Aldo Leopold was an American writer, professor, environmentalist and conservationist - but not his whole life. He spent his youth working to eradicate one of the most efficient predators in North America. When Aldo Leopold was young, he believed what many wildlife managers and hunters still say today:

Fewer predators = more deer
More deer = better hunting
Therefore: killing predators helps both people and “game.”

So he shot hundreds of wolves. One old female in particular changed his mind about the role of predators in North America.

On a rocky New Mexico hillside, Leopold and his colleagues fired into a pack. When they reached the old female, he watched “a fierce green fire dying in her eyes” and realized, as he later wrote in his essay Thinking Like a Mountain, that “neither the wolf nor the mountain agreed with such a view.”

That moment cracked open a truth our policies still haven’t caught up with:

You can’t simply remove predators, hand humans a rifle, and expect nature to work the same.

Human hunting will never be an adequate stand-in for predation, and if we care about forests, rivers, songbirds, and even the long-term health of deer and elk, we need predators back in the story.

Aldo Leopold wrote that essay in 1949. We’ve known for 75 years that removing predators harms ecosystems, and have done very little to introduce predators back into their native regions, with the exception of the Yellowstone Wolves, of course.

Let’s unpack why.

1. Predators don’t just “eat deer.” They structure the entire food web.

Ecologists use the term trophic cascade to describe what happens when changes at one level of the food web (like top predators) ripple through herbivores, plants, and even soil and water. In a classic top-down cascade, predators keep herbivores in check, which allows vegetation to recover, which then supports more insects, birds, and other life. Remove that top level and the whole structure warps.

A 2017 study on an island ecosystem put it bluntly: removing an apex predator triggered a trophic cascade that extended “from the ocean to the land.” Once the predator was gone, herbivores exploded, plant communities shifted, and ecosystem processes changed across habitats.

Predators shape systems in at least three big ways:

Numbers
They reduce prey populations, especially in years or places where food is short.

Behavior (“the ecology of fear”)
Wolves, big cats, sharks and others change where prey feel safe to feed, how long they linger, and when they move. That can give sensitive areas (like streambanks or young forests) a chance to recover even when total prey numbers aren’t dramatically lower.

Evolution
Predators tend to catch the young, the old, the injured and the unlucky. Over evolutionary time, that pressure shapes agility, vigilance, herd behavior and other traits that keep populations robust.

When we erase predators, we’re not just subtracting “X wolves” from a system. We’re pulling out a stabilizing pattern that touches everything else.

2. What happens when predators vanish? Forests, rivers, and birds pay.

Yellowstone: the famous (and complicated) cascade

In Yellowstone National Park, wolves were eradicated by the 1930s. Without them, elk numbers surged; at one point there were roughly 18,000 on the northern range. Elk browsed young aspens and willows so heavily that for decades, surveys found essentially no new aspen trees growing above browsing height.

Wolves were reintroduced in 1995. Since then, multiple studies have documented changes in elk numbers and behavior, plus big vegetation shifts. A 2025 paper in Forest Ecology and Management found that in Yellowstone’s northern range:

Aspen sapling density increased more than 152-fold between 1998 and 2021

For the first time in over 80 years, many stands had new young trees with trunks >5 cm diameter at chest height

In other words, after wolves came back (alongside predation by bears and cougars), the aspen forests finally began to regenerate.

Not every scientist agrees that wolves alone “saved” Yellowstone’s aspens; some 2024–2025 studies emphasize that climate, bison, elk hunting outside the park, and other factors matter too, and point out areas where aspen recovery is still limited.

But taken together, the evidence does support the idea that bringing back large carnivores helped break an 80-year logjam in forest regeneration. That’s a trophic cascade in real life: predators → elk → trees → beavers → birds → rivers.

Europe & eastern North America: deer without enemies

Yellowstone is not unique. Where large predators are absent and people protect or subsidize herbivores, similar patterns pop up.

In Switzerland’s strictly protected Swiss National Park, wolves and other large carnivores were absent for most of the 20th century and hunting is banned. Red deer densities climbed sharply after the 1920s and stayed high (around 21 deer per km² in one study area). Heavy browsing on spruce and Swiss stone pine saplings led to poor forest regeneration and a skewed age structure, with very few young trees.

