How One Invading Ant Species Sent Ripples Through A Food Web

12:16 minutes

Three large ants are surrounded by swarms of smaller ants on the surface of a tree.
Native acacia ants are killed by the smaller invasive big-headed ants, leaving whistling-thorn trees defenseless against elephants. Credit: Pat Milligan

When people talk about the interconnectedness of nature, the usual example involves a little fish that eats a bug, a bigger fish that eats the little fish, and an even bigger fish at the top of the chain. But in reality, the interconnected relationships in an ecosystem can be a lot more complicated. That was certainly the case in a recent study, published in the journal Science, which describes how the arrival of an invasive ant species changed the number of zebras that get eaten by lions on the Kenyan savannah.

The unwelcome ant is known as the big-headed ant. It’s on a list of top 100 invasive species around the world. When it arrived on the African savannah, the ant newcomer muscled out a native ant species known as the acacia ant—which, though tiny, was able to help defend acacia trees from being grazed upon by elephants (picture getting a trunkful of angry ants while snacking).

With the trees undefended, hungry elephants feasted, resulting in fewer trees on the savannah and more open space. That made the hunting environment less favorable to stealthy lions, and more favorable to fleet-footed zebras. But to the surprise of the researchers involved with the study, that didn’t mean hungrier lions. Instead, the lions shifted their hunting from targeting zebras to targeting buffalo instead.

Dr. Jacob Goheen and Douglas Kamaru of the University of Wyoming join guest host Sophie Bushwick to describe their research, and how a small ant can have a big effect on an ecosystem.

Further Reading

Donate To Science Friday

Invest in quality science journalism by making a donation to Science Friday.


Segment Guests

Jacob Goheen

Dr. Jacob Goheen is a professor in the Department of Zoology and Physiology at the University of Wyoming in Laramie, Wyoming.

Douglas Kamaru

Douglas Kamaru is a graduate student in the Department of Zoology and Physiology at the University of Wyoming in Laramie, Wyoming.

Segment Transcript

SOPHIE BUSHWICK: This is Science Friday. I’m Sophie Bushwick,

KATHLEEN DAVIS: And I’m Kathleen Davis. We’re sitting in for Ira Flatow today. Later in the hour, we’ll dive into some shark research.

SOPHIE BUSHWICK: And we’ll learn about some of the legal challenges of space. But first, when people talk about the interconnectedness of nature, the basic example usually goes something like the little fish eats the bug, the big fish eats the little fish, and then an even bigger fish eats that. But in reality, the relationships can be a lot more complicated.

Take the example of a recent report in the journal Science, which describes how the arrival of an invasive ant species in Africa changed the number of zebras that get eaten by lions. Joining me now to help connect those dots are two of the researchers on that project. Jacob Goheen is a professor and Douglas Kamaru is a graduate student both in the Department of Zoology and Physiology at the University of Wyoming in Laramie, Wyoming. Welcome to Science Friday.

JACOB GOHEEN: Thanks, Sophie. Good to be here.

SOPHIE BUSHWICK: This is a bit of a twisted path, so let’s step through it point by point. First to set the scene, where were you studying?

JACOB GOHEEN: So this research occurred at Ol Pejeta Conservancy, which is right on the equator in Central Kenya. And it’s about a 250 square kilometer property. It’s managed jointly for wildlife conservation and also cattle ranching.

SOPHIE BUSHWICK: And can you describe the landscape there for us?

JACOB GOHEEN: I’d call it a classic African savanna. So you’ve got too many trees to call it a grassland and you’ve got too many grasses to call it a forest. So it’s kind of between those two extremes. And both of those plant forms kind of co-occur, but neither really outcompetes or dominates the other.

SOPHIE BUSHWICK: And so that’s the situation. And then these invasive ants, they arrive on the scene. So who are these guys?

JACOB GOHEEN: This is the bigheaded ant, pheidole megacephala. And we’re really not sure about its origins. It likely was introduced in bushels of produce imported from somewhere in the Indian Ocean, perhaps Mauritius. These are roughly one milligram ants, and they’re labeled as one of the globe’s top 100 invaders.

