Creating Resilience: Trophic Cascades and Climate Change by Moderator Cristina Eisenberg

Discussion Topic:

The Wolf's Tooth: Keystone Predators, Trophic Cascades, and Biodiversity]

Ecologist Robert Paine summed up the effects of keystone predator removal by saying, “You can change the nature of the world pretty simply. All you need to do is remove one species.”

In The Wolf’s Tooth, I compare ecosystems from which carnivores have been removed to a game of Jenga. This game involves removing wooden blocks one at a time from a tower without causing it to collapse. As you remove them, the tower starts to teeter, loosening some of the blocks, making them easier to remove. Eventually the whole system collapses.

In an ecological game of Jenga in an aspen forest, you might begin by removing species that may seem redundant. If you remove the black-capped chickadees first, not much happens, the system continues functioning more or less as usual. The number of chestnut-backed chickadees increases to fill the gap left by the black-caps. If you remove short-tailed weasels next, the mouse population initially increases, but then the coyotes start eating the surplus mice, as do the northern harriers, and mouse numbers go back down to the level they were at before you removed the weasels. A casual observer might not notice much missing. If you continue by removing white-tailed deer, the other ungulate species carry on, filling in the gap by producing more elk and moose.


Aspen with barren understory
in a wolfless area in the
Northern Rocky Mountains/ Photo credit: Cristina Eisenberg

However, when you remove predators in your ecological game of Jenga, the whole system starts to teeter. Ungulate numbers explode, because cougars and bears can’t put much of a dent in them. The ungulates start running out of food. Birds that nest and feed in the aspen canopy’s mid-story leave. With sustained intense browsing, shrubs and wildflowers disappear. And in less than a decade you are left with an impoverished aspen stand that contains maybe one third of the species it once did.

Resilient, healthy understory in an area
with abundant wolves in the
Northern Rocky Mountains/
Photo credit: Cristina Eisenberg

Can you think of other keystone species removals that could cause ecosystems to tip into “alternative” states?

When you put the impacts of wolf removal in the context of climate change, things become even more serious. Scientists Evelyn Hutchinson and E. O. Wilson have stated that there is no such thing as a redundant species. All species have a role, and multiple species of the same type of animal or plant exist to ensure ecological stability and resilience. Keystone predators increase biodiversity. Accordingly, landscapes from which keystone predators have been removed will experience a loss of stability. As global changes in climate occur, which involve sudden weather events or catastrophic fires, such systems may lack the resilience to adapt to change and continue to function in a healthy manner.

How do normally occurring disturbances (e.g., fires, floods) different from the disturbance patterns associated with climate change?

Metaphors work well to explain the concept of ecosystem resilience. In their seminal book Panarchy, ecologists Gunderson and Holling use the metaphor of a raft to describe an ecosystem. The resilience of the raft depends of course on its biophysical context. But it depends equally on its occupants—which may be human—their objectives, and their social institutions.

How does the concept of ecological resilience relate to resilience in human societies?

Biodiversity loss has become a crucial issue in the past two decades, as human-caused ecosystem modifications continue to precipitate extinction. Human actions that create islands of habitat in a sea of development exacerbate the effects of keystone predator removal. Saving all the pieces, as Aldo Leopold put it so long ago, can do much to help ecosystems stay resilient in the face of climate change. But doing so, as we will discuss next week, involves our social and political institutions, public policy, and humans opening their minds and hearts.

How can we incorporate the concept of resilience into natural resources policy?

Yellowstone and Beyond: Fear as an Ecosystem Engineer by Moderator Cristina Eisenberg

Discussion Topic:

The Wolf's Tooth: Keystone Predators, Trophic Cascades, and Biodiversity]

Imagine that you are walking alone in downtown New York City at 11 p.m. You move quickly and don’t linger, looking over your shoulder regularly, your senses on high alert. In this high-crime city, every shadow, every dark alley contains the potential of danger. Now imagine you are walking downtown in the small town in Kansas where you grew up. It is 11 p.m. Your pace is relaxed, as you stroll from block to block. You pause and look at shop windows and take your time. And so it is for elk when wolves are in an ecosystem versus when they are not. Ecologist Joel Berger termed this effect the ecology of fear.

The ecology of fear has deep roots. Staying alive during the early Pleistocene involved escaping large creatures with sharp teeth and claws, such as saber-toothed tigers and dire wolves. Thus prey species evolved behavior driven by survival. Vigilance—time spent head up, looking for threats—is essential for survival in systems with apex predators, but comes at the expense of time spent eating. Fear inspired by the threat of predation also influences feeding choices animals make—and this in turn can affect how plants grow.

Vigilant elk/

Photo credit: Cristina Eisenberg

For example, in Yellowstone National Park, William Ripple and Robert Beschta found that wolves keep elk more wary and alert, thereby reducing consumption of plants such as aspen and willows in places that have escape impediments. And also working in Yellowstone, researcher John Laundre and his colleagues described landscapes inhabited by apex predators such as wolves as landscapes of fear.

How does fear shape ecosystems?



