Yellowstone wolves, elk, willows… and beaver

By Kristin Marshall

Everyone loves a good story. But all good writers know that a really great story is a balancing act between creating a trajectory that your audience can connect with, and including all the relevant facts and details. Stray too far in one direction and the story becomes overly stylized and not believable, too far in the other leaves you with a pile of facts and no story at all. The same can be said for doing science.  We want to boil down all of our observations to a simple theory that explains what we observe, but we also want to balance that simple theory with the complexity we know exists in the natural world.

This post is about finding the sweet spot in telling an important conservation success story: the reintroduction of wolves to Yellowstone National Park.

Wolves chase an elk on the northern range.  Credit: Rebecca Raymond, Yellowstone Wolf Project

Wolves chase an elk on the northern range. Credit: Rebecca Raymond, Yellowstone Wolf Project

I have a paper that just came out in Proceedings of the Royal Society B on some of my recently completed PhD research in Yellowstone (co-authored by my two former co-advisors David Cooper and Tom Hobbs). Our work suggests that the reintroduction of wolves has not caused all of the subsequent changes to the ecosystem that ecological theory would suggest.  Instead, we conclude that the story is more complicated and requires a few more characters.

If you watch nature programming on tv or read basic ecology textbooks, you are bound to have come across the Yellowstone wolf story.  Wolves were hunted to extinction in the Greater Yellowstone Ecosystem in the early part of the 20th century.  Decades and decades went by without this top-predator.  As a result, the northern range elk population (the larger of two elk herds in the park) eventually sky-rocketed.  All these elk needed A LOT of food, which is especially limited in the winter, and so the vegetation was overgrazed and subsequently declined.

Trophic cascades in action


This is a classic example of a popular theory in community and food web ecology– trophic cascades.  The story goes like this: top predators (in this case, wolves) prey upon herbivores (elk) and control their population size.  Herbivores feed on plants, and when herbivores are controlled by predation, plants do better. If top predators are removed, herbivore populations increase, more plants are consumed, and overall plants do worse.

Back to our story. In 1995, something really miraculous happened in Yellowstone. There was enough interest and political will to allow Park biologists to reintroduce a few wolves, and then a few more the following year (you can find more backstory in this book).   Wolves quickly became established on the northern range, and their population grew.  They preyed upon the large elk herd, and elk numbers declined (other factors contributed, like people hunting elk outside the park boundaries).

Declining elk numbers should mean that plants should do better, right?  That’s what the ecological theory predicts.  But it turns out the story is a bit more complicated.

So far, I’ve been talking about plants, in general.  Now I want to be specific and talk about willows.  Why willows?  Well, willows make up the majority of the woody plants that line small streams in Yellowstone, and across the semi-arid western US.  Almost every animal that you can imagine needs access to water, and willows provide habitat and cover to those critters. Willows are also an important winter food source for elk, and for beaver (they provide other ecosystem services I won’t go into here).

Increasing complexity with beaver

A beaver swims in Slough Creek.  Credit: Kristin Marshall

A beaver swims in Slough Creek. Credit: Kristin Marshall

What you may not have heard about in those nature documentaries that seem to document the wolves down to every sneeze, is the poor, under-appreciated beaver.  Beaver are fascinating; they cut willow to store as a winter food source and to build dams.  Beaver dams have key feedbacks to willow stands.  They raise water levels behind the dam, giving willow roots easier access to water, and increase flooding, a disturbance required for willow reproduction.

When wolves were eliminated from Yellowstone, beaver disappeared shortly thereafter.  A likely explanation is that as elk numbers increased, they reduced willow stands enough that beaver could no longer make a living.  Beaver require tall, thick willow stems for food and building dams.  Willows taller than about 2 m are big enough for beavers to use, and plants this tall are likely to have stems that are out of the reach of hungry elk mouths. Short, stumpy willows just will not do.

Willows returning?

As you might imagine, the impacts of the wolf reintroduction in Yellowstone has been the subject of much ecological research. The first round of results on willows suggested that the tallest willows were getting taller, and willow stems were growing thicker.  A good initial sign for willows, and maybe even for beavers.

But there was no sign of beaver returning to the small streams that they used to dam. In 2001, my co-authors and a previous graduate student (Danielle Johnston) set up an experiment in Yellowstone to figure out what was more important to willows: the absence of beaver dams, or browsing by large herbivores, like elk.  So, they fenced some plots and built some simulated beaver dams to find out.  I hit the graduate student jackpot when I inherited the lead role on this study in 2008.

One of our experimental sites on East Blacktail Creek

One of our experimental sites on East Blacktail Creek. Credit: Kristin Marshall

After 10 years of monitoring these plants, we showed in our recent paper that willows behind a dam grew just as tall as willows that were protected from browsing.  More importantly, plants that were behind fences for 10 whole years didn’t make it to the 2 m height threshold, on average (remember 2 m is about the height where we think elk, beaver, and willow can coexist).  Instead, we found that it takes both dams and fences to get really tall willows, the kind that can sustain future populations of willows and beavers.


