Think Out Loud

Humans have been relocating beavers for a century. We are only now learning some of the effects.

By Rolie Hernandez
Sept. 9, 2021 3:50 p.m.

Broadcast: Thursday, Sept. 9

A beaver in Bannister Creek Greenway.

A beaver in Bannister Creek Greenway.

Tualatin Hills Parks & Recreation District


When we think of beavers many of us think of the dams they build, but along the Oregon coast, some just aren’t doing that. Little research has been done to explain why, but it is a part of a larger story of relocation and reintroduction of Oregon’s state animal. Beavers have a long history within the United States, and over the years, the animals have been extensively relocated. They’ve been moved by bus, train and even parachuted out of planes from Wyoming to New York. Oregon State University’s Dr. Clint Epps and Vanessa Petro wanted to know what genetic effect all that relocation has had on these large rodents. They join us to share the results from their latest study on coastal beavers.

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. Oregon may be the Beaver State, but there is still a lot that we don’t know about these industrious creatures. That’s especially true in certain habitats like the steep slopes and fast water of the coast range. But now we know more about beavers in this area thanks to a new study by scientists at Oregon State University. The results were published recently in the Journal of Wildlife Management. Clint Epps is the lead author of the paper. He’s a professor of Mammalian Ecology and Population Genetics there. Vanessa Petro is a senior faculty research assistant in the OSU College of Forestry. Welcome to you both.

Clint Epps: Thanks very much.

Dave Miller: Clint Epps, what were the big questions you wanted to answer for this study?

Clint Epps: Well, we were really interested in understanding dispersal. So, how do beavers move around the landscape? And that matters because that influences where beavers show up, whether they can colonize habitats that are currently empty. Once they’re in these different places, how much do they interact, which is important for population health, for maintaining genetic diversity and for understanding things like the spread of diseases. So we were looking for tools to address that in some pretty tricky terrain where it’s not easy to directly measure long distance movements.

Dave Miller: Vanessa Petro, my understanding is to answer the questions essentially to get genetic samples from hundreds of beavers. You had to trap them, 200 or so of them. And this was one of your jobs. How do you do it?

Vanessa Petro: Yes, that’s a great question. It takes a lot of effort and time and coordination. So, as many folks are already aware with our study results, our study area itself, in terms of where we implemented live trapping capture to collect these DNA samples, I mean our study area was almost as the size of the state of Vermont. And so we were coordinating with watershed councils, federal agencies, state agencies, small and private landowners trying to figure out where we could find all the known beaver colonies that were currently occupying our study area as fast as we could because essentially we had only one summer to run three live trapping lines to collect as many samples as we could throughout this area.

Dave Miller: Do you essentially have to think like a beaver to trap a beaver?

Vanessa Petro: Yes. In fact, that’s one of the great things I enjoy about this type of work is that not only do you learn how to recognize their behavior but also how to interpret it. And the better you are at interpreting beaver activity and their behavior, the more likely you are to increase your successes of filling your live traps. So for us in research, it’s important to have as many samples as possible for any of our studies.

Dave Miller: Clint Epps, in the big picture, what did you find when you looked at the different beaver genomes from different parts of the coast range?

Clint Epps: Well, in this study, we were using a sort of a small number of genetic markers. So we weren’t looking at whole genomes, but these markers can help us understand how much the beaver are interacting across these areas. And there’s really only one other study that’s looked at gene flow, as an index of dispersal for beavers in North America that I’m aware of. And they didn’t find a lot of genetic differentiation over hundreds of kilometers. In this landscape, which is highly dissected by all these steep drainages, we actually found a lot of genetic differentiation across the system. So first of all, that’s interesting, it’s a much more complex system. Beavers aren’t just moving freely all the way up and down the coast range.

Dave Miller: Meaning that they stay, in general, they’re staying in a more narrow band of land?


Clint Epps: Yeah, what we found is that genetic variation tended to cluster by watershed, but, and this is also a key point, there is clearly some dispersal occurring between watersheds. So, we did a bunch of different things to understand that. We looked at some correlation of genetic similarity over distance. We ran models looking at the influence of landscape on genetic structure. We also tested for related individuals. And those tests aren’t perfect, but we infer related individuals occurring distances from one kilometer out to many, up to about 15 kilometers and occasionally many more kilometers apart than that. So they are moving around. So it’s also an interesting place because there are a lot of translocations. Hundreds of beaver were moved into the coast range starting around 1940. Big efforts before 1950 and then subsequently, still some movement and we see some evidence that those translocations had an impact. We see some genetic signature that I think is tied to those translocations. But it also looks to me like there were native beaver there probably the entire time and that those natural dispersal processes have been occurring.

