Mutual benefits: Stressed-out trees boost sugary rewards to ant defenders


By Jim Erickson
Jan 31, 2014 Bookmark and Share

An Azteca ant patrols the surface of an Ecuador laurel tree in Jalisco, Mexico. When a patrolling ant encounters a leaf-eating insect, it bites the insect until it falls from the tree.

An Azteca ant patrols the surface of an Ecuador laurel tree in Jalisco, Mexico. When a patrolling ant encounters a leaf-eating insect, it bites the insect until it falls from the tree.

When water is scarce, Ecuador laurel trees ramp up their investment in a syrupy treat that sends resident ant defenders into overdrive, protecting the trees from defoliation by leaf-munching pests.

The water-stressed tropical forest trees support the production of more honeydew, a sugary excretion imbibed by the Azteca ants that nest in the laurels' stem cavities. In return, ant colonies boost their numbers and more vigorously defend the life-sustaining foliage.

The mutually beneficial interaction between laurels and ants, which also involves tiny, sap-sucking bugs called scale insects that make the honeydew, is a well-known example of what ecologists call a mutualism. Theoretical studies predict that mutualisms should be stronger under resource-poor conditions, but until now there was little evidence to support that theory.

In a paper published online Nov. 5, 2013 in the journal PLOS Biology, University of Michigan ecologist Elizabeth Pringle and her colleagues identify a clear-cut case of a stress-strengthened ant-tree mutualism and suggest a possible mechanism underlying it, one based on interspecies carbon exchange. Their results suggest that trees at drier sites buy insurance for their leaves, in the form of beefed-up ant protection, and pay for it with carbon, the coin of the realm.

All plant-animal mutualisms may employ a similar "insurance model," according to Pringle, a Michigan Fellow and an assistant professor in the Department of Ecology and Evolutionary Biology and at the School of Natural Resources and Environment. And the type of water-mediated stress response observed in the study may be more common in the future, if droughts become more severe with climate change, she said.

"We show that trees and their defensive ants invest more in one another in drier, more stressful conditions," Pringle said. "We saw this happening along the coastline from Mexico to Costa Rica, and then we realized that carbon exchange can explain it."

The study received widespread media attention including in the Spanish language press. Coverage included the Los Angeles Times, WAMC Northeast Public Radio, El Diario Madrid, and was previously on the U-M Gateway.

Michigan News press release