ANTS AND ACACIA TREES HAVE COMPLEX RELATIONSHIP

by Whit Gibbons

October 11, 2015

After a recent period of major rainfall, I visited a wooded area to see what animals I could find, wondering if any had left for Noah's Ark. I turned over an old piece of roofing tin and noticed that the ant colony beneath had their eggs stacked on the surface, presumably to protect them from rising waters.

Ants are incredible creatures. Of the more than 15,000 species of these highly social insects (that includes some yet to be described by scientists), those that have been studied have displayed extraordinary behaviors. Watching how ants had responded to the dramatic change in environmental conditions to protect their reproductive effort reminded me of another behavioral response of ants related to reproduction of a tree.

When insects benefit from getting nectar from a flower, they inadvertently perform an essential function for the plant: pollination. Such mutually beneficial relationships between plants and animals are fascinating and ecologically complex, and understanding how they work can be a scientific challenge. Research on ants and acacia trees (members of the mimosa family) revealed a special alliance in which each helps the other survive.

Many fuzzy-looking acacia flowers are fragrant, a lure to pollinating insects, and also have ants that live on them and protect the trees from plant-eating insects. The ants attack an invading beetle or aphid that plans to make a meal of acacia leaves. In return, the tree provides shelter for the ants, which also thrive on nutrients produced by the tree. A mystery surrounding the observed bond between acacias and ants is how other insects manage to pollinate the flowers when the tree is guarded by ants. How does the ant distinguish between a food-searching insect that would harm the tree from one seeking nectar that will benefit the tree?

One study focused on ants protecting African acacia trees whose flowers are pollinated mainly by bees during the midday period. Like any other insect, a bee is not welcomed by ants guarding a tree. Ants were observed to protect the buds and older flowers from insects in the same manner that leaves were protected. The guarding ants did not permit any insect to stay in the vicinity of the plant before flower development and during seed production. However, once the young flowers were ready to be pollinated, the ants began to avoid the area around the flower, allowing bees and other insects to gather nectar and serve as pollinators.

As each flower aged, however, the guarding ants began to march back onto the scene. The scientists concluded that when the flowers are ready to be pollinated they produce a chemical that acts as an ant repellent. To test their hypothesis, they picked newly developed flowers at a stage ripe for pollination and wiped some of the old flowers with them. The ants present around old flowers, which would normally be protected, retreated from those wiped with new flowers. The behavior of the ants supported the hypothesis that a chemical produced by the flower temporarily deterred ants and allowed bees to pollinate the flowers without being attacked.

One interpretation of how the acacia-ant system developed is that a genetic change occurred in the ancient past so that some acacia trees began to produce a chemical that kept ants away from the flowers during pollination. Acacias without the genetic change of an ant-deterring chemical would still be pollinated but at a lesser rate than trees with the genetic change that would result in their becoming much more prolific. Nearly every flower would get pollinated and thus more seeds and, ultimately, more trees would be produced. Eventually, the trees most likely to have offspring would possess the genetic makeup to produce the chemical.

So how does an ant know not to attack during pollination season? In one sense, the ant doesn't know, but the tree does. Such intricate ecological systems working in favor of two species are intriguing, and ants continue to be remarkable creatures.

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