How ants thwarted lions on the African savanna Science, Sean Cummings January 25, 2024 Filter David and Goliath through a Rube Goldberg machine, and you’ll get a sense of a bizarre ecosystem shake-up happening in the African savanna. In the grasslands of Kenya, an invasive ant has displaced an insect that protects the region’s acacia trees, allowing elephants to overgraze them. This, in turn, has denuded the landscape, robbing lions of the hiding spots they need to stalk zebras and forcing them to switch to a more difficult—and dangerous—prey: buffalo. “This study was a beautiful snapshot of how complicated ecosystems can be—this idea that you pull on a single thread and the whole system reacts,” says Meredith Palmer, an ecologist at Fauna & Flora International who was not involved in the work, published today in Science. Across tens of thousands of square kilometers of eastern Africa, a type of acacia called the whistling thorn tree (Vachellia drepanolobium) sits at the heart of the ecosystem, accounting for 70% to 99% of woody plant mass wherever it grows. The trees provide nectar to native acacia ants (Crematogaster spp.), which live in the hollow, spiky bulbs growing along their branches. In exchange, the ants defend the tree from herbivores, swarming up the nostrils of African bush elephants (Loxodonta africana) hungry enough to get past the spines. Increasingly, however, the tiny insect sentinels are disappearing. At some point in the early 2000s, big-headed ants (Pheidole megacephala), thought to be native to an island in the Indian Ocean, showed up in Kenya. Although humans likely played a role in their arrival, no one knows exactly how they got there. But their impact is clear: They attack native acacia ants and consume their young, leaving whistling thorn trees defenseless against munching elephants. For Jacob Goheen, a wildlife ecologist at the University of Wyoming (UW) and senior author of the new study, the observation sparked questions about how a landscape of denuded trees might affect other creatures in the community. “Lions are much more effective at killing their prey in dense, woody thickets,” he says. “Would … more open savannas make it so that lions had a harder time catching zebra?” The Ol Pejeta Conservancy, a reserve in Laikipia County in Kenya, offered a perfect setting to seek the answer. There, across more than 360 square kilometers of savanna, Goheen and colleagues set up a dozen 2500-square-meter study plots. Within each one, the team tracked landscape visibility, plains zebra (Equus quagga) population density, big-headed ant occurrence, and zebra kills by lions (Panthera leo). They also captured six lionesses and outfitted them with GPS collars to monitor how their activity differed between plots. “These pieces of research have long histories: We’ve been studying ant-tree mutualisms, elephant-tree interactions, the role of visibility in lion hunting,” Palmer says. “Seeing this study put all those pieces together makes it more powerful than we could have hoped for.” “It’s not usual to have an ant-plant study that also caught and collected data on lions,” agrees Megan Frederickson, an evolutionary ecologist at the University of Toronto. “That’s pretty cool. I’ll probably use this paper for teaching.” After 3 years of monitoring, Goheen’s team arrived at some striking conclusions. Elephants stripped and toppled trees that lacked native ants up to seven times faster than those that had ant protectors. Tree and bush cover shrunk dramatically, making it difficult for lions to sneak up on and ambush zebras. In contrast, areas free of invasive ants had plenty of whistling thorn foliage where lions could hide, enabling them to kill zebras almost three times more often. “In ecology, it’s always hard to say, ‘This caused that,’ because there are so many species interacting,” says Kaitlyn Gaynor, an ecologist at the University of British Columbia who was not involved with the research. But in this case, she says, directly comparing invaded and uninvaded areas helped pin the blame on the ants. “The species interactions involved were so critical to the state of the ecosystem that when they were disrupted, you could watch those dominoes fall.” The surprise, for some experts, was that lion populations in the study area held steady despite reduced access to their primary prey. So far, the big cats have adapted by diversifying their diet: Drawing on other research, the team found that zebra kills made up only 42% of the lions’ hunting activity in 2020, versus 67% in 2003. Over the same period, kills of African buffalo (Syncerus caffer) rose from 0% to 42% of the lions’ hunting activity. But it’s unclear whether this dietary shift is sustainable. “We don’t know what’s going to happen going forward,” says Douglas Kamaru, a wildlife ecologist at UW and lead author of the new study. “It’s very difficult for lions to kill buffalo. It’s a lot of energy compared to [hunting] zebra, and sometimes buffalos kill lions when they’re fighting.” Other experts wonder whether buffalo numbers will decline if lions continue to hunt them, or whether plant communities and wildfire patterns will change as grazing zebras face less hunting pressure. Some, Kamaru included, see a need to study how big-headed ants spread to inform efforts to stem their advance, which currently clocks at about 50 meters per year. “Thinking about ecosystems as a whole instead of just individual species … can help us to better anticipate the effects of some kind of disturbance or change,” Gaynor says. “Losing one species can totally change the way the system looks.” https://www.science.org/content/article/how-ants-thwarted-lions-african-savanna