To test whether water limitation strengthens the defensive mutualism between Ecuador laurel trees and Azteca ants, Pringle and her colleagues studied the interaction at 26 sites in seasonally dry tropical forests along the Pacific coast of southern Mexico and Central America. The coastal sites span 1,426 miles, with annual precipitation increasing fourfold from the northernmost to the southernmost site. The findings reported in PLOS Biology are based on observations as well as field experiments, physiological data and an evolutionary model.
Ecuador laurels are deciduous, shedding their leaves during the dry season and growing new ones each rainy season. Pringle and her colleagues found that the strength of the tree-ant mutualism as measured by investment of trees in sugar for ants and defense of leaves by ants was greater at sites with longer dry seasons.
Laurel trees don't feed ants sugar directly. Instead, they host scale insects, familiar to gardeners as common backyard pests, which produce the honeydew. Scale insects are the middlemen in this protection racket: through the scale insects, the trees indirectly pay a carbon fee, in the form of sugar-rich sap that is distilled into honeydew, to the ants in exchange for guard duty.
"When ants patrolling the surface of the tree encounter a leaf-eating insect, they bite the insect until it falls from the tree," Pringle said. "We found that at the drier sites, the larger ant colonies were more likely to find such intruders, and the colonies sent more ants to attack the leaf-eaters and chase them away."
The fact that laurel trees at drier sites pay their ant protectors higher "wages" suggests that the potential costs of defoliation outweighs the relatively modest price of supporting more ants. Pringle and her colleagues used a mathematical model to test this idea, looking at the relative costs and benefits of c
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