One of the greatest lessons in life that the natural world has taught me is that no living organism can stand alone. Each living entity is greatly dependent upon a whole host of factors that contributes to and often ultimately becomes their environment. Sometimes living organisms develop a relationship with another living organism where the two organisms are mutually dependent on each other. Some call this symbiosis.
Often symbiotic relationships develop between unsuspecting partners. Many of these go unnoticed. Such is the unusual relationship between the black cherry tree and a particular species of ant. Ants and cherries? Two more different organisms could not be found, so how are they mutually dependent?
Most people know the black cherry as a large hardwood tree that is native to the deciduous forests of the northern United States. This tree fills a very important ecologic niche in our temperate forests. Black cherries grow to a height of 80 to 100 feet and can attain a diameter of three feet. It is a very important source of food producing large crops of fruit every year. The berries are used by numerous species of birds and mammals and are available in the late summer and early autumn at a time when these animals need this sustenance to help them survive the long, cold winters of the north. The mature clack cherry is part of our northern hardwood mature forest and is prized by many humans for it’s fine grained wood often used in furniture making and as woodwork and flooring in select homes. The tree is somewhat susceptible to disease and predation, especially when it is young. And yet, it is difficult to imagine that this common tree might be rare, indeed, if it weren’t for a particular species of ant.
Ants are a very large group of insects that have many different species. They are known amongst entomologists as one of the most social and creative insects. They usually live in structured colonies where each ant is aware of his or her walk in life. Like honey bees, ants have a social order and the burden of certain tasks assigned to members of that order. This is their lot in life, so to speak. Some ants within a colony are responsible for reproduction, some are responsible for food gathering, and some ants are responsible for protection, amongst other chores that may be assigned to them. Certain species of ants may even raise and farm other insects, for instance, one species of ant actually tends aphids and collect the sweet honey dew nectar that the aphids produce for food for the colony. All of us who have been on a picnic know that some ant species love sweet morsels of food. It is this love of sweets that helped one species of ant develop a symbiotic relationship with black cherry tree.
It seems that high up in the branches of the towering black cherry the tree produces a honeydew through a gland at the base of a stipule located near the joint of the branch. This honeydew is highly prized by ants and is ferociously gathered by the ants in the spring. The production of the black cherry honeydew begins when the branchlets are budding and lasts through the early maturation of the newly formed black cherry leaf. The ant has but a short time to take advantage of this delightful and necessary food supply.
During this same time period leaf predators, primarily the tent caterpillar, becomes active in hopes of finding tender young leaflets to satify their budding enormous appetite. The tent caterpillar is capable of annihilating a tree’s young leaf crop. If this occurs over a period of consecutive years it can be devastating, even fatal, to a tree.
It is this coincidence of timing that proves advantageous to the black cherry tree. At the same time the ants are foraging for honeydew, the tent caterpillar is hatching and seeking young leaves for forage. The tent caterpillars have to pass over the honeydew gland to reach the leaflet, a dangerous route that places them in great danger.
The ants detect a threat by the caterpillar advance, likely seeing the caterpillar as a competitor for the honeydew. The foraging ants, in response to the caterpillar invasion, organize large armies of ants from their colony to do battle with the tent caterpillars. The battle can go on for days, pitting the will of the ants against the overwhelming numbers of tent caterpillars. Wholesale slaughter of tent caterpillars takes place as the ants wrestle with these large insects, often throwing the caterpillars from a high branch where it plunges 60 to 80 feet to the ground. The battle continues until the tree stops producing honeydew. At that point, the ants go on their way, leaving the carnage of caterpillars behind, where the survivors, far fewer in number, forage on the black cheery leaf. By this time the leaf of the black cherry is mature, and the tent caterpillar can do only minor defoliation due to the fact that they are significantly reduced in numbers while at the same time the leaf biomass has greatly increased as the leaves have matured. The black cherry goes largely unharmed from the remaining leaf foraging.
And so the ant and the black cherry, two organisms that seem unlinked to the untrained eye, are critically dependent on each other for survival. The black cherry tree provides critical food for the ant colony, wild the colony protects the tree from defoliation at a time when the tree needs a full set of leaves for photosynthesis to take place.
This is just one more elegant example of mutual dependency in the natural world; a plain and simple truth that humans should come to understand and take to heart in our quest to understand our real position on our living planet.
Originally written in May of 1996.