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Nature’s Fear Factor

Ecologists argue balance is maintained by predators on the prowl.

When Mozambique’s civil war ended in 1992, more than 1 million people had lost their lives. Another 5 million were displaced. And…By Brandon Keim

When Mozambique’s civil war ended in 1992, more than 1 million people had lost their lives. Another 5 million were displaced. And the carnage was not restricted to humans: Gorongosa National Park, a 1,500-square-mile mosaic of habitats that was home to a richness of life almost unparalleled on Earth, had become a battlefield. Almost every large animal had been killed by soldiers and either eaten or sold. The destruction was so complete that many people doubted whether recovery was even possible.

Just a generation later, it’s a conservation success story, teeming once again with wildlife—but something vitally important is still missing. Apart from lions, other big predators have yet to return. Ecologists studying Gorongosa say that’s created an imbalance, and not only because predators regulate populations of their prey by eating them. “The really interesting idea,” says Sean B. Carroll, “is that predators can shape behavior.”

Carroll is the author, most recently, of A Series of Fortunate Events: Chance and the Making of the Planet, Life, and You. He leads the Department of Science Education of the Howard Hughes Medical Institute and is chair of biology at the University of Maryland. Carroll is executive producer of Nature’s Fear Factor, which tells the story of how ecologists are trying to bring endangered African painted dogs back to Gorongosa. The title refers to how their presence creates what is known as a “landscape of fear”—an ominous-sounding term, but those dynamics may be vital to restoring Gorongosa to its former glory. Nature’s Fear Factor has been nominated for best long-form film in the “Science in Nature” category at the 2021 Jackson Wild film festival.

I recently caught up with Carroll to talk about the film, the science behind it, and in his own life.

predator pile: Ecologists are trying to bring endangered African painted dogs, seen here as a pile of puppies, back to Gorongosa National Park.HHMI Tangled Bank Studios


How did you come to be interested in science and in Gorongosa National Park?

I’m an indoor biologist. I made my living studying genes involved in making animal bodies and the evolution of animal body form. But I was a log-flipper as a kid. I liked to flip things over to see if I could find a snake or a salamander, or scamper into ponds to find frogs.

But a weird thing is that the indoor world of biology, the world of cells and genes, doesn’t meet the world of ecologists and paleontologists. We’re generally housed in different departments. We publish in different journals. We go to different meetings. And as the conversation about the fate of the biosphere grew more intense, I really felt ignorant.

I don’t care if people understand the basis of DNA. I’d rather they understand what species do in their communities.

As I started to meet more ecologists, and I learned that even some of the most protected parts of the world were showing major stress, I really wanted to understand more. I was interested in the science; I was interested in first principles. And when I came across work on trophic cascades and keystone species—the fact that predators, or lack of them, have many dramatic and indirect effects on ecosystems—at first I was somewhat embarrassed that I didn’t know about it. But then I realized that many other biologists didn’t know about it, either. In 2015 I visited Gorongosa for the first time. That wound up in my book, The Serengeti Rules, which highlighted those discoveries. I am an enthusiast for the people who are undertaking the hard work of restoring and protecting what we’ve got left.

You were saying how the principles of ecology are not well-disseminated through the scientific world. Isn’t that crazy?

One hundred percent.

Here’s a discipline of science devoted to understanding how life on Earth works—and it’s off in the corner somewhere.

You can look at a syllabi of general biology courses from high school and college, and often ecology gets short shrift. It just doesn’t make any sense. I don’t care if people understand the basis of DNA. I’d rather they understand what species do in their communities and start addressing their energies to figuring out how we’re going to reverse the tide.

Was there any crosstalk between the world of ecology and your own field?

There was. I learned the story of sea otters in kelp forests on the Pacific coast: sea otters, by consuming sea urchins, allow kelp forests to flourish. It’s what we refer to as double-negative logic. I’ve seen so much of that inside cells and in molecular circuitry.

That example captures in a nutshell what people need to know about trophic cascades, but could you elaborate a bit more on the science?

There’s a great phrase from a physicist in the early 20th century that the purpose of science is to explain the visible with simple invisibles. I’ve always loved that.

When you look out your window at nature, or go to a true wilderness, it’s overwhelming. It seems chaotic. What are the rules? But if you can discover some of those hidden connections between creatures in the system—if you understand that the otters, by that simple act of consuming urchins, allow the kelp forest to flourish, which is then habitat for fish and invertebrates, and that they’re food for birds and seals—then you recognize all the knock-on effects.

I don’t think that we’re trying to recreate the past as much as reestablish some kind of stable present.

What are ecologists trying to accomplish by reintroducing painted dogs to Gorongosa?

Because of the extermination of large predators in Gorongosa, herbivore populations were numerically out of balance. Behaviorally they were doing things that were odd. The film shows how bushbuck—which are named bushbuck because they’re often hiding behind termite mounds in little patches of foliage—were wandering out onto the floodplain and eating different food. And then waterbuck, another ungulate, were at least tenfold higher in number than they had ever been.

While lions made a comeback in Gorongosa, other predators were missing. Historically wild dogs had been there, so the thought was to introduce wild dogs. There would be two benefits: One, restoring some of the balance between predators and herbivores. The other, because Gorongosa is a relatively secure place, was that it would be a stronghold for this species in Africa. There are only six or seven thousand painted dogs left. They need some well-protected habitat.

The dogs would be doing something by consuming prey. But the really interesting idea of this landscape of fear is that predators can shape behavior, too. If you have to be aware of danger, you behave differently. In the case of grazing animals, they spend less time grazing. They must have their heads up a lot. They have to think about camouflage or cover. They may not be able to eat some of the foods that would otherwise be available. Evolutionary biologist Rob Pringle describes it as staying in their lane. In Gorongosa, the idea was that a lot of herbivores had gone out of their lanes, and that the presence of predators might knock things back into balance.

