By fighting off predatory lady beetles, ants create a safe haven for plant-eating insects.

By fighting off predatory lady beetles, ants create a safe haven for plant-eating insects.

Joshua Grinath

An ant, a plant, and a bear, oh my

In a mountain meadow in Colorado, ecologists have come across yet another example of the amazing interconnectedness of nature’s flora and fauna. Black bears, by eating ants, help one of the meadow’s key plant species thrive.  

“This is yet another example of the hugely important and sometimes unexpected roles that top predators play in ecosystems,” says Todd Palmer, an ecologist at the University of Florida in Gainesville who was not involved with the work. The bears’ influence is indirect, but may be significant enough that land managers should take a broader perspective when making decisions about bears in their territories, adds Joshua Grinath, an ecology graduate student at Florida State University in Tallahassee who discovered this connection.

Ecologists are increasingly realizing that no species exists in a vacuum, but understanding the effects of their interactions can be challenging. For example, researchers had thought that the reintroduction of wolves into Yellowstone National Park in the United States 20 years ago led to an increase in willows and aspens there because the wolves made elk fearful of browsing the saplings. But recent data suggest that elks aren’t really intimidated by wolves. So the forest’s revival is a mystery.

Grinath came upon another predator-plant connection while studying the partnership between ants and treehoppers on a common plant called rabbitbrush. These tiny sap-sucking insects secrete a sugary liquid the ants eat in return for taking care of the treehoppers. One summer, a bear moved into Grinath’s study site and started digging up the underground ant nests, eating both larvae and adults. So he decided to see what effect the bears had on his study subjects. 

Over 4 years, he and his colleagues monitored 35 ant nests in this subalpine meadow for bear damage. During that time, bears damaged or destroyed 26% to 86% of the nests. He soon realized that plants lacking ants grew better and produced more seeds.

Now Grinath knows why. The ants aren’t directly harming the plants, he and colleagues concluded after a series of field experiments. Instead, the presence of the ants scares off predatory insects, in turn enabling treehoppers and other plant-munching insects to thrive and take a serious toll on plant growth. “The ants are providing an enemy-free space for all these herbivores,” Grinath says.  Where bears have eaten the ants, predators return and help protect the plants, he and his colleagues reported online ahead of print in Ecology Letters.

“The study really highlights the complexity of effects that a predator can have on a whole community of species that are interacting with each other,” says Corinna Riginos, an ecologist at Teton Science Schools in Jackson, Wyoming, who was not involved with the work. “Most likely, other big predators also have just as many surprising and complex effects on the many species they live with.”

Judith Bronstein, an ecologist at the University of Arizona in Tucson, was also impressed, as such interactions can be hard to tease out. “In this case, many of the strongest effects are indirect ones whose importance couldn’t have been predicted in the absence of controlled experiments,” says Bronstein, who was not involved with the work. (The experiments performed by Grinath’s team included removing all ants from some rabbitbrushes, allowing just a few ants on others, and leaving  the ants alone on still other plants. In other tests, they manipulated the number of insect predators on the plants.)

In this situation, it’s not just who eats whom, but also a question of who helps whom. Such mutualisms, as these partnerships are called, “can play a powerful role in the organization and functioning of nature,” Palmer says.

Bears are changing their diets—with some eating more ants and others becoming increasingly reliant on garbage and human food—so ant populations may increasingly be affected. But “it’s hard to say” whether the bears’ eating habits have a big enough impact “to affect the abundance of rabbitbrush across the landscape,” Riginos says. Adds Palmer, “The jury is out until large-scale and longer term experiments are conducted.”

But Grinath thinks there is cause for concern: “These types of ecological relationships could unravel” if bears and their habitats aren’t managed carefully, he says, and plant populations could change as a result.