Transcript of Interview with Bill Ripple
Professor at Oregon State University in the College of Forestry:

ES: Thanks for talking with Earth and Sky today, Dr. Ripple. Can you give me some background about your study of wolves and their role in the ecology of Yellowstone?

BR: Back in 1997 I became aware of a mystery, a scientific mystery of declining aspen trees in Yellowstone National Park. And this mystery has been going on for decades. And people were uncertain as to why aspen were declining in the park. So, back in 1997 and 1998, I departed for the park with my graduate student Eric Larsen, and we went in studied the declining aspen tree to try to understand why these trees were diminishing over much of the 20th century. So what we did was we did these tree ring counts. We used a coring device, and inserted it into the trees, and pulled out a core, and counted the rings on the aspen trees. And, what happened with that is, after summing this up for the northern part of the park, we found that aspen trees stopped regenerating back in the 1920s, which is about the same time that wolves were eliminated from the park. So there, we developed this wolf hypothesis that states, aspen forest regeneration may be dependent upon wolves being in the ecosystem. So, at that point, we published it, and continued to study the aspen trees. Then in 1995 and 1996, the wolves were reintroduced into the park, and it took some time for the population to build up. And by 1998, 1999 there got to be a large population of wolves in northern Yellowstone, and I started noticing some of the plants that had been suppressed for years, they were starting to grow again. So, Bob Este and I, another professor here at Oregon State University College of Forestry, we got together and started measuring the new growth on the plants – just recently in year 2003 and 2003. And we looked at the growth for both young cottonwood trees and the willow shrubs, and we’re finding new growth since wolves have been reintroduced into the ecosystem. So we asked the question – why are the willow and cottonwood trees growing in some sites and not others? And we wanted try and understand this patchiness of the growth of the plants. So we developed another theory which is based on predation risk. And that’s simply part of the ecology of fear where that the main browsers of willow and young cottonwood trees are elk. They are fearful when wolves are in the area, and the elk have what’s called predation risk sensitivity. So what they do is that they move around a lot more and they don’t’ forage as much in certain areas that are high risk of being preyed upon by wolves. So in areas that are risky on the landscape, these are where we’re seeing the plants growing at the present time. So that’s what’s in the article that was published in the fall of 2003 about the recovery of some of the plants after the wolves have come back. So, to give you a summary of it – we had wolves in Yellowstone up until 1926. And we also had aspen, willow, and cottonwood growing well up until the 1920s. And then we went 70 years without wolves in the ecosystem. And much of the vigor in these plants declined over those 70 years. And there was really no new recruitment of young plants such as young aspen trees and cottonwood trees that wood grow into large trees. So then 70 years after the wolves were eliminated, we had wolves back in the system through the reintroduction program, and now we’re starting to see some of these same plants growing again. So, I think we have a pretty good idea that wolves influence the plants through changes in the main browsers, which would be the elk in the ecosystem. So I think we’re getting close to understanding the mystery of the disappearing aspen, which we really had no idea that wolves would be part of that equation, until we got into it, looking at the tree rings, and looking at the browsing patterns of elk, and then seeing the wolves come back into the system. So it makes quite an interesting win-win story for both the ecosystem, and the elk. Because once these plants start growing in severe weathers, the elk can have more forage for their nutritional needs. So I think we’re seeing the ecosystem come back into balance after such a long period without this top predator available to play its part.

ES: What kinds of changes have happened to the landscape since wolves have been reintroduced to Yellowstone?

