Could Restoring Animal Populations Store More Carbon?

16:27 minutes

a group of sea otters on their backs floating in the ocean over some sea gass
Southern sea otters rest on a bed of seagrass in Morro Bay, California. These sea otters play critical roles in natural food webs, keeping important ecosystems like kelp forests and seagrass beds in balance. Credit: Lara Drizd/USFWS Pacific Southwest Region/Flickr

Did you know that land and ocean ecosystems absorb about half of the carbon dioxide we emit each year? But what if the earth had the capacity to absorb even more? With the help of some furry, scaly, and leathery critters, maybe it can.

A recent study in the journal Nature Climate Change claims that by restoring the populations of just a handful of animals—like gray wolves, bison, and sea otters for example—the Earth could capture around 6.41 more gigatons of CO2 each year. This idea of restoring wildlife is called rewilding.

Ira talks with the co-author of this study, Dr. Trisha Atwood, an associate professor at Utah State University, based in Logan, UT. They chat about what critters make the rewilding list, and how they fit into the carbon cycle.

Further Reading

Segment Guests

Trisha Atwood

Dr. Trisha Atwood is an associate professor at Utah State University based in Logan, Utah.

Segment Transcript

IRA FLATOW: Did you know that Earth’s land and ocean ecosystems absorb about half of the carbon dioxide we emit each year? Take mangroves, they’re one of the best carbon sinks around. They store carbon in their long, thick roots and branches and trap it away in the sediments and soils around them. Or grasslands, they hide away most of their carbon underground in their tangled web of roots, and in the soil, which is another carbon sink in and of itself.

The Earth has an incredible capacity to sequester carbon. But what if it could absorb even more with the help of the critters living on the land and in the sea? Well, a recent study in the Journal Nature Climate Change found that restoring the populations of just a handful of animals, like gray wolves, bison, and sea otters, for example, if that could be done, the Earth could capture nearly 6 and 1/2 more gigatons of CO2 each year.

This idea of restoring wildlife is called rewilding. Joining me to talk about this study and what rewilding could look like is co-author of the study, Dr. Trisha Atwood, associate professor at Utah State University, based in Logan, Utah. Welcome to Science Friday.

TRISHA ATWOOD: Thank you, Ira. I’m super excited to be here.

IRA FLATOW: Nice to have you. Let’s start with a definition. What does rewilding mean?

TRISHA ATWOOD: So for us, we specifically called it trophic rewilding in our paper because we’re interested in some of the higher order animals. So things like wolves, sharks. So trophic rewilding is to restore or protect animal functional roles in ecosystems. And what that essentially means is either protecting or restoring animals to ecologically meaningful densities so that they’re doing things in their ecosystems like they used to before humans came on the scene.

IRA FLATOW: And when you say they’re doing things, how do these animals then fit in to the carbon cycle by what they’re doing?

TRISHA ATWOOD: Yeah. So especially top predators– so things like sharks and wolves– they can have amazing influences on the behavior of their prey. Their prey oftentimes eats things like plants. We all know that plants are a really important aspect for accumulating carbon or sequestering carbon. So the way that these predators change the behavior of these herbivores can influence how much carbon is being accumulated and then stored long term within these ecosystems.

IRA FLATOW: So if you have the bigger animals eating the smaller animals that eat the plants, then the plants remain?

TRISHA ATWOOD: Yeah, exactly.

IRA FLATOW: There’s a list of animals in this study that could have the most impact on carbon storage. Tiger sharks are on it. What other critters are on that list? And how do you decide on them?

TRISHA ATWOOD: So we decided on this list based off of some research that has already been done and shown that these animals can significantly increase carbon accumulation– or carbon storage– within the habitats that they live in. And so the animals that we have chosen are wildebeests. Wildebeests are really important because they can eat a lot of grasses. And that helps suppress fires in savannas.

Sea otters– sea otters are really great. A lot of people know the story about sea otters coming back and how bringing back sea otters help suppress sea urchins along the coast. And as a result, we got our amazing kelp forests back, especially along the coast of California.

Wolves– most people are familiar with Yellowstone and bringing wolves back and how that’s really changed Yellowstone as a whole ecosystem by influencing the behavior and the densities of elk and other deer.

Tiger sharks, as you already mentioned.

IRA FLATOW: Yeah, tell me about the tiger sharks.

TRISHA ATWOOD: Yeah, tiger sharks, other animals, their prey are scared of sharks just like we are. And sometimes they choose herbivores. Things like dugongs and sea turtles and herbivorous fish will choose to not forage in an area or forage at different times because they are fearful of sharks and being caught out by a shark and of course eaten by it. So that can change where they forage and how they forage.