SOPHIE BUSHWICK: And when they invade, what happens?

JACOB GOHEEN: In general, not much. [LAUGHS] But in this instance, the trees that I was just describing earlier are what we would call myrmecophytes. They’re ant plants. They defend themselves with these tiny little bodyguards. These are called acacia ants or sometimes cocktail ants. They’re tiny, but they’re three times the size of a bigheaded ant.

So they’re about three milligrams. And believe it or not, they defend trees from the world’s largest extant land mammal, that being the African elephant or the savanna elephant. So you’ve got these tiny three milligram ants that are defending trees against 3 billion milligram elephants.

SOPHIE BUSHWICK: Hang on. How are ants defending trees against elephants?

JACOB GOHEEN: So that’s a great question. If you think about elephants, they’re unique in a number of things. One of which is their nostrils are about six or eight feet from their mouths. So their nostrils are on the tips of their trunks. And so when they’re feeding on a tree, they’ve got to stick that trunk into a bunch of foliage and then grab a trunk full of leaves and pull. And that whole process takes about three seconds before the elephant is putting those leaves into its mouth. And in that time, you’ve got lots of these acacia ants that are swarming up into the nostrils of elephants.


JACOB GOHEEN: Yeah, ouch. And because of that trunk, it kind of exposes elephants to this unusual defense by the tree. Things like giraffes would just use their tongues to swipe ants away from their eyes and nostrils. Things like black rhinos will just plug up their nostrils and eat. But the system is kind of rigged against elephants because of that trunk.

And so to get at your earlier question, what the bigheaded ants do is they form these super colonies of tens to hundreds of thousands of individuals. And unlike the acacia ants, they don’t defend trees. They don’t live on trees. They live in cracks and crevices in the soil. And just by virtue of their numbers, they can overwhelm and just clobber the native acacia ants and render those trees defenseless against elephants.

SOPHIE BUSHWICK: So essentially what happens is the bigheaded ants come in, they destroy the defenders of the trees, these larger ants that have been protecting the trees from elephants, and then the elephants reduce the tree cover. Is that right?

JACOB GOHEEN: Yep, you got it. Absolutely.

SOPHIE BUSHWICK: So when you lose that tree cover, what happens to the other animals?

JACOB GOHEEN: I mean, the ones that we addressed in this study were lions and their primary prey, those being plains zebra. And zebra require kind of big, open expanses to detect lions. They want to see lions and then just run away from them. So in areas that are open where tree cover has been reduced, zebra can more effectively avoid lions.

On the flip side of that, lions like areas that are bushy. So they like these trees. They like to hide behind them and use them as stocking cover. So you can imagine that when you go from a pretty dense savanna with lots of trees to one that is suddenly now more open, the lions are exposed and they can’t ambush zebra as effectively as they once did.

SOPHIE BUSHWICK: OK. So I think I understand the theory here. But Douglas, could you tell us a little about how you did go about proving this in practice?

DOUGLAS KAMARU: Yeah, thanks, Sophie. What we did actually, we used a combination of methods. We assessed the tree cover to see how comparing areas that are invaded by the bigheaded ants and the areas that are not invaded. And what we noticed or what we realized is that in invaded areas, we had about five to seven times higher visibility or low tree covers compared to areas that we didn’t have bigheaded ants. We also collared or fitted GPS collars on lions to be able to tell where they are going in real time or where they are moving.

SOPHIE BUSHWICK: Oh wow. Wait, what, sorry, I have to interrupt. What was it like to put a tracking collar on a lion?

DOUGLAS KAMARU: It’s quite some work.


Yeah, it involves a lot of logistics. Because we did that in collaboration or partnership with Kenya Wildlife Service. So there’s a lot of moving parts that we had to put together. With lions, you have to dart them using a dart gun, like kind of make them sleep. And then you fit the GPS. And also it’s a lot of work going out early in the morning looking for them, see where they’re occupying within the conservancy or the vegetation.