BR: Since wolves have been reintroduced, there hasn’t been a lot of time that has passed. Yet this is only seven or eight years since the wolves have been in the ecosystem. But one of the first things that we noticed within the last few years was, some of the plants in just certain parts of the landscape are starting to grow again after many decades of suppression from elk browsing. So, what we came up with is that we think that the elk are changing their foraging patterns based on the ecology of fear, which would say that elk become fearful of wolves in certain types of situations. And let me explain what kind of situation this would be. For example, the risk of being preyed upon would be higher if the elk are in a canyon or along a steep ravine or the meander of a large river, where they may feel trapped if they become spotted by wolves or chased by wolves. So these high-risk places on the landscape are the same places that we’re seeing the plants grow. So we’re early into the release of these plants, but this is the first signal that we’re seeing. For example, let me give you some more ideas on how this works. Near the west end, we’re seeing plants growing taller. There we feel like the risk of predation may be higher. And, along the large ravines – for example the islands in the rivers – the plants may be growing taller. Or along the steep canyons. Any place that would limit the escape possibilities for the elk, they’re not feeling as comfortable according to what we’re seeing to what in the landscape foraging in these sites. Therefore, those are the first places we’re seeing plants growing taller. So now we have some willow that are 12-15 feet above the ground, where before the willow were less than three feet above the surface of the ground. So, quite a flush of vegetation is happening, but it’s only happening in certain spots at this time. And, as time goes on, this will take decades for it to unfold, for things to settle out in this ecosystem, for predator numbers and prey numbers, and the predator-prey interactions. And so we’ll just have to watch for this. It’s a grand experiment that this country has set up. And it’s probably one of the biggest conservation efforts ever taken in the 20th century. And I think we’re just in a great position to just watch and see how it just unfolds over the next few decades.

ES: I don’t quite follow the connection between wolves and tree rings.

BR: The elk will be eating the small sprouts. So the young sprouts are just coming out of the ground, that are maybe less than two or three feet tall, the elk find those as comfortable forage, so they’ll go ahead and eat those before they get a chance to grow into a tall bush or a tall tree, like an aspen tree or a cottonwood tree. So, it’s on the smaller sprouts that they eat. And they prevent the regeneration into larger trees. So that’s how we get this lack of some of the intermediate sized trees that you might expect. Since when wolves were gone, we have this record from the tree rings that there just aren’t any intermediate size or age trees during that period of wolf absence. That’s how the tree ring’s tied into it. But very simply, the elk eat the small, young sprouts.

So, what we did is we took a sample of trees in northern Yellowstone Park, and cored them, and counted the tree rings to determine the date of origin for each one of the trees. So, for example, we could tell if a tree was seventy years old versus 110 years old, just by counting each of the rings in the core. So, each ring represents one year of growth. So, therefore, when we got it all done, we were able to determine the age structure of Aspen in northern Yellowstone Park. And what that shows us is when trees originated. And trees stopped originating in the 1920s, which is the same time that wolves were eliminated from the park. And the way that works with this hypothesis is that the trees were no longer able to grow into tree form. The sprouts were no longer able to grow into tree form because our theory was that the elk would to eat these young sprouts before they could grow into a full sized tree. So that’s how the tree rings tie into the story and explanation of this connection that we have found between wolves, elk, and vegetation.

ES: Okay, you recently published your work linking cottonwoods, elk, and wolves. Can you talk a little more about this study?

BR: Well, I got together with Dr. Robert Beschta who is a professor at Oregon State University in 2001. And what we did is, we wanted to document what the growth is on the vegetation since wolves were reintroduced into the park. So in the year 2001-2002, we went and did a survey in the Lamar Valley of Yellowstone Park and measured the elk browsing intensity on the willow and the cottonwood plants, and also the height of these plants. And then we also looked at what type of landscape these plants were growing in – whether these plants were growing in high-risk landscapes or low-risk landscapes. High-risk being sites that are risky for elk to browse in because they would have a higher probability of being killed by wolves. And low risk would be just the opposite, where the elk would have good visibility and good escape terrain in case there was an attack by wolves. And what we found was that the plants were growing taller, and there was less elk browsing, in these high-risk places on the landscape. And we found just the opposite on the low-risk places where we found more elk browsing on shorter plants. So, what we did was that we wrote this into a journal article and published it in October of 2003, relating to predation risk, wolves, elk, and the willow and cottonwood recovery that we documented.

