Between 2017 and 2020, University of Washington scientists placed GPS collars on dozens of bobcats and coyotes as part of a study looking at how the presence of wolves, cougars and humans affected their behavior and mortality rates. One of the bobcats the researchers tracked in Northeastern and Central Washington is shown in this photo after it was fitted with a GPS collar in February 2020.
Courtesy Zachary Wardle
Scientists at the University of Washington placed GPS collars on dozens of bobcats and coyotes and tracked their movements and behaviors in Northeast and Central Washington. The animals moved from areas uninhabited by humans into rural communities to escape being killed by wolves and cougars with whom they compete for food. But the strategy ultimately proved lethal, as the bobcats and coyotes were three to four times more likely to be killed by people than by those apex predators. Laura Prugh is an associate professor of wildlife ecology at the University of Washington and the lead author of this study which was published last month in the journal, “Science.” She joins us to talk about the research findings and what they say about the so-called “human shield effect” on predator-prey relationships observed in Yellowstone and other national parks.
The following transcript was created by a computer and edited by a volunteer:
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. As wolves continue to reintroduce themselves onto western lands, scientists at the University of Washington had a big question: what will this mean for predators that are lower on the food chain? Basically, if wolves, or cougars for that matter, move into an area, how do bobcats and coyotes respond? It turns out, according to research published recently in the journal science, these smaller predators skedaddle. They get out of the way of apex predators by going into areas more dominated by humans. But then, their strategy backfires as humans end up killing them.
Laura Prugh is an associate professor of wildlife ecology at the University of Washington, and the lead author of this study. She joins us now to talk about it. Welcome to the show.
Laura Prugh: Thank you, it’s great to be here.
Miller: What exactly did you set out to learn with this new study?
Prugh: Well, the main goal is to find out how large carnivores like wolves and cougars affect smaller ones like coyotes and bobcats. These midsize carnivores that we refer to as mesopredators are really important in food webs because they tend to have really diverse diets. What happens with these critters can really have a big effect on the whole ecosystem.
Miller: How much did researchers already know about what happens, or what were your suppositions for the effects that the largest predators have?
Prugh: Well, there have been quite a few studies that have shown that larger carnivores will sometimes kill smaller carnivores. But on the other hand, the smaller carnivores often scavenge from large carnivore kills. So there’s prior evidence to suggest that the relationship could kind of go either way. But one key thing to note is that most of these studies that have looked at interactions among carnivores within the community have happened in protected areas like national parks where there’s either no or very little human hunting of these animals. How humans would affect these dynamics is really not well known at all.
Miller: So for this new study, you focused on different kinds of lands?
Prugh: We had two study areas in Northern Washington, but there were no protected areas in either one, meaning no national parks or areas where hunting activities were restricted. All four of these carnivores were subject to human caused mortality in both of our study areas.
Miller: Can you just give us a sense for the mixture of lands, what they were used for, what they were like?
Prugh: Sure. Both study areas were fairly typical of what you find in rural western areas where there is a mix of private land that included ranch land, farm land, small towns, and private timber land, as well as a lot of forest service land and tribal land as well.
Miller: So even though this is Northern Washington, what you’ve just described would actually be a pretty good description for huge swaths of Oregon as well, and big parts of the rest of Washington.
Prugh: Yeah, I don’t think there was anything particularly unique about these study areas.
Miller: Can you describe how you set up this research?
Prugh: Well, it relied primarily on putting radio collars on all of these species. We captured the animals using a variety of techniques that were approved by our animal care committees at the university. We put GPS collars on them that record their locations every four hours. And then if the collar is still for six to eight hours, the collar will transmit a mortality signal. Typically, the only reason why a collar would be still for that long is if the animal is dead.
And so once we’d get a mortality signal, we’d go to the site as fast as we could and do a mortality investigation to figure out why and how the animal died.
Miller: There is a pretty fascinating video on the university website announcing this research, it’s sort of a time lapse video of a bobcat getting collared. I guess you have different methods to collar the different animals, but what do you do for bobcats?
Prugh: Well for bobcats we would use cage traps, just a large wire mesh trap that we put some bait, like a chunk of beaver meat or something like that, in the back of it, and some scent attractant. They really like shiny objects, so sometimes we’d hang a compact disc on the end of a stick hanging from a string, or as was shown in the video, basically a cat toy, a fluffy ball of tinfoil-like material. And that attracts the bobcats, and then they investigate the trap and hopefully go in.
Miller: I understand it’s actually more challenging for coyotes to catch them and collar them. Why?
Prugh: Coyotes are extremely wary, and extremely smart. I’m not saying bobcats aren’t smart, but relative to coyotes, it’s much harder to catch them. A coyote would generally not go anywhere near a trap that’s set like I just described for a bobcat. They generally don’t go into cage traps. For coyotes, they need to be captured either net gunning or darting from a helicopter, or with a foothold trap or a snare.
Miller: All right, so you in a variety of ways collared all these animals and then let them go around and move into each other’s habitat or move away. What did you find?
Prugh: So when we looked at their movements, the wolves and cougars very strongly avoided areas with a high level of human activity. We were using something called the human footprint index as a measure of human modification and presence on the landscape. And so cougars and wolves both avoided areas with a high human footprint quite strongly.
And for the bobcats and coyotes, they avoided them strongly, but there is a little bit of variation there. When cougars and wolves were around, the coyotes and bobcats shifted their movements towards areas with twice as high of a human footprint index value. So they pretty strongly shifted their movements towards areas with more people. And the only real logical way to explain that behavior is it indicates that the mesopredators perceived the large carnivores to be a greater threat than they thought people were. So when the large carnivores were around, they shifted towards areas with more people.
