Gray wolves were reintroduced to Yellowstone in 1995, restoring a native species and functional
component of the ecosystem.
Heralded as one of the greatest conservation success stories ever, the reintroduction received the strongest public input response in the history of the Endangered Species Act. It set standards, broke records, and raised the conservation bar. It also brought an important ecological concept to discussions taking place in classrooms, around conference tables, and in households across the world.
Trophic cascades in Yellowstone—the idea that the interactions of animals like wolves, elk, and beavers
that are hunting, fleeing from, or competing with one another causes landscape-shaping effects—have
engaged the wonder and curiosity of scientists, policy-makers, and the general public alike. Dozens of
scientists have investigated the idea with hypothesis testing in Yellowstone, though perhaps the best
indicators of widespread interest in the subject are the 21.5 million views that the short YouTube video,
“How Wolves Change Rivers,” received in its first two years on the Internet. Wolves reducing the
willow-browsing elk population, beavers damming streams with the rebounding willows, the landscape
changing as a result of changes to its riparian communities and stream flows—it’s an engaging story that
is both comprehensive and comprehensible to a worldwide audience.
But as with everything in ecology, it’s actually a lot more complicated—and wonderful—than that.
Bison—a significant browser of willow—using a riparian area in Yellowstone’s Lamar Valley. Carter Gowl photo.
Numerous environmental changes—not just the restoration of a top predator—have affected the Greater Yellowstone Ecosystem over the past three decades. A severe 100-year drought punctuated by years of high winter snowpacks;record spring flooding events in the mid-90s; growing populations of bison and grizzly bears; changing fire regimes affected by a changing climate as well as changing policies: these are just some of the factors that have to be considered to fully understand the recent transformation of Yellowstone’s riparian communities.
Long-term drought affects fire regimes which can result in significant ecological change.
Thanks to our diversity of longterm monitoring projects in Yellowstone—as well as our remote sensing
data capturing a satellite's eye view of ecological conditions on the ground—we’ve been observing these
changes and hypothesizing about their causes and effects for a long time.
In addition to the one-of- a-kind wild canine data YERC is known for, we have a multitude of datasets on the ecosystem's other players: Songbirds, geomorphology, stream characteristics, we even have an extensive dataset on small mammal communities—an important level on the trophic chain often overshadowed by the carnivores and elk at higher levels. And we're lucky, as we initiated many of the projects before wolves were reintroduced, forging comparative “before and after” periods in our datasets. Among these are data on Yellowstone’s riparian growth and browsing history, which YERC has been collecting since 1989.
Bears and wolves, geysers and waterfalls, wilderness solitude—of all the reasons people come to Yellowstone, hardly any of them are there for the willows (even though the wildlife everyone enjoys depend on these riparian communities). But at YERC right now, we couldn't be more excited about these bushes and our data showing their response to a changing ecosystem.
After documenting a major seedling establishment event of willows in 1996, we realized that reconstructing decades of past growth and browsing history would provide a timeline of environmental events affecting both the ability of willows to grow and the ability of browsers to browse them. By carefully examining and aging shoots and scars on several thousand living willows—some have grown to trees while others are stunted from heavy browsing—we've documented year-by- year patterns of growth and browsing at plots across the Northern Range. Understanding the years when heavy browsing occurred, the months when browsing peaked, the species that were browsing—elk snip their shoots as if they had a hand pruner, while bison gnaw and tear at the willows—and the species being browsed: all of this is necessary to draw correlations between changes in the environment and responses in the riparian communities. It is also needed to understand secondary impacts on small mammals and songbirds, and their impacts on the wider ecosytem, and so on and so on...
This figure from Don Belile's M.S. thesis studying environmental effects on browsing in Yellowstone's Northern Range shows how even though the number of elk and wolves—their primary predator—has decreased over two decades (red and yellow lines, respectively), the level of browsing (black line) has increased corresponding with an increase in bison (blue line). This is just one example showing greater complexity in Yellowstone's riparian ecosystems than the simple wolf-elk-willow trophic cascade is able to capture.
Using YERC’s data, University of Montana graduate student Donald Belile just completed his thesis,
“Spatio-Temporal Analysis of Ungulate Browsing on Willow (Salix spp.)
Communities Within the Northern Range of Yellowstone,” specifically examining
how snow characteristics have strongly affected browsing and height structure in riparian plant
communities. But using browsing history remains rare in similar projects: Studies on wolf trophic
cascade effects on riparian vegetation have still not considered the timing of willow establishment and
browsing history in their analyses, while winter elk behavior has also been excluded. Fortunately, with
NPS funding YERC began monitoring plots during the winter to determine exactly when, and by what
species—elk or bison—had browsed riparian shrubs destined to be trees. We also are currently
‘counting rings’ on riparian trees to determine exactly what year they grew to a height that elk and bison
could not reach them, called the “release height” since that is when the shoot was released from
But with both research and social interest piqued by these projects and by the “How Wolves Change Rivers” video—much of which was in fact based on YERC's research over the past 24 years—YERC is in an advantageous position to put our longterm datasets, our advanced analysis capabilities, our experience translating science to researchers, policy-makers, and the general public alike, and our experience living, working, and collaborating in Yellowstone, to work helping further unravel these complex, controversial issues. And why not? Good science should be the ultimate arbiter.
Like everything in ecology, the trophic interactions taking place on Yellowstone’s Northern Range are immensely complicated. But that doesn’t mean that we cannot or should not attempt to dive deep and understand and explain these complex systems. Indeed, that is just the job that Adaptive Ecologists are meant for: Look within, attain a deep understanding, translate and then adapt to the ever-changing nature of ecosystems.
Continuing this longterm data collection and monitoring effort is a current priority for YERC, and it’s one area where you can help. Traditional funding mechanisms tend to fund studies for two to four years and sometimes that is hardly enough time to even being to begin to scratch the surface of how ecosystems work. With training and commitment, volunteer opportunities may be possible, while financial support for data collection in the field and data analysis in the lab is greatly needed. Along with creating this invaluable longterm dataset, our goal is to produce predictive models that managers can use and adjust to anticipate scenarios related to changing environmental conditions both inside Yellowstone and beyond its boundaries. We also want to produce popular media products like YouTube videos to bring an already interested public up to the next level of understanding and engagement. As we continue to seek funding sources for this project, we would welcome and greatly appreciate any donations.