Alpha female wolf on an elk carcass

in a high predation risk area/

Photo credit: Dave Moscovitz



As a scientist, one of my leading research questions has to do with whether elk avoid risky places (thick forests, downed wood, steep embankments) in which it is more difficult to detect and escape an apex predator, such as a wolf. To answer this question I put in 180 miles of transects in Glacier National Park, Montana, and Waterton Lakes National Park, Alberta, in which I measured elk presence. One morning, when I resumed work in an area where the previous day I had put a transect into thick forest and quantified it as having high predation risk, I found an alpha female wolf standing in my transect, feeding on a freshly killed elk. She calmly looked up at me and my field crew, the heart of the elk in her mouth. The following year I found that elk were not spending much time browsing on the aspen in this particular area, thereby enabling young saplings to grow into adult trees. This vivid demonstration of the ecology of fear in action provided me with a powerful reminder of how wolves and other apex predators can be ecosystem engineers.

Wolf on a carcass/Parks Canada/

Photo credit: Cristina Eisenberg

What are the long-term consequences of the ecology of fear?

How might managers simulate the ecological effects of fear in landscapes where it is impractical to have apex predators?

Do you think cougars have different effects on prey behavior than wolves?

Why or why not?

Why the Earth is Green: Trophic Cascades on Land and Water by Moderator Cristine Eisenberg

Discussion Topic:

The Wolf's Tooth: Keystone Predators, Trophic Cascades, and Biodiversity]

We have been discussing the powerful and essential ecological link between apex predators, their prey, and the foods prey eat. Based on the revolutionary ideas of Hairston, Smith, and Slobodkin, who in 1960 ingeniously proposed that the world is green because predators limit their plant-eating prey, trophic cascades science has since then explored the consequences of predator removal from ecosystems worldwide.

In all ecosystems, researchers have found a strong link between predator removal, plant community simplification, and reduced energy flow. Lacking apex predators, ecosystems become capable of supporting fewer species, because the trees and shrubs that create habitat for these species have been over-browsed. With top predators in them, they contain richer and more diverse habitat, and thus can support a greater number of species such as songbirds and butterflies. Apex predators are thus thought to exert top-down effects on ecosystems.

A picture is worth a thousand words. The pictures below illustrate the effects of predation—and predator removal.

Without Sea Otters (Photo credit: Jim Estes)

With Sea Otters (Photo credit: Jim Estes)

For example, in Alaska Jim Estes and his colleagues have found that removing the sea otter, the apex predator in the Aleutian archipelago, causes sea urchins to increase in number to the point that they greatly reduce kelp forests.

Kelp forests provide habitat for many other species, such as fish. Returning sea otters to the ocean helps reduce sea urchin numbers, and in turn enables kelp to thrive.

Predators Absent (Photo credit: John Terborgh)

In Venezuela, John Terborgh found that on islands too small to support large predators, such as jaguars and eagles, leaf-cutter ants eventually denuded the forest floor, greatly diminishing habitat for songbirds and other species.

Predators Present (Photo credit: John Terborgh)

In all ecosystems, researchers have found a strong link between predator removal, plant community simplification, and reduced energy flow. Lacking apex predators, ecosystems become capable of supporting fewer species, because the trees and shrubs that create habitat for these species have been over-browsed. With top predators in them, they contain richer and more diverse habitat, and thus can support a greater number of species such as songbirds and butterflies. Apex predators are thus thought to exert top-down effects on ecosystems.

Aspen Recruitment Gap

In my own research in Glacier National Park, Montana, I have found that since wolves have returned, aspen formerly browsed to death are now growing above the reach of hungry elk. The gap in aspen linked to wolf removal and their subsequent return, called a recruitment gap, is one of the trophic cascades patterns written on the landscape I study in the Northern Rocky Mountains.

Where else might these effects be manifested? And in places where apex predators might not be tolerated, how can managers create healthier ecosystems?

Other effects, besides predation, also make energy flow through ecosystems and enable vegetation thrive. Some of these effects, which can be caused by environmental factors such as moisture and sunlight, are termed bottom-up effects.

How might climate change, a bottom-up effect, contribute to apex predator effects, a top-down effect?

Aldo Leopold and the Mark of the Wolf’s Tooth by Cristina Eisenberg, Moderator

Discussion Topic:

The Wolf's Tooth: Keystone Predators, Trophic Cascades, and Biodiversity]

In the early 1900s, while cruising timber as a young forester, American conservationist Aldo Leopold, founder of the science of wildlife biology, encountered a female wolf with her pups. The common wisdom of that era was that the only good predator was a dead one, so he and his crew opened fire. But as he stood there watching the “fierce green light” fade in the wolf mother’s eyes, he felt a sharp, surprising pang of remorse. It would take him decades to parse out his feelings about her death.

In 1935, Leopold bought an abandoned farm in southwestern Wisconsin as a hunting reserve. Today known as the Leopold Memorial Reserve, this land, which he and his family dubbed “the shack,” became the site of some of his deepest lessons about the ecological value of predators. In shack journals between 1939 and 1940, he noted that deer were nipping plants and trees down to eighteen inches in height. In a 1940s game survey, he found that humans had eliminated wolves throughout North America, causing an explosion in deer and elk numbers, and resulting degradation of forests through over-browsing. Meanwhile, back at the shack, deer calmly stood their ground in the absence of wolves, chronically browsing tender young saplings to death.