ES: So, what are some of the things that came out of the study?

BR: Well, these are early results and preliminary results for a smaller area. So this study area was several miles long, and we should be cautious in extrapolating this to the entire northern part of the park, So, we’re continuing to look at the vegetation response in other parts of the elk winter range, and with additional studies we’ll be able to determine if these same trends hold up. But still, I say this is very early in the process, and these results are somewhat preliminary, although the signature is pretty strong in terms of the findings that we have and the photographs that we took show that the plants are definitely growing much taller than they have for decades in northern Yellowstone. We found some old photographs in this part of the park that were taken before wolf reintroduction, and then we re-photographed these same sites after reintroduction, so we could look at before and after. And in six out of the eight sites that we re-photographed, there was significant growth in the woody vegetation. And we’re attributing this directly to the reintroduction of wolves and their effect they’re having on elk browsing.

ES: What role do these trees have in the ecosystem? Are they all that important?

BR: Willows, cottonwood, and aspen are all considered woody brow species for elk and other large mammals that forage on them in the park. But much bigger, these are indicator species of ecosystem health, they’re indicators of biodiversity, and when we have healthy willow shrubs and cottonwood and aspen trees, biodiversity will be flourishing at a much greater level. For example, with more and taller willow and aspen and cottonwood, we’ll have more birds, there’ll be greater opportunity for beaver to be using the wood from the trees in their lifecycle. For example, with the willow growth now in northern Yellowstone, we’re seeing new and expanding colonies of beaver coming into the ecosystem. The beaver are feeding on the willow, and they’re making dams and backing up water, making these ponds, which are good habitat for a variety of all types of wildlife – insects, birds, small mammals, amphibians. So on the whole, I think these plants are a big indicator of biodiversity. These areas along the streams are very important in terms of fish habitat, for example when the willow grow tall over the bank, they hold the bank in place, keeping it from eroding and maintaining habitat for important fish species. They also provide shade on the water, which cools the water, which is important for fish like the cutthroat trout that exist in northern Yellowstone. So I think there’s lots of ecosystem benefits that can be ecological chain reactions from first the reintroduction of wolves, the changes in the elk browsing, the growth of the woody plants, then the increase in the birds and beaver and better habitat for fish, less erosion – so it’s a complete ecosystem story that really shows us the links and connections in the food web and how each one of these components are important for a healthy ecosystem. I must say that the work that I’m doing with my colleagues and the views I’m giving you today are my views. There are other scientists that may have other views as to what the cause is for the growth of the plants going on out there right now. So I’m emphasizing in describing the hypothesis and the theories that I’ve been working on and writing about. I do want to acknowledge that there are other potential explanations that are being considered. For example, changes in moose browsing might be one of the other theories. Or else another one might be changes in winter conditions, winter severity, the changes in snow pack during recent winters. So I do want to acknowledge other theories that are out there, but at the same time I think that we’re on to something. And in terms of wolves affecting the plants, we have some pretty strong direct evidence of that.

ES: So, what are some of the downsides of bringing wolves back into and ecosystem like Yellowstone?

BR: In general, there can be concerns about bringing wolves into an ecosystem for the first time. For example, elk hunters may be concerned that the wolves will be preying upon their game, or there may be a competition problem with the sport industry – would be one thing. Another one would be the wolves occasionally interfere with the private ranches outside the park. And those typically I relate as social-economic concerns. In terms of downside of wolves in the ecosystem, It seems to me that they were there historically, and they’re a natural part of the system, and that there’s no real long-term downside, or problems, that result from having this missing component or link added to the ecosystem.

ES: How many wolves are we talking about that live in Yellowstone?