Dave Miller: When you say that you can actually recognise these different populations, for example, of native beavers who have been in a particular area for probably hundreds of years or ones that were introduced maybe within the last 90 years. How big a genetic difference are you talking about?

Clint Epps: Probably not a genetic difference that matters in terms of adaptation. This is on the order of differences at the DNA fingerprinting level. So we can see suggestions that, okay, these beavers at one time were part of a different gene pool than these over here. But I wouldn’t expect there to be really important adaptive differences at this point. They come from different habitats, so you can’t rule out that there wouldn’t have been some differentiation originally, but things have mixed now, and so I wouldn’t anticipate any observable difference between these populations at this point.

Dave Miller: If you’re just tuning in, we’re talking right now about beavers in the Coast Range and also beavers more generally. Clint Epps is a professor of mammalian ecology and population genetics at OSU. Vanessa Petro is a senior faculty research assistant for the College of Forestry at OSU.  Vanessa Petro, one of the things that have, to be honest, that I learned from your report and from articles about it, is that a lot of the beavers we’re talking about here don’t make dams. I have to say. I just thought that was an integral part of beaver life, that they lived in dams. That was part of what made beavers beavers. So what are beavers doing if they’re not making dams?

Vanessa Petro: Well, that’s a great question. And you’re not alone on that thought because honestly, that is such a huge misconception when it comes to American beaver that a lot of people actually get wrong and between our various studies and then even other studies that have been conducted across their range here in North America, a lot of people are finding that not all beavers contribute to dam construction. And so that’s something that’s definitely of interest here in our study area, particularly in Oregon itself because there’s a lot of desire to work with beavers for stream habitat enhancement purposes.

Dave Miller: Right to actually introduce them to re-tangle streams or to change whole aquatic ecosystems.

Vanessa Petro: Right.

Dave Miller: And you’re saying that you could put a beaver in a certain watershed and they may not make dams and may not change their landscape?

Vanessa Petro: Correct.

Dave Miller: So why, why would they make a damn? And why wouldn’t they?

Vanessa Petro: So that comes down to the cues associated with when beaver exhibit that damming behavior. And so there’s two primary reasons that beavers construct dams. One is to increase water depth for protection from predators. And two, it’s also to increase the amount of impounded water to gain more access to other food resources. So you can assume that if both of those needs are already being met, then there’s no need for a beaver to expend that energy and those resources to construct them. And so what we’re seeing with other research here in western Oregon is that most of the population, I should say the majority of the population, are actually not contributing to their construction. And that even includes beaver family units or colonies that are already occupying stream regions that are considerable or considered suitable for dam construction. So, it kind of leaves this question of all right, well, is the reason why they’re not constructing dams, is it just simply explained by the fact that their needs are met? Or is there more to the story there? There’s certain habitat characteristics associated with why they’re not potentially damming in areas that we desire that dam construction activity or are there other factors impacting that?

Dave Miller: Clint Epps, we just have about a minute and a half left. But what does your research suggest about how ecologists should think now about introducing beavers in the Coast Range or anywhere else and where they should be doing that?

Clint Epps: Yeah, that’s an important implication from the study. So, in this Coast Range system, based on the results of these analyses, the study, I think beavers have demonstrated that they will probably find their way into most habitats that they’re interested in. And it may not be obvious when they do that, if they’re not building dams. If people want to translocate beavers to address wildlife conflict, beavers often plug culverts or cut down trees that people want. We’ve recommended that those translocations occur within the same watershed. So then you’re not moving them out of sort of the region where they’d be likely to move in and we think that’s good practice for not spreading diseases and things like that. Alternatively, if you’re in one of these pretty dissected landscapes and you’re trying to restore beavers where they really are absent, you’re probably going to need to bring them into each major watershed because it will take a while for them to move between watersheds. So we think this will be useful for management at even places whether we already have lots of beavers or in places where they’re really trying to do more wholesale restoration.

Dave Miller: Clint Epps and Vanessa Petra, thanks very much.

Clint Epps: Thank you.

Vanessa Petro: Thank you for having us.

Dave Miller: Clint Epps is a professor of Mammalian Ecology and Population genetics at Oregon State University. Vanessa Petro is a senior faculty research assistant for the College of Forestry at OSU.

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