No insect-eating birds inside weaver ant territories, say scientists

Standing in the forests at the foothills of mountains across Asia, Australia, and parts of Africa, hungry insect-eating birds seem to cry out: “Where have all the insects gone?” To their surprise, the weaver ants have devoured them all. These reddish, giant ants swarm the forest floor, feasting voraciously on insects—a familiar scene often portrayed in nature documentaries. Weaver ants consume so many insects that they leave little to nothing behind for the birds.

A team of scientists, led by Umesh Srinivasan from the Centre for Ecological Sciences at the Indian Institute of Science, found that in the foothills of mountain ranges across the world where weaver ants reside, the number of insect-eating birds is low. “Oecophylla [weaver ants] and insectivorous birds have highly overlapping diets,” said Srinivasan, Assistant Professor at CES. “As a result, where Oecophylla ants are present at the base of mountain ranges, the species richness of insectivorous birds is low, while the species richness of all other birds is high.”

Srinivasan, who researches climate change and bird interactions in the eastern Himalayas of India, was ignited to explore the relationship between weaver ants and insect-eating birds after a colleague in Darjeeling, India, prevented these ants from infesting trees. It was found that trees without weaver ants had more insects than those with ant infestations. Srinivasan said he had been curious ever since learning about the experiment, especially since weaver ants are considered the “oldest biological control agent.” He noted that Chinese farmers have long used weaver ants in their orchards to control insects and pests. “So they’re very effective insect predators,” added Srinivasan.

So, Srinivasan’s team categorized birds into three groups: insect-eating, omnivorous (which eat both insects and fruits), and fruit- and nectar-feeding. They then analyzed global datasets on bird elevation and recorded the lower and higher elevations at which birds lived in the observed mountain ranges. Afterward, they counted the number of bird species at different elevations in each mountain range and correlated it with the presence of weaver ants.

The research team found that where weaver ants are present, the number of insect-eating birds was lowest at lower elevations, with the bird population stabilizing only after reaching an altitude where the ants no longer exist. The population of these birds peaked at mid-elevations, around 900 meters, which is “over 400 meters higher than in mountains without Oecophylla,” the researchers wrote in their paper published in August in Ecology Letters. They also observed a “similar but weaker” pattern for omnivorous birds, with their population peaking around 700 meters higher, indicating that omnivores are less affected than insect-eating species. For other bird species, Srinivasan noted, “species richness is highest at low elevations,” suggesting that fruit-eating birds are not significantly impacted by the presence of weaver ants, as their population peaks at lower altitudes and steadily declines with increasing elevation.

“Birds eat a lot of things. Some species eat only insects, some eat only fruits, others are omnivores, and some are scavengers like vultures,” Srinivasan explained. Since weaver ants fiercely compete with insectivorous birds for the same food, only a few insect-eating birds thrive in areas where the ants are present: “Less food, less birds.” 

For hundreds of years, people have proposed numerous ideas to explain the causes behind the uneven distribution of species across elevation gradients: “Temperature, rainfall, area, and various other environmental factors,” Srinivasan said. Another factor is competition between species for resources, which Srinivasan noted hasn’t received much attention—especially competition between species that are distantly related to each other. In fact, the researchers also analyzed factors like precipitation and productivity, but they found these had less impact on the birds compared to the influence of the ants!

This new research—which exposes the link between weaver ants and insectivorous birds—is significant because it presents a new perspective on the question of why species numbers vary in different parts of the world, which is a “fundamental scientific curiosity,” added Srinivasan. Furthermore, he emphasized the rapidly changing global climate that is pushing species higher up the elevation gradient. “Because of rising temperatures, species are shifting their ranges to higher elevations,” he added. “This is not migration; this is actually a permanent shift in the range of species to higher elevations,” Srinivasan clarified. “So if these ants shift their ranges upwards, what will happen to the bird communities?”

Ben Freeman, a biologist at the Georgia Institute of Technology who was not involved in the study, told Science Magazine that he considers this new hypothesis “a big-time idea, that it’s ants that shape insect-eating bird communities.” Although Freeman finds the idea interesting, he expressed a desire to see it tested experimentally in the future.

“This is a pattern that is consistent with one hypothesis,” Srinivasan added. “It's difficult to get proof of a hypothesis like this when what you’re actually asking questions about are at the global scale.” He stated that since it is extremely challenging to prove this hypothesis through large-scale experimental studies, “We can only say that this might be one of the reasons why we see a difference in species richness patterns across a global scale.”