BR: The number of wolves in Yellowstone, I haven’t studied that specifically. There are other wolf biologists that are concerned with that topic and are studying it. But what I do know is that after the wolves were reintroduced into Yellowstone in 1995 and 1996, the population has been going up, and the wolf density is quite high. On this northern winter range, currently there’s over 80 wolves just in this northern part of the park. And one of the packs was just over 30 wolves – that’s just one pack. So, we’re seeing very high wolf densities. Now, how those compare to historical times, I don’t know and I’m not able to comment on that. But what I’m hearing, that now is that these wolf densities are quite high, and the elk densities are pretty high too, so that may be the reason why there’s so many wolves, is that there are a lot of elk out there.

ES: So, are the elk numbers actually going up after the reintroduction of wolves?

BR: No, the elk populations aren’t going up since wolf reintroduction. They’re actually down a little bit. So, the population is down slightly since wolf reintroduction. But it’s been high, elk densities are some of the highest in North America, and they’ve been high for decades. So this isn’t anything new, it’s just been a place with a lot of elk for a long time, for a good part of the 20th century. And again, these populations are something that I don’t study specifically. There are others that do this kind of work, the population biologists.

ES: Tell me a little bit about the trees and elk, when are trees vulnerable to elk browsing?

BR: For a young aspen tree or a cottonwood tree, they need to grow up to 10-15 feet high before they are above the browse line for elk. But even when they get that high, their main stem is not thick enough, elk can eat the trunk of the tree or push it over, and eat all of the tops out of the trees. So, the trees need to get quite large, to the point where they can’t be pushed over by elk. And, so once they get maybe 3-4-5 inches in diameter at their trunk, and 15-20 feet tall, then I think they are safe from some of the browsing. Although on the other hand, elk are known to eat bark off of aspen trees and they can strip the bark right off of a mature tree and cause injury to that. But in a lot of cases, when they do strip some of the bark off, the aspen are able to seal it up with a scar and the tree survives just fine. In some cases, the tree may develop disease.

ES: What were some of the surprises you encountered in your study?

BR: Well, I think in terms of surprises, after we counted the rings on the aspen tree cores, and we plotted those data, we plotted the graph, I saw that the aspen stopped regenerating into trees within several years of when wolves were removed from the ecosystem, I was just fascinated and surprised at that point. And I knew right there that this was just strong evidence of the link between the wolves and the trees. So that was a big “aha” milestone point in this whole research endeavor.

ES: What are some of your critics saying about your work linking wolves and trees in Yellowstone?

BR: Well, there is some healthy skepticism out there on this, but at the same time there is a lot of support. We’re not the first ones in the world to look at a connection between tree rings and wolves. There are researchers that “looked at the connections”:http://www.isleroyalewolf.org/overview_of_research.htm between wolves, moose, and balsam fir tree rings, and found when wolves were numerous, and the moose were rare, that the trees grew more than when the wolves were rare, the moose were abundant. So when there was abundant moose, these balsam fir trees in Isle Royale did not produce as much. So there is a small body of literature out there, this very same thing in one or two other places. So, I think there’s some good, healthy skepticism, but at the same time we are getting quite a bit of acceptance from both the lay public and the scientific community on our findings.

ES: Is there anything else that you’d like to share with the listeners of Earth and Sky?

BR: One thing that I want to say is that this is a grand experiment that’s happening at Yellowstone National Park at this time. This is the first time in seven years that we have all of the major predators – the large carnivores – in place with the mountain lions, the bears, and the wolves all in this ecosystem. And that what we see here is going to take many decades to play out, in terms of how we can learn and understand how this system works with the full compliment of predators in it. So this is a rare opportunity in the history of this country to be able to watch this. So what we’re seeing early on are just the very first signals of what we call “trophic cascade”:http://www.defenders.org/wildlife/wolf/speakup/benefits.html, the cascading effect of wolves to elk to vegetation. There are many other things that scientists are studying. And, it’s going to take a long time for this to play out. And I think that we’re poised to sit back and learn from nature when it has it’s full compliment of animals. I think we’re in a great position to really learn a lot as to how these natural ecosystems function.

The following person was interviewed for today’s program. Our thanks to:

Willliam J. Ripple
Professor
Department of Forest Science
Oregon State University
Corvallis, OR