In eastern North American forests, decades of overabundant white-tailed deer have:

Stripped forest understories

Prevented many tree seedlings from reaching the canopy

Reduced the abundance and diversity of understory plants, especially woody species

One natural experiment in British Columbia showed that high deer densities resulted in strong declines in songbird abundance, likely because deer consumption of shrubs removed nesting sites and reduced insect habitat.

So when we talk about “too many deer,” we’re often really talking about:

Too many deer for a landscape with no functioning predators and lots of human-provided food and edge habitat.

You can try to fix that entirely with rifles. But it’s like trying to retune an orchestra with one very loud trumpet.

3. Human hunters are “super-predators,” not ecological stand-ins.

There’s a popular myth that human hunters “replace” natural predation. The story goes something like: wolves are gone, so hunters keep deer numbers in check, which keeps forests healthy. Convenient, simple… and wrong in crucial ways.

A 2015 global analysis in Science looked at more than 2,100 predator–prey exploitation rates and coined a term many ecologists now use: humans are “super-predators.”

Some key findings:

Humans kill adult prey at much higher rates than natural predators, with median exploitation rates up to 14 times higher in some systems.

We also kill large carnivores (wolves, bears, big cats) at rates up to 9 times higher than those carnivores kill each other.

We don’t behave like just another predator. We behave like an extremely powerful, highly selective force that reshapes populations in ways nature never did.

Who we kill: big, healthy, impressive

Natural predators tend to pick off:

The young

The old

The injured

The out-of-place

That strengthens populations by removing individuals least likely to survive and reproduce.

Human hunters, especially in systems built around trophy values, prefer the exact opposite:

Largest antlers or horns

Heaviest bodies

Prime-aged adults

A growing number of studies show this size-selective harvest is changing the evolutionary trajectory of wild populations.

Work on bighorn sheep in Canada found that intense trophy hunting (favoring rams with the biggest horns) led to a genetic decline in horn length over time. When heavy selective hunting was reduced, the decline stopped, but it didn’t reverse.

In fisheries, the same pattern shows up in dramatic fashion: a 2025 study on eastern Baltic cod found that overfishing of larger individuals has helped drive the average mature body length from 40 cm in 1996 down to 20 cm in 2019, effectively halving the size of mature cod. The authors found genomic evidence that fishing pressure has reduced gene variants associated with large body size.

We are not “culling the weak and leaving the strong.” We are often culling the strong and rewarding traits that help animals avoid us (smaller bodies, earlier reproduction), not necessarily traits that make populations resilient in the long run.

When and where we kill: seasons and access, not food-web feedback

Predators are on the landscape all year. They hunt where prey are abundant, where they’re easiest to catch, and when conditions are worst for the prey (deep snow, drought, etc). Their impact is continuous and tied to ecological feedback.

Hunters are constrained by:

Short, fixed seasons

Access (roads, private land, ruggedness)

Social preferences (close to town, nice weather, weekends)

That means even high hunting pressure can fail to reduce browsing where it matters most. Studies in Europe show that deer control targeting broad areas doesn’t always prevent localized overbrowsing of young trees and shrubs, because animals simply shift activity into less hunted zones or times.

And we don’t create the same fine-scale landscape of fear. Large predators can make specific valleys, streambanks, or thickets feel risky for prey, giving plants in those spots crucial breathing room. Fear of hunters, by contrast, often just shifts activity to nighttime or deeper cover, not necessarily away from sensitive plant communities.

So even if harvest statistics say “X% of the deer herd removed,” the ecological impacts of that removal can be very different from the impacts of wolf or cougar predation.

4. Predator control doesn’t always “help the deer” anyway.

Here’s the other twist: even if you accept the narrow goal of “more ungulates for hunters,” decades of predator removal haven’t always delivered.

A 2021 analysis in the Journal of Applied Ecology reviewed predator control programs aimed at boosting ungulate numbers. The authors found that predator removal had highly variable and often modest effects on ungulate survival and recruitment, with outcomes depending heavily on context and the broader food web.

