Res Extensa #55 :: A story about wolves, trophic cascades, and far-reaching consequences
"When wolves were brought back to Yellowstone, the beavers came back."
I heard this somewhere recently and was fascinated. It's counterintuitive — most would probably think beavers are wolf prey. So how could they rebound with more alpha predators around?
I wanted to find out more about the causal chains that could affect systems so seemingly disconnected. So I did some reading.
From the late 19th century to the early 20th, as we expanded westward and populations grew, gray wolves were considered a nuisance and a threat to livestock. And the locals back then were more concerned with surviving than wildlife conservation. This led to widespread eradication efforts, and by the 1920s, wolves had been virtually eliminated.
Eventually, there were no more wild wolves in the northern Rockies:
The last wolves were killed in Yellowstone in 1926. After that, sporadic reports of wolves still occurred, but scientists confirmed that sustainable wolf populations had been extirpated and were absent from Yellowstone during the mid-1900s.
Their removal had profound and unpredicted consequences on the park's ecosystem, leading to overpopulated herbivore species, degraded vegetation, altered streambeds, erosion, and a series of hundreds of negative impacts. Recognizing the ecological imbalance eventually, the US government undertook a major effort in the mid-90s to reintroduce wolves to Yellowstone.
Over the course of a year in 1995, park rangers captured and moved 2 small wolf packs from western Canada, put them in an acclimation area in the park for a few months, then released them. Today there are around 10 separate packs in various areas of Yellowstone, most descended from the original group from the 90s.
The most direct impact was on the elk population. With the absence of the wolves for several decades, elk populations exploded and spread over areas they traditionally didn't spend time. Elk overgrazing was one of the pivotal events that caused so much negative impact. When the wolves came back, the elk started avoiding wide open areas again, and their numbers diminished. Fewer elk meant more berries and shrubbery, more aspen, willow, and cottonwood trees. More undergrowth meant more rodents and rabbits. More rabbits benefitted the hawks and eagles. Tree regrowth meant more material for beavers to build their dams. Beavers returned and dammed up streams and creeks. This (along with recovering vegetation) stabilized the erosion of streams, and recreated ponds. More ponds meant more otters and fish. About the only species negatively impacted were the poor elk. And maybe some rancher’s cattle now and then.
The story is a textbook example of the profound complexity of natural systems, and serves as a yellow flag against attempts to intervene with hubris about what we can predict and manage. Certainly the officials and park rangers who set out on these "extirpation" campaigns didn't also predict the movement of mountain lions, erosion of riverbanks, or the eagles having less food. It turned out wolves were a "keystone species", as it's called.
You can follow extended chains tracing the far-reaching consequences of 10 wolves and a few decades. The changes compound and spread with time. This sort of non-linear chain of events proceeding from a small change is known as a trophic cascade — a blast radius of events stemming from the addition or removal of a single element.
It's hard to imagine another intervention we could make so small that could have such far-reaching, lasting impact on the natural ecosystem. A single trailer of wolves brought in from Canada created a seismic chain of natural consequences. It wasn't really a change to the natural rhythms. It was really a return to the historical equilibrium of the biome.
Not only are complex systems destabilized by small changes. This example shows how resilient they can be when allowed to find equilibrium. Reinsert the keystone species into the slot and the surrounding, connected systems will rebound on their own.
Humans have an incredible (some would say terrible) ability to bend our surroundings to our needs. But even with a wealth of resources and technology, we can't even predict the consequences, let alone actually muster the resources to mitigate each one of them individually. If we tried to manually interfere to replant the aspens or supply the beavers or restore the riverbanks ourselves, each of those interventions would create its own smaller cascade.
We find keystone elements and cascades like this in other systems, too, not just the zoological. Look at our attempts at quantitative easing, printing currency, or tweaking interest rates on the economy. Or the succession of impacts on supply chain snarls of a few years ago. Or the decades-long chains of urban impacts downstream of zoning laws and failed "urban renewal" projects.
I’m not making a judgment about whether reintroducing wolves, or changing interest rates, or creating a particular regulation is categorically right or wrong. Everything has its own butterfly effect with hard-to-predict consequences. We should make trade-offs and intervene expecting the unexpected consequences. Wikipedia has a whole page on perverse incentives. Attempts to engineer outcomes don’t always work out the way we intend.
This story is a reminder of what can happen when experts tinker with complex systems, thinking they can engineer particular results. Without a doubt, we've gotten better since the 1920s at both recognizing this fact, and at using computational power to guess at probabilities in useful ways.
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