IRA FLATOW: I never thought of that.

TRISHA ATWOOD: Yeah, it’s a really cool study.

IRA FLATOW: That’s kind of interesting. So they’re scared of some of the animals so they don’t go eat the vegetation where those animals are hanging out.

TRISHA ATWOOD: Yeah, exactly. And we’ve seen this in places like Shark Bay, Western Australia, which is aptly named because it has some of the highest tiger shark densities in the world. And we see there that sea turtles and dugongs forage very differently in what we call these high-risk areas, where they’re likely to lose in a fight against a shark. And they instead forage in these more safer areas, where they can see a shark coming more and evade a shark if one is starting to attack them.

IRA FLATOW: That is cool. Now, let’s talk about how big of an impact rewilding can make on the climate. Give me some numbers or a timeline.

TRISHA ATWOOD: Yeah. So you hit it on the head earlier. So we gave a back-of-the-napkin or a first calculation of, if we were to bring back or protect just a handful of animals– so this included wildebeests, sea otters, wolves, sharks, many pelagic fish species that have declined due to intense fishing, African elephants, bison, and baleen whales– that we could sequester about 6 and 1/2– 6.4– gigatons of CO2 per year more. And that’s really significant because that number is extremely close to the 6.5 gigatons of CO2 per year that we would need in negative emissions to keep us below 2 degrees Celsius if we were to reach net zero.

IRA FLATOW: That’s amazing. And do any climate models account for the animals?

TRISHA ATWOOD: They don’t really currently. There’s a couple of new models coming out where people are interested in including animals. But traditionally, NASA models, all of our predictions about climate change, none of those include animals.

IRA FLATOW: I understand now. How do you figure out, then, the exact impact one species could have? It seems like that takes a lot of math.

TRISHA ATWOOD: It definitely takes a lot of math. But I assure you, it takes no more math than what we’re already doing in terms of figuring out plants– and especially microbes. If you think, we’re already including microbes into these calculations, and we can’t even see them.

IRA FLATOW: Right. I understand that. What about ecosystems? Are there some better at carbon storage than others?

TRISHA ATWOOD: Yes. I’m a little bit biased because I’m a marine scientist. And so I, of course, love talking about marine systems. And it just so happens that coastal vegetated systems, which includes sea grasses, tidal marshes, and mangrove forests, are some of the best at storing carbon, both in terms of how fast they accumulate carbon and how long they can store it. So they can store carbon sometimes 100 times faster than a temperate forest can. And they can store it for tens of thousands of years.

IRA FLATOW: That is just amazing. So OK, let’s get down to practicality, if I may. How would this work? I imagine it’s more complicated than just moving some animals around.

TRISHA ATWOOD: Yes, exactly. It’s a very complicated thing. Rewilding, of course, has a lot of other aspects that we have to think about that include both the animals and the ecosystems, but also the communities that are around those– and by communities, I mean people– and who is going to benefit and who may not benefit from protecting or restoring an animal population.

And so we have to think carefully about what the organism is that we’re going to rewild. Can it go back to its ecosystem in a way that we think it is going to? Or has that habitat been so modified by other human activities that, even if we bring that species back, it’s not going to have the same effects that it used to have because that system is no longer the same?

IRA FLATOW: OK. So give me some examples of rewilding that have gone right– that have worked.

TRISHA ATWOOD: Yes. So one example is wildebeests. And so wildebeest populations severely declined as a result of a disease that was actually brought in through cattle. And because wildebeest populations declined so much, we saw that savannas overgrew with grasses. And those grasses catch fire really, really easily. And they help things burn longer and hotter. And as a result, the burning of that grass led to higher CO2 emissions.

When the wildebeests started to come back after we protected their populations, we saw that fires were more suppressed in savannas. And as a result, those savannas were holding more carbon than when the wildebeest populations were really low.

IRA FLATOW: That’s interesting. I’ve always seen film, movies, videos of sea otters frolicking in kelp forests. I mean, are they also an example of things that could go right?

TRISHA ATWOOD: Yeah, sea otters are absolutely a organism that could go right– or has gone right. So sea otters love to live in kelp forests. And they were hunted almost to extinction as a result of their fur. And after they were put on the Endangered Species List and their populations started to come back, we saw that kelp forests also started to come back. And that was because the sea otters, as their populations grew, were starting to eat more and more sea urchins. Sea urchins’ favorite food along the coast oftentimes was kelp.