Near the end of his life, in possibly his most famous and poignant essay, “Thinking Like a Mountain,” Leopold reflected on his early encounter with that mother wolf and the wildlife management implications of her death: “While a buck pulled down by wolves can be replaced in two or three years, a range pulled down by too many deer may fail of replacement in as many decades.”

Today ecologists refer to the powerful ecological link between predators, their prey, and the foods prey eat as trophic cascades. We have learned much about the conservation value of keystone predators, which have the ability to touch everything in a food web, thereby creating healthy, resilient ecosystems. Now more than sixty years since Leopold’s death, we are actively restoring these predators to oceans, streams, forests, and prairies worldwide.

Trophic cascades are an ecosystem’s stories writ large upon terrestrial and aquatic landscapes. Please join me over the next month to explore some of the ecological stories I write about in The Wolf’s Tooth. Ecosystems speak to all of us—researchers, managers, students, and plain members of the biotic community—through the glyphs left by the interactions of keystone predators and their prey. These stories are part of the rubric of conservation and can inform an alternative to single-species conservation. And if we play close attention, they will tell us what to do as we strive for sustainability in our rapidly changing world.

Here are some questions to which you can respond via the "Comment" link below.

How do keystone predators structure ecosystems

What are some of the patterns in an ecosystem linked to trophic cascades?

In places where we can’t have keystone predators, do you think hunting by humans can be used to create similar effects?

British ecologist Charles Elton referred to the food web as an “integrated economy” in which members exchange energy. Beyond keystone predators, their prey, and the food prey eat, what other kinds of exchanges could be created by trophic cascades?

How have Elton’s and Leopold’s ideas about the ecological value of predators influenced wildlife management today?

Acting Like a Mountain by Will Stolzenburg

I add my apologies for the electronic gremlins who made off with several rich and thoughtful comments from yesterday’s discussion. And my thanks to the intrepid WildRead team who so gallantly rescued them from the ether.

I also want to introduce next week’s moderator, Anne Post, the U.S. Fish and Wildlife Service's Conservation Librarian. Anne and the WILD READ team will be your guide as we take what may seem an epic leap of subject—on levels of taxonomy, geomorphology, and philosophy—as we go from the wolf of Aldo Leopold’s Thinking Like a Mountain, to the Anthill of E.O. Wilson. But of course there’s a common heart to these stories, as Anne and the rest of you will no doubt discuss.

But for the last post on Thinking Like a Mountain, I’d like to raise a prickly issue that comes from that cynical and skeptical chamber of my journalist heart. We’ve talked about philosophy and ethics and thinking like a mountain. But how many of us really live like a mountain? Do we gravitate to Leopold’s poetic vision of compassion and community as a nice, soothing ideal, but as an impractical way of life in the crowded, dog-eat-dog world we’ve inherited? And what about the convictions of Leopold himself?

I’m speaking here of a point Curt Meine brought up a week ago in his opening post on this blog, about discovering the date that Leopold actually shot the now legendary heroine of Thinking Like a Mountain. It’s an important question, because until recently, none of us—not even Curt, who wrote the book on Leopold—could be sure the shooting actually happened. I first spoke with Curt about that incident when I was writing my book, which examined the consequences for life on earth from losing our great predators. An essay written in 2005 by a biologist from Berkeley named Dale McCullough—which unfortunately may never have been published—raised the question of whether Leopold did indeed shoot the iconic wolf in that canyon on that fateful day. The records were missing. Leopold, the consummate journaler, apparently hadn’t recorded it anywhere that scholars like Curt Meine could detect—until recently. Curt emailed shortly after my book came out—my published position on the matter sitting squarely and cowardly on the fence–that he had found Leopold’s smoking gun. “Just recently some new Leopold family letters surfaced,” wrote Curt. “One of them contains the first extant documentary evidence of the wolf-killing incident.”

So it turns out Leopold did shoot the wolf, in 1909. As a journalist and admirer of Leopold’s thinking and writing, I’ll admit I was relieved to hear that one of the conservation movement’s most iconic martyrs had not been manufactured for the sake of good story. But there remains that unfortunate gap, those years between the shooting and Leopold’s actual change of heart. As McCullough so baldly stated: “Leopold's conversion about the inherent value of predators didn't come until the middle of the 1930s, some 20 years later.” And as we all know, Thinking Like a Mountain was written in 1944. Leopold’s legendary line, as he peers into the fading green fire of the wolf’s dying eyes—“I realized then, and have known ever since, that there was something new to me in those eyes—something known only to her and to the mountain"—is, to be charitable, a bit of a stretch.

So I wonder, do we grant Leopold his poetic license, ignore the messier details, and accept the larger lesson of Thinking Like a Mountain as a sincere and achievable goal in our ethical evolution? Or do we chalk it up as one more piece of hypocrisy that helps explain why we’re losing the campaign for wildness in this world?