If you were a tree, you wouldn’t have to worry about taxes or traffic or most of the things that stress out a human. Your problems would run more to things like having enough water and being found by critters that will eat you and your leaves. Those are the two main stressors that the Ecuador laurel tree has to deal with. And when the dry season comes, herbivores start munching more leaves. What’s a tree to do? This one invests in its ant defenders.
Ecuador laurel trees (Cordia alliodora) are populated with populations of tiny Azteca pittieri ants. The ants nest in stem cavities that are also home to pink scale insects. Those scale insects feed on sap from the laurel tree and produce a sugary substance called honeydew that the ants eat. The ants may be small, but they provide adequate defense for the tree by ganging up on leaf-eating caterpillars and biting their undersides until the herbivores fall off the tree.
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Providing sugar for the pink scale insects comes at a cost for the tree, and it might seem better if the tree hoarded its meager carbon supply during dry months instead of giving it away.
After all, if that supply is exhausted, the tree will die. And even if it didn’t die, the tree needs that carbon to make new leaves when the rainy season comes. But a new study in PLOS Biology finds that trees in drier areas have an even stronger relationship with their ant defenders than do trees in wetter regions.
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The research team studied the relationship between Ecuador laurel trees and Azteca ants at 26 sites in Mexico and Central America. The sites all had different levels of precipitation, with the wettest site receiving four times as much moisture as the driest. They found that trees in drier sites had more scale insects that in turn supported larger ant colonies. And the ants were better defenders in the drier locations: They responded to threats to their host tree faster than did ants that lived on trees in wetter places. Instead of dry times loosening this plant-ant relationship, it makes it stronger.
Investing in ant defenders is like tree insurance, the researchers say. The cost of herbivory is high. If a tree’s leaves are eaten, the tree has to replace them, and it loses some of the machinery it needs to get energy from the sun and make the replacements. Investing some of its precious carbon in its defense system just makes good sense.