And so when those sea otters weren’t there, the sea urchins were eating all the kelp. But now that the sea otters are back, they’re keeping those sea urchins in check and our kelp forests are beginning to flourish.

IRA FLATOW: Wow. Who would have thought? You don’t think about one or two levels up or down the chain, do you?


IRA FLATOW: It really helps. Are any of the animals that we’re talking about on this list endangered? Does that make a difference?

TRISHA ATWOOD: Yes, it does make a difference. So we do have some endangered animals. We also have some animals that are protected through other policies. So for example, sea otters are protected through the Marine Mammal Protection Act. And baleen whales are also protected as part of that. And that’s one of the animals that we also listed as being significant. That influences it because it suggests that there’s already policies in place, saying that we need to be focusing on restoring these populations to some set level, to some trigger level.

And so that’s great. Because it means that we have already laid the foundations for doing so. It’s much harder to probably bring back species that aren’t listed because there’s not policies in place to force people to have to do that. And some of these organisms, or course, are somewhat contentious in bringing back– things like sharks and wolves.

IRA FLATOW: Yeah. Well, that’s what I was going to ask next, about cases where rewilding has gone wrong. Are there cases there?

TRISHA ATWOOD: Yes. So the wolves in Yellowstone has been a trigger point for a lot of states that butt up against Yellowstone National Park, especially because we have ranchers that are very concerned about wolves moving outside of the Park and starting to decimate their livestock. Which, of course, is really important for their livelihoods.

And so we can’t control the boundaries of animals. We can rewild them, but we can’t force them to stay in natural habitats. We’re not creating a zoo. And there is a chance that they will move outside of those habitats and we will get into human-wildlife conflict.

IRA FLATOW: Yeah, I want to get back to something you mentioned before. And that’s the huge social component in rewilding, especially when people’s businesses or livelihoods are affected– like if they’re ranchers or fishers– or for Indigenous nations that have their own practices. How do you take all of this– how do you take all of them– into consideration?

TRISHA ATWOOD: Yeah. One of the most important thing whenever making a management or conservation decision is to make sure that all of the stakeholders are at the table. That includes your ranchers, your conservationists, your fishermen– if you’re talking about marine systems– any Indigenous cultures that would be influenced by that decision, and make sure that everybody there has an equal voice so that we can really look at, what are the potential problems? What is the potential gain? Is there any way that we can potentially mitigate the problems that we’re seeing?

So for example, with the wolves, we might be able to give some ranchers what we call cattle dogs that will actually help protect their flocks against wolves.

IRA FLATOW: So that’s like a win-win situation.


IRA FLATOW: And can you get more of those, like in protecting marine areas?

TRISHA ATWOOD: We do have some that actually protects seabirds. And so you could put them out there. So we have– I think Australia it is– actually has dogs that protect penguins.

IRA FLATOW: Is that right?


IRA FLATOW: I’ve never heard of dogs protecting penguins.

TRISHA ATWOOD: Yeah. So they have guard dogs, basically, that live on these islands with them. And their entire job is to drive away foxes and anything that’s trying to eat what they’re protecting.

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

You’ve gone from America to Australia to sea creatures all over the world. It does seem like this really has to be a global effort, right, for it to work?

TRISHA ATWOOD: Yes. Our list certainly includes animals from almost every continent. But the US and some of these bigger countries– Australia– can do a lot with the land that they already have. The US has a lot of wild lands still available that we can use to help rewild animals without running into conflict with already built cities or urbanization.

And so the US-Canada is in a great position to begin rewilding with less effects than places like England.

IRA FLATOW: Right. And you’re saying also that we’re not going to solve the CO2 problem by just sequestering it artificially. We need nature to help us.

TRISHA ATWOOD: Yes. So we definitely have to reach net zero fossil fuel CO2 emissions or all of this is somewhat pointless in terms of carbon. So that means stopping CO2 emissions from the burning of fossil fuels.

But after we do that– and we really will do that– after we accomplish that, we need negative emission strategies. And that means we need to suck CO2 back out of the atmosphere and store it long term. And this is where nature can play a key role. And we call these strategies either nature-based solutions or natural climate solutions.

IRA FLATOW: Well, Dr. Atwood, we’re very happy to hear about these solutions. And thank you for your research and for taking time to be with us today.

TRISHA ATWOOD: Yeah, great. Thank you so much, Ira. And we’re super excited to talk about this. And we love that you guys are interested in this story.

IRA FLATOW: You’re quite welcome. Dr. Trisha Atwood, associate professor at Utah State University, in Logan, Utah.

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