Sometimes killing predators lowered predation in the short term. In other cases, it led to:

Mesopredator release (smaller predators increasing)

Compensatory mortality (other causes of death filling the gap)

Limited or no long-term increase in ungulate densities

In plainer language: you can spend a lot of money and bullets and still not get the “more deer” outcome you were promised, while degrading the ecological role of predators and stirring up new problems.

5. Why we actually need predators back

Bringing predators back isn’t about romanticizing wolves or demonizing all hunters. It’s about putting the right players back in the right jobs.

Predators are uniquely good at:

Regulating herbivore numbers in tune with food availability

Shaping where and how long herbivores browse

Maintaining the evolutionary fitness of prey populations

Supporting forest regeneration, river stability, and biodiversity through trophic cascades

Humans, meanwhile, are uniquely good at:

Completely erasing predators from landscapes

Killing large animals at unsustainably high, selective rates

Convincing ourselves that this is “management” rather than a massive experiment in rewiring evolution and ecology

We don’t need to remove people from the system. We need to change our role:

Allow or actively support the return of wolves, cougars, bears, and other large carnivores where habitats can sustain them

Protect and reconnect habitats and migration corridors so both predators and prey can move

Shift hunting away from pure trophy selection and toward ecologically informed harvest (age/sex structures that don’t undermine genetic resilience)

Accept that a truly healthy landscape will sometimes mean fewer deer in the short term and more forest, birds, and long-term stability

Leopold’s “green fire” moment wasn’t just regret. It was a realization that mountains, forests, and rivers “think” at scales the human ego struggles to grasp. Our job isn’t to replace predators. Our job is to make room for them to do their work, and then design our own lives, economies, and recreation around that reality.

Human hunting can be one tool in a toolbox.
Predators are the operating system.

If we want rivers lined with willow and aspen instead of bare banks, forests with young trees and singing birds instead of over-browsed silence, and deer and elk herds that are robust a century from now, we don’t need a bigger rifle.

We need to let the green fire back onto the mountain.

Sources & further reading

Leopold, A. “Thinking Like a Mountain,” in A Sand County Almanac (1949).
https://ecotoneinc.com/ (Ecotone, Inc)

“Trophic cascade.” Wikipedia and ecology textbooks summarizing predator–prey cascades.
https://en.wikipedia.org/wiki/Trophic_cascade

Morris, T. et al. “Removal of an apex predator initiates a trophic cascade that extends from the ocean to the land.” Scientific Reports (2017).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5345092/

Painter, L. E. et al. “Changing aspen stand structure following large carnivore restoration in Yellowstone National Park.” Forest Ecology and Management (2025).
https://www.sciencedirect.com/science/article/abs/pii/S0378112723001234
https://www.washingtonpost.com/science/2023/04/15/yellowstone-wolves-aspen-regeneration/

Fluri, J. The influence of wild ungulates on forest regeneration in Swiss National Park (Master’s thesis, 2023).
https://www.mdpi.com/ (search for thesis or related articles)

Habeck, C. W. & Schultz, B. C. “Community-level impacts of white-tailed deer on understory plants in North American forests.” Ecosphere (2015).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4562469/

Allombert, S. et al. “A natural experiment on the impact of overabundant deer on forest birds.” Biological Conservation (2005).
https://www.sciencedirect.com/science/article/abs/pii/S000632070400336X

Clark, T. J. & Hebblewhite, M. “Predator control may not increase ungulate populations in the long term.” Journal of Applied Ecology (2021).
https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2664.13958

Darimont, C. T. et al. “The unique ecology of human predators.” Science (2015).
https://www.uvic.ca/science/biology/people/faculty/darimont/index.php
https://phys.org/news/2015-03-unique-ecology-human-predators.html

Pigeon, G. et al. “Intense selective hunting leads to artificial evolution in horn size.” Proceedings of the Royal Society B (2016).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4807444/

“Overfishing has caused cod to halve in body size since 1990s, study finds.” Science Advances coverage in The Guardian (2025).
https://www.theguardian.com/environment/2025/mar/12/overfishing-cod-body-size-halved-study

BioInteractive. “Trophic Cascades & Keystone Species” educator guide. HHMI BioInteractive
https://www.biointeractive.org/classroom-resources/trophic-cascades-keystone-species