The carbon cycle is essential to life on earth, but scientists still struggle to grasp its complexities. Most research to date has focused on major sources of the greenhouse gas carbon dioxide, such as deforestation and the use of fossil fuels. Now some scientists have begun to explore subtler factors, such as the interplay between plants and animals. A new study has come to the counterintuitive conclusion that plants might accumulate more carbon in the presence of predators and herbivores than they do in isolated locales, where they are less likely to be eaten or trampled.

To tease out interactions between the plant and animal worlds, Yale University ecologist Oswald Schmitz and his colleagues built three enclosed grassland environments—one with meadow vegetation only; one with vegetation and herbivorous grasshoppers; and one with carnivorous spiders, along with the grasshoppers and plants. The researchers found that plants in the third environment, the home to both herbivores and carnivores, stored 40 percent more carbon than plants in the grasshopper enclosure.

It makes intuitive sense that arachnid predators, which eat the herbivorous grasshoppers, would limit the consumption of vegetation and thus free plants to store more carbon than they would in the enclosure where herbivores grazed unchecked. Surprisingly, however, the plants sharing an enclosure with grasshoppers and spiders also packed away significantly more carbon—20 percent more—than did the isolated plants. “You would think that an environment with no herbivores or predators would lead to peak carbon storage,” says Schmitz, a co-author of the study, published in June in the Proceedings of the National Academy of Sciences USA. “But that's hardly the case.”

Why? Perhaps a degree of herbivory stimulation—a light nibble here and there—causes some unidentified physiological change in plants that boosts carbon uptake, Schmitz says, “but the truth is that we really don't know.”

The research highlights how ecological changes can have significant, and unforeseen, climate impacts. “Right now there is a crisis in terms of predator diversity loss,” Schmitz says. “And that may mean that we are losing the potential to help regulate the carbon cycle in ways that go far beyond just growing more trees.”