Ant-eating bears help plants

Ants show up on the bear menu surprisingly often — if you only think of bears as scary creatures with big teeth. But bears are omnivores. Some black bears even eat a diet of mostly vegetation. Others, though, such as the black bears of Rocky Mountain National Park in Colorado, consume a diet heavy in ants. The insects are a high-energy, abundant food source — bears eat them for many of the same reasons some say humans should eat more insects.

The bears’ insect diet affects more than just the ants. The impact of the food choice ripples through the ecosystem, helping out the dominant plant in the area, a type of shrub called rabbitbrush, Joshua Grinath and colleagues from Florida State University in Tallahassee report in the February Ecology Letters.

It’s not that unusual to find that the presence or absence of a top predator can have wide-ranging effects. But figuring out all those interactions can get pretty complex, in part because “predator eats prey” isn’t the only interaction that can occur between two species. With the ant-eating black bears, what’s most important is a mutualism between the ants and plant-sucking treehoppers: The ants protect the treehoppers from predators and in return get a sugary liquid called honeydew, which the ants eat.

Grinath had been monitoring ant nests in a Colorado meadow when he discovered that bears sometimes disturbed the nests (and scientific equipment — “bite-marks to field equipment” was listed among the evidence for the presence of bears in the meadow). Soon he and his colleagues noticed that rabbitbrush grew better near the disturbed ant nests. Over several years, the researchers conducted experiments in which they removed ants or treehoppers from rabbitbrush plants. And eventually they deciphered a web of interactions.

When the bears eat a lot of ants, the scientists found, species that are treehopper predators increase and treehoppers correspondingly decline. Without so many plant-suckers around, the rabbitbrush thrive.

But the situation in the meadow didn’t play out the same way every year. Bears tended to eat an ant-heavy diet only when there were tougher climatic conditions, such as a late spring or a drought. “This suggests that bears forage more heavily on ants in harsh environments, indicating the potential for strong cascades in these conditions,” the researchers note. “Additionally, many species are involved in the ecological web connecting bears and plants, and changes in interactions among component species will contribute to variation in the cascade.”

It’s complicated, but that’s why the term “food web” has become more fashionable than “food chain” in some circles.