Miller: But your study shows that they actually greatly misjudged that, right? What did you find when they went to areas with more people?
Prugh: So that’s what the movement information showed us. But then as I said, the callers let us see what the causes of death were. And when we looked at causes of death, bobcats were four times more likely to die from humans than they were from large carnivores, and coyotes were over three times more likely to get killed by people.
Miller: So they’re trying to escape from territories of wolves or cougars, going to where humans were, and then humans were killing them. But is it possible to know if these bobcats or coyotes would have had a better chance of surviving if they had stayed in the territory of wolves? In other words, would wolves or cougars have gotten them anyway, maybe gotten them in the same numbers if they had stayed?
Prugh: That’s a really fascinating question, and I wish that we could look at that. I was surprised at how few mesopredators were killed by wolves and cougars because we’ve radio collared coyotes up in Denali National Park in Alaska, and wolves were a main cause of mortality. In that landscape, coyotes really have no option of escaping wolves because one wolf territory abuts against another one. They use these areas on the borders of wolf territories to try to minimize their risk, but they can’t get away from them quite as much as they can down in these landscapes, like in Washington and Oregon, where there’s areas that wolves just absolutely won’t go in.
I do expect that, say coyotes and bobcats had just stayed put, that some of them might have been killed anyway just by wolves and cougars instead of people. But, it also can be a lot harder for the animals to accurately judge the risk from humans.
Miller: And so why is that?
Prugh: We use these technologies that have been developed just in the past several hundred years, like guns and modern traps and such. When an animal is caught or shot, often a human isn’t actually within range of the animal to perceive them. A long range rifle, someone can be quite far away to where a coyote might not see or hear or smell them, whereas if a wolf or a cougar is close enough to be an actual threat to a coyote or bobcat, there’s going to be much more reliable cues for them to clue in on to alert them.
Miller: If you’re talking largely about rifles here, then I’m assuming that people are intentionally killing coyotes and bobcats as opposed to, say, running them over by mistake.
Prugh: Right. We radio collared 72 total, only a single one was hit by a car. So they seemed to be remarkably skilled at navigating roads. There were definitely roads and a fair amount of traffic in these areas. 14 coyotes were killed by people, one was hit by a car, and then 13 were shot. And for the bobcats, 11 were killed by people and 8 were trapped and 3 were shot.
Miller: Why are people killing bobcats? I guess I know that coyotes are not that much of a beloved species right now, there’s fear of maybe predation or eating chickens or maybe together going after smaller livestock. I don’t think of bobcats as being a huge threat to cattle, say. Why are people killing them?
Prugh: Well, that’s certainly true, it would be quite a bobcat that could take down a cattle, and even coyotes couldn’t do that, possibly a calf.
The main reason that the bobcats were killed is because they are considered valuable fur bearers. Most of the bobcats were trapped, and that was for their fur. There were a few that were shot, and a couple of those were getting into chicken coops.
Miller: My understanding is that your new findings complicate something known as the human shield effect or the human shield hypothesis. What is this? And how do your new findings change it?
Prugh: This idea of a human shield effect, that is a phenomenon that has been found in areas like national parks where sometimes prey animals will start using areas along hiking trails or areas that humans are pretty common to try to avoid predation. Even in a national park, carnivores tend to be a lot more wary of people than prey animals tend to be. And so that has been shown to actually lead to higher moose calf survival in one study, or elk calf survival in Banff, where the elk will kind of congregate in town and have their calves. So there’s a few cases like that where it seems as though some of these prey species are successfully associating with people to reduce their predation risk. And so that’s called a human shield.
What we found is that when you’re outside of a national park or a major city where people tend aren’t gonna be shooting these animals on site, when you’re in a rural area where there is a much higher risk of getting killed by people, then this human shield can kind of backfire, or lead to being killed by people rather than the predator.
Miller: Coyotes and bobcats are pretty common, that doesn’t seem like their populations are endangered at all right now. But are there other middle of the food chain predators that might behave in similar ways, might flee from cougars or wolves, but whose populations could be more at risk if they then end up being killed by humans?
Prugh: I think it certainly could be possible. One example that comes to mind is that in Washington in 15 years we’ve reintroduced fishers, which are considered a mesopredator. They’re in the weasel family, a bit larger than a marten and smaller than a wolverine. They’re a forest carnivore that is pretty sensitive to predation by larger carnivores like bobcats and coyotes. And so if these reintroduced fishers are moving towards a human impacted areas to avoid those other carnivores, then it could lead to a similar issue. For fishers, they’re more threatened by things like rodenticide poisoning in terms of humans, humans don’t tend to directly kill them, and I believe that that would be illegal in Washington because they’re protected.
It is also possible because you have this kind of dominance chain. Fishers actually are sometimes killed by cougars, but more often killed by bobcats. So it is possible if the presence of these large carnivores is actually limiting the populations of coyotes and bobcats, which are the dominant mesopredators, that could end up actually having a beneficial effect on some smaller carnivores like foxes and and fishers, or more rare carnivores like lynx that compete with bobcats and coyotes for food, they all like to eat snowshoe hares. So I think there are some positive effects basically limiting the abundance of these dominant mesopredators that could be happening.
Miller: Laura Prugh, thanks very much.
Prugh: Thank you.
Miller: Laura Prugh is an associate professor of wildlife biology at the University of Washington and the lead author of this study that found that middle of the food chain predators, like bobcats and coyotes, tried to evade cougars and wolves by going into human dominated landscapes, only to be killed in greater numbers by those humans.
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