Just to push back a little, I can imagine someone saying, “Well, maybe balance is in the eye of the beholder.” When we talk about bushbuck doing something unnatural, it just means something we haven’t seen them do before. Is it necessarily bad? Is it something that needs to be corrected?

Those are great, tough questions. The perturbations to Gorongosa have happened in the last 50 years and were certainly human caused. Really, Gorongosa was broken. By the year 2000 wildlife was very scarce, and many doubted whether Gorongosa could come back at all. So in many ways, people are thrilled that Gorongosa now abounds with wildlife. But now, you look around and say, “OK, what’s going on?”

There’s no hyenas where there used to be. There’s only one transient leopard that’s been seen from time to time. And the sense was that the huge over-proliferation of waterbuck and lack of fear among other antelope were not normal. There was a case for bringing back painted dogs, which we knew were there before, and seeing whether that was both good for the system and good for the dogs.

There’s always a risk that we are tinkering too much or trying to get things back to something that may no longer exist and may not be able to exist again. But this experiment was not done in a void. Probably the most famous reintroduction experiment is Yellowstone, where wolves were introduced after a 70-year absence.

Wolves influenced the number of elk and also their behavior. And there are knock-on effects on vegetation and on other creatures like beavers, which have their own influence on the ecosystem. And from both small-scale experiments that have looked at the ecology of fear with spiders and grasshoppers, to various reintroductions of predators in Africa, the sense was that this was something worth trying.

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Yellowstone is the archetypal example of trophic cascades and landscapes of fear. But it’s also been a subject of contention. The criticism is that it’s not entirely clear whether changes in elk and beaver populations were consequences of the wolves or other factors. I want to believe in that beautiful, straightforward cascade—but how confident are we that it will happen?

There’s lots of moving parts. I don’t know that there’s any kind of trophic cascade expected from the dogs where the change in bushbuck diet is going to yield a major change in Gorongosa vegetation. It’s going to change bushbuck diet, which might change the way bushbuck compete with some other browsers and grazers, which might influence the ability of some other things to get better established.

I don’t know what they expect to see in terms of major vegetative or hydrology changes in Gorongosa, the way that’s alleged to have happened in Yellowstone. But you certainly now have another predator. And these are coursing predators, which are different than ambush predators like lions. Painted dogs and lions are divvying up territory because they are natural enemies.

I would imagine that as lions and painted dogs apportion the landscape to themselves, then you would get different kinds of vegetation depending on their different predation influences. And even if it’s not this big dramatic Yellowstone-of-our-dreams cascade, it can still be a good thing, right? It doesn’t need to be all that complicated to be positive.

I think that’s a good outlook on it. I don’t think that we’re trying to recreate the past as much as reestablish some kind of stable present. In the film, we also wanted to bring attention to the whole body of thought around the landscape of fear. Fear has been a part of ecosystems for very long periods of time. It may be an even stronger factor than consumption in terms of regulating animal numbers and the balance between various populations. Hopefully that’s a revelation people take away from the film.

It does seem that our ancestors didn’t necessarily try to wipe out all the large predators that might give them trouble from time to time.

I wanted to ask about that. There has been debate among researchers studying landscapes of fear over how to define the term. Some people emphasize the felt experience of fear by prey. Others say it’s not so much about living in fear, but rather a mental map of risk perception. Like, there’s some neighborhoods I don’t go to because bad things would happen to me—but I’m not in constant fear of them. How do you think about what a landscape of fear is?

Creatures have to be aware of their surroundings. And how vigilant they are may depend on their encounters with predators. If they’re in a herd, obviously there’s lots of mechanisms that animals use to warn other animals that there may be danger afoot. So it’s not necessarily fear where cortisol is at the maximum and they’re just living stressed-out lives. I don’t know that we know if it’s physiologically a constant state of fear as much as it is a state of awareness. Awareness that may be shared, for example, among all the members in a herd. And it’s just woven into the fabric of their daily lives. It could just be a landscape of awareness—but that’s not a compelling movie title.

Going in the other direction with fear: What about the human fear of big, scary dogs? Wherever people talk about reintroducing or protecting large predators, there’s always ecological and logistical challenges, but ultimately the greatest challenge may be sociological.

The dogs apparently don’t have a lot of interest in human habitats. That’s not where their prey is. The hope is that dog conflict will be relatively minimal. Unless there’s a lot of livestock there’s not a big attraction, and the livestock densities are not that great. There’s more pickings inside the park. I’m under the impression that the dogs are not interested in humans. But if that were to change, we could have a crisis.

I’ve got a folder on my computer of stories involving cultures of coexistence between humans and big predators. Do you think something like that could emerge to help people and painted dogs live alongside one another?

As I understand, in Mozambican culture, people do identify with animals. There may be a better word for it, but the idea is that there’s a strong connection with certain animals. I would love to know how deep that goes in human history. It does seem that our ancestors didn’t necessarily try to wipe out all the large predators that might give them trouble from time to time. Perhaps the understanding was that they were a part of the world, and you just didn’t do that. But as we as a species moved across the world, we found wolves and bears and lions and tigers and everything inconvenient—and we nearly didn’t understand their place in these ecosystems. There’s been a huge rethink of the place of apex predators in ecosystems. Hopefully that rethinking is in time to reverse the tide.


Brandon Keim (Twitter / Instagram) is a science and nature journalist. He is presently working on Meet the Neighbors, a book about what understanding animals as fellow persons means for human relations to wild animals and to nature.

Lead image: Still from Nature’s Fear Factor, a NOVA production by HHMI Tangled Bank Studios for WGBH Boston.

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