As the Forest Fries, Brazil’s Formidable Formicidae May Falter

Story by Jason Bittel

Brazil’s Caatinga dry forest is about as inhospitable as it gets. Daytime temperatures are sizzling. The thorn-covered shrubs and cacti seem equipped for battle. And if you brush against the wrong plant, you’ll quickly meet the forest’s territorial guard—more than 40 species of ants ready to defend the forest with powerful bites, venomous stings, and, ultimately, their itty-bitty lives.

Fortunately, neither you nor I are the ants’ intended target. Rather, the insects deploy their formidable armaments to protect their host plants from herbivores, such as caterpillars, that crunch through the plants’ leaves. In return for their protection, some plants pay their mercenaries well by exuding a nutrient-rich, sugary substance for the ants to eat from tiny, nubby glands known as extrafloral nectaries.

The bond between ants and plants in the Caatinga dry forest is an example of what scientists call mutualism. It’s the same kind of quid pro quo that arises between flowers and their pollinators, or between cleaner fish and their clients. 

“It’s a plus-plus interaction,” says Talita Câmara, a biologist at the Federal University of Pernambuco in Brazil. “It means that both parties benefit from each other.”

The trouble is, after taking millions of years to develop, the Caatinga dry forest’s ant-plant partnerships may now be in danger of disappearing. 

In a recent study, Câmara showed that when the surface temperature of plants’ leaves in Caatinga rises to more than 44 degrees Celsius (111 degrees Fahrenheit)—a plausible spike, given that she once recorded a temperature of 50 degrees Celsius (122 degrees Fahrenheit) on her own shoes—fewer species of ants show up to defend their plants.

Ants are extremely diverse creatures, says Câmara, so it’s expected that different species have different heat tolerances. But as temperatures in Caatinga and elsewhere rise with climate change, more and more ants will likely be pushed beyond their thermal limits. And that’s bad for the plants, Câmara says, as it’s the ants’ collective effort that keeps herbivores from gnawing them down to nothing.

While Câmara’s study didn’t investigate what would happen to the 14 species of Caatinga plants that rely on ants should their hired bodyguards quit altogether, previous work shows just how important the ants’ pest-control services can be. A study published in 2023, for instance, showed that even for trees that don’t produce ant-attracting extrafloral nectaries, just living next to a tree that does means a more peaceful life with fewer hungry caterpillars.

“Ants in subtropical ecosystems are functionally very important, simply because there are so many of them. Not in terms of species, but in terms of individuals,” says Michael Staab, an ecologist at the Leuphana University of Lüneburg in Germany, and lead author of the neighboring-tree study.

While Staab is excited to see where the researchers take this work next, he suspects some of Caatinga’s ants might prove surprisingly resilient in the face of global warming. Many of the ants that form mutualistic relationships with plants in the dry forest are generalists, Staab says, meaning they’re not strictly carnivores or herbivores. That flexibility may insulate many of the ant-plant partnerships from climate change. While more specialized ant species may suffer, he says generalists will likely find a way to persevere.

Interestingly, Câmara notes that while Caatinga is known for its brutal heat, studies show that the region’s ants are less adapted to high temperatures than ants in other arid areas such as in Africa and Australia. This may also mean they’re more vulnerable.

But just because these long-standing partnerships are under threat does not mean either party is necessarily doomed. If the ants stop patrolling, Câmara says, the plants may invest more in chemical or physical defenses. Likewise, the ants may respond to rising temperatures by altering their behavior, such as becoming more nocturnal. At the same time, climate change could hurt the herbivorous caterpillars—which might help keep the scales in balance. 

When push comes to shove, Staab won’t bet against the ants.

“One thing I’ve learned about ants is that they are there for the unexpected,” he says, “and some species can be very, very resilient.”