Willow responses after 10 years. (a) Unbrowsed willows grew taller than browsed willows, and dammed willows grew taller than undammed plants (bars = mean heights from the fit model). Unbrowsed, dammed willows exceeded the 2 m height threshold (dashed line). (b) Dams and browsing removal had similar effects on height after 10 years, but prediction intervals on the interaction between the two treatments overlapped zero. (c) Biomass accumulated at similar rates whether or not willows were browsed, but damming plots increased biomass accumulation through time. (d) Dams had a large positive effect on biomass, whereas removing browsing had no effect. (a,c) White bars, undammed; grey bars, dammed. Error bars indicate 95% prediction intervals. (b,d) Heavy line, mean; horizontal bars, 50% prediction intervals; whiskers, 95% prediction intervals. From Marshall et al. 2013 Figure 2.

Does that mean that the wolves are not the saviors of Yellowstone’s willows?  Maybe, maybe not.  Are wolves having a positive impact on the ecosystem? Absolutely.  Does that translate to lush riparian zones? Not yet. The future is always uncertain, but our experimental results suggest that on average, willows may be stuck.  Plants can’t grow taller without beaver dams, but the beavers probably won’t recolonize the streams unless there are enough tall willows.

All is not lost, though.  Beaver populations are slowly on the rise on the northern range in the bigger rivers. And if I know anything about beaver, it’s that they are incredibly industrious and resourceful. And if I’ve learned anything about ecology, it’s that as soon I trick myself into thinking I know exactly how an ecosystem works, someone or something will prove me wrong.  It’s almost always more complicated than we think.  Thankfully, more complicated can still make a good story, it might just take a little longer to tell.


14 thoughts on “Yellowstone wolves, elk, willows… and beaver

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  2. Very nice post Kristin. Your work is very intriguing, and really makes me think about the importance of beavers in trophic dynamics. How can we develop our understanding of top-down vs. bottom-up when we have complicated animals like bears (omnivores) and beavers (ecological engineers) knocking on the sides of the linear food chain? We talk about food “webs”, but in reality, we have a difficult time actually modeling, describing, and understanding non-linear interactions. I look forward to seeing what else you come up with regarding the Yellowstone Ecosystem!

    • Thanks, Jerod! I think your bison photo recognition work is really cool, too. It is a difficult problem, dealing with complexity. I was originally trained in aquatic/marine systems, and there “webbiness” and omnivory tends to be more widely acknowledged, I think. I would guess it’s just a matter of tradition and tools, e.g Ecopath is widely used food web modelling software, typically applied to aquatic communities. Not that similar tools aren’t available for terrestrial systems, it just tends to be a different audience (soil food webs, for example).

      Non-linearities certainly are a challenge in and of themselves. Describing them is one thing, but predicting them in advance is a completely different matter. But, that’s what keeps things interesting, right?

      Thanks again for your comments.

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  4. Wouldn’t it be wonderful if Governor Bullock and the Montana legislators care about science rather than politics, as we were promised when wolves were delisted?

  5. As remarkably interconnected as this system is, I think ecologists will always find beavers to be near the center of things. Even in urban settings we’ve seen an explosion of wildlife since beavers colonized our local stream. I hope your next research follows the topic into the cities to find out what a remarkable resource is thrown away and scoured from the landscape when beavers are trapped. Thanks for your great work.

    Heidi Perryman, Ph.D.
    Worth A Dam

    • Thanks for your note, Heidi! I would love to study urban beavers– it’s not in the cards at the moment, but maybe one day. We have a few industrious colonies in Seattle (where I live) that provide some good viewing opportunities, and some headaches to the parks department I’m sure. Beavers do a lot of great things for ecosystems, but it’s often hard for people to recognize that when they’re damming culverts and flooding roads, etc. Like with wolves, beavers illustrate important conservation trade-offs for society.

  6. Interesting findings! As Heidi said, beavers appear to create biodiversity in systems. I wonder why beavers, who change their surroundings much like humans, do this, but humans themselves do not. In fact, we appear to negatively impact biodiversity. Perhaps we can apply the Lesson of the Beaver in our society.

    • That’s an interesting point, Megan. Although perhaps if beavers were capable of flooding the entire planet, and did so, they would not be increasing diversity on a planetary scale. I do like your term “Lesson of the Beaver,” though, I’ll have to figure out how to slip it into everyday use…

  7. I just had a brief chat with James Estes about this paper and additional trophic levels in similar systems. Carrying these findings further into the stream and aquatic foodwebs really brings out the non-linearities involved in the restoration of ungulate-grazed riparian systems. There is much to consider and there are many experiments yet to be carried out. Good stuff Kristin!

    • Thanks, Nate. I saw on twitter you were going to Estes’ lecture, and wondered if he would talk about Yellowstone since it was in his 2011 Science paper. Agreed on the unknown consequences for streams– I particularly think of Yellowstone cutthroat that face a number of challenges. Definitely many more questions to be asked!

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