And also, we use the GPS collars to see where they are killing. The lion, we realized the lion is staying in one place for long. Then we go there and then we assess what actually– the question is what the lion is doing there. So sometimes you go there, you find them with a kill. And we are able to identify the kill. And also sometimes you could go and find some little cubs, because lions, once the lions they have little cubs, they tend to stay in one place for quite a while. We also did count animals like zebras within the conservancy to be able to determine their densities and so on.

SOPHIE BUSHWICK: OK. And you mentioned that so the lions aren’t able to prey on the zebras as easily when they’re in these areas where the bigheaded ants have moved in. So did you find that the lions were going hungry or did they end up eating something else?

DOUGLAS KAMARU: Our hypothesis was if the lions are not able to eat zebras, which is their primary prey, then they will go hungry. And then after they go hungry, then we expect their population to decline. But that’s not what we saw after we did our analysis. So we were a little bit surprised. And then after analyzing some more data, that’s when we realized that lion actually shifted their diet to eating more buffaloes.

So the lions actually were able to switch their diet and killing more buffaloes. Although it’s actually difficult to kill buffaloes, because it takes up to 5 to 10 adult lions to bring down a buffalo, because the buffaloes are aggressive and they tend to fight back compared to zebras. And it takes, let’s say, two to three adult lions to bring down a zebra.

SOPHIE BUSHWICK: Wow. So I guess that’s good news for the zebras. But Jake, can you tell us what’s going to happen to them? Is their population just going to get really big in these treeless landscapes?

JACOB GOHEEN: That’s a good question. We think that probably is not likely in that zebra numbers are controlled by grass. So they’re controlled by their food supply, which is in turn controlled by rainfall. So they seem to vary independently of what the lions are doing. On the other hand, it is possible that this prey switching from zebra to buffalo that Douglas just described could reduce the number of buffalo on this property.

SOPHIE BUSHWICK: And is the cascade of effects going to keep going beyond even reducing buffalo? Do fewer zebra kills also affect the vultures or some other scavengers?

JACOB GOHEEN: Yeah. That’s another great question. We really don’t know. Whenever there are big species invasions like with this bigheaded ant, there are winners and losers, and sometimes those take a while to reveal themselves. So I could imagine something like that happening. The thing that I think is on both Douglas’s and my radar is that this particular tree, this myrmecophyte, is a key food for globally endangered black rhinos. So that I think is the next thing that we likely would look at.

SOPHIE BUSHWICK: Where do you go from here? So what’s next for your research in this area?

JACOB GOHEEN: The next things we’re really interested in looking at is whether the removal or the eradication of bigheaded ants will revert those invaded areas back to something that resembles their kind of natural or pristine state. It’s possible that we remove the invasive ant and the acacia ants recolonize. They defend trees and we get something resembling the savanna prior to that bigheaded ant invasion. It’s also possible that we remove the bigheaded ants and then nothing happens, in which case it would say that it’s more difficult to restore this ecosystem than we might have thought originally.

The second thing, and Douglas mentioned this in one of his earlier answers, is that it takes three to four times the number of lions to bring down a buffalo than it does a zebra. And frequently in the Serengeti greater ecosystem to the south, male lions are involved in hunting buffalo, which is pretty unusual. They typically are not involved in hunting other prey. So we’re actually interested in whether this bigheaded ant invasion is causing changes to the social structure of lion prides and different hunting groups of lions.

SOPHIE BUSHWICK: Thank you both so much for coming to tell us about how these teeny tiny ants have had such a massive effect.

JACOB GOHEEN: Absolutely. Thanks a bunch for your interest.

SOPHIE BUSHWICK: Jacob Goheen is a professor and Douglas Kamaru is a graduate student both at the Department of Zoology and Physiology at the University of Wyoming in Laramie, Wyoming.

Copyright © 2023 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/

Meet the Producers and Host

About Charles Bergquist

As Science Friday’s director and senior producer, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

About Sophie Bushwick

Sophie Bushwick is senior news editor at New Scientist in New York, New York. Previously, she was a senior editor at Popular Science and technology editor at Scientific American.

Explore More