When water is scarce, Ecuador laurel trees (Cordia alliodora) ramp up their investment in a syrupy treat known as honeydew imbibed by ants (Azteca pittieri) that nest in the laurels' stem cavities. Honeydew is not produced directly by the trees, but by tiny, sap-sucking bugs called scale insects. The higher availability of honeydew when conditions are dry sends resident ant defenders into overdrive, boosting their numbers, and the ants protect the trees more vigorously from defoliation by leaf-munching pests.
The mutually beneficial interaction between laurels and ants in the tropical forest 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 5 November in the open access journal PLOS Biology, University of Michigan ecologist Elizabeth G. 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.
All plant-animal mutualisms may employ a similar "insurance model," according to Pringle, a postdoctoral fellow in the Michigan Society of Fellows and an assistant professor in the Department of Ecology and Evolutionary Biology and the School of Natural Resources and Environment. In addition, Pringle suggests that 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.
"We show that trees and their defensive ants invest more in one another in drier, more stressful conditions," says Pringle. "We saw this happening along the coastline from Mexico to Costa Rica, an
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