences large herbivore populations, like moose or deer or mountain goats, that has consequences for plants, has consequences for bees, for bears, that can reverberate throughout the system.

Norcross: Gretchen, have you seen a wolf hunt a sea otter?

Roffler: So what we’ve seen in our study site is we occasionally see wolves walking along the beach, alert, looking for different prey items. We haven’t gotten the smoking gun of seeing a wolf actually kill a sea otter on Pleasant Island or in the Gustavus Forelands or Glacier Bay National Park. We have captured on trail cameras, wolves dragging freshly killed sea otters up above the high tide line. But we’re still trying to capture the act in the process.

Norcross: Okay. But do you have any doubt that it’s actually happening?

Roffler: I don’t have any doubt based on the evidence that we find at the kill sites, there are very definitive signs of wolf predation, especially the drag marks coming from the beach and blood, blood trails, bite marks and hemorrhaging on the carcass, especially around the skull. And of course all the wolf sign around the sea otter carcasses and the locations of the collared wolves at these sites.

Norcross: If anything, doesn’t this show that wolves are just resilient and adaptable, even when they decimate their favorite food source?

Roffler: Yeah, I think that this just really underscores how adaptable wolves are. And before we really thought that, based on other research, that wolves couldn’t survive without ungulate prey, for example, deer, especially on small islands and in this case at least on this island, this shows that wolves were able to continue to survive and we know that they are very resilient animals and this shows that even more.

Norcross: Taal Levi, what do you think could be the consequences for other animals that are there, that are part of this food web, that predators like wolves like to eat?

Levi: Our initial results from paired study areas in Western Alaska, the first surprise was that in Katmai National Park - this is a PhD student in my lab also working with Gretchen, Ellen Dimmitt - she’s found the sea otters are again the most common prey item in a mainland study area. And that’s the site where direct observation of sea otters are being killed by wolves was observed. I think Gretchen was also at that site.

At the other study area where there are no sea otters, we are looking, but I think we’re finding that there’s substantially lower wolf density, so fewer wolves are packing along the coast. And there are many reasons you could have different prey communities, but for example, the site with high wolf density has much lower moose density than the site without sea otters, and has no coyotes. The site without sea otters has lower wolf density, it has coyotes, has black bears and high moose abundance, so linking the actual causality here is challenging. But I think the first step is going to be to understand whether wolf density really is much higher in the presence of sea otters. And if that’s the case and and occurs on really large spatial extents, I think that could have effects on all sorts of things that wolves eat because they can remain abundant and prey on beavers, black bears, moose and so on.

Norcross: Wolves, there’s a range of opinions about them. They can inspire fear and loathing, especially in rural areas and especially in ranch lands. Gretchen, I’m wondering how wolves are regarded in Southeast Alaska.

Roffler: It seems that wolves in Southeast Alaska, as in most parts of the world, do not inspire neutral feelings by people. People tend to have strong opinions about them on either side of the spectrum. In Southeast Alaska, Sitka blacktail deer are the primary subsistence food that is hunted by local people. There are thousands of deer that are taken every year by people and that’s what, that is how they fill their freezers and that is what they eat and so wolves can be seen as a source of competition with people for deer.

At the same time, people also do like having wolves in the environment. They find them to be interesting. I think there are some misconceptions about wolves that people have, but we’re constantly trying to learn more about this species and to share this information with people.

Norcross: What about the sea otter part of the equation? I mean they were almost wiped out in Alaska because they were hunted for the fur trade. What about their reintroduction and their impact on the ecosystem and what this new finding might mean for their recovery?

Roffler: Yeah, that’s correct. They have made a remarkable recovery and many people have contributed to understanding how sea otters have transformed the nearshore environment. I think that this study is one of the first that shows that sea otters can also transform the terrestrial ecosystem.

Norcross: Taal Levi, last question is for you. I have to admit, I’m not entirely sure how to feel about this. I mean it’s evolution, it’s survival of the fittest and wolves are gonna be wolves and otters are gonna be otters. But when you think about these two populations and their impacts on the land in Southeast Alaska, is this a good thing or a bad thing?

Levi: Well, I’m not really sure how one defines a good thing or a bad thing. I think it depends on your perspective. It is a thing, though, that we didn’t know happened, that we now know does. And I think it’ll be interesting to see how this unfolds over the coming years, but it might not be a good thing. It leads to lower deer density for deer hunters, for example, but it might be a really good thing for bees and birds and bears.

Norcross: And Gretchen, just to kind of put a coda on this. What research are you doing that might build on these findings about sea otters and wolves? What happens now?

Roffler: We’re going to continue collecting data in this study system on Pleasant Island, in adjacent Gustavus and in Glacier Bay National Park. Our field season starts up in just two weeks. We’re going to be going to Pleasant Island two or three times a week to track the wolves and look for kill sites. And of course we’re going to keep collecting poop.

[Laughter]

Norcross: Of course you are. Thank you both very much. I appreciate it.

Levi / Roffler: Thank you.

Norcross: Gretchen Roffler is a wildlife research biologist at the Alaska Department of Fish and Game and Taal Levi is an associate professor at Oregon State University.

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