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Do ants benefit plants? Like so many things in nature, the interrelationships among organisms are far more complicated than it first appears. Plants rely on ants for many things but not pollination, the sexual act of transferring pollen from one flower to the other. Only 12 plants (5 of which are orchids) have ants as the proposed pollinators. Even in these dozen we might be dealing with guilt by association. In most of these 12, one or more of the following critical pieces of information is missing: the observation of pollen actually being transplanted from one flower to another and/or the demonstration that ant pollinators produce live, i.e. fertile seeds. This second piece of information is particularly important because ants excrete antibiotics that coat their outer surface. Such excretions have been repeatedly shown to reduce pollen germination. Thus, even if the pollen gets to the female portion of the flower, it may not germinate.
Ants do appear, however, to be important distributors of seeds, particularly in our area. Seeds of many of the herbs that brighten our forest floors with a carpet of spring flowers have food bodies (elaiosomes) attached to their seeds. In New York beech/maple forests, over 50 percent of the spring blooming herbs have seeds with elaiosomes. Biologists speculate that these food packets have essential compounds (steroid-like substances) needed for normal development of the ant young. The seeds produced by these herbs are toted back to the ant hill. The food packet is devoured, but not the seed. This is the reward offered to the plant. The soil surrounding the surviving seeds is enriched by ant droppings and aerated and drained by the tunnels. Asarum canadensis (Wild Ginger) Claytonia virginiana (Spring Beauty), Erythronium americanum (Trout lily), Hepatica acutiloba (Hepatica), Hexastylis sp. (Ginger, Little Brown Jugs or Little Brown Pigs) Sangunaria canadensis (Bloodroot), Trillium grandiflorum (Trillium), Viola blanda and Viola canadensis (Violets), and Uvularia sp. (Bellwort), common members of our moist forest spring flora are some of the plants species that appear to be distributed by ants.
Ants also provide important defense mechanisms against herbivores and other plant predators. This role was brought home to me quite forcibly while visiting Costa Rica. I was walking along a dirt road sandwiched between two cultivated fields when a wonderfully shaded spot was offered by a weedy Cecropia tree. Its large umbrella-shaped leaves beckoned me to stay awhile, rest, mop my brow, and have a sip from my canteen. Propping on the truck, I suddenly became aware of a war being waged on my hand by a fiery attack force of tiny ants. The pain was sudden and intense. There was no competition; I lost. I found out later that these fierce creatures were members of the aptly named genus Azteca. The stems of this shrubby Cecropia are hollow. Colonies of ants live within these apartments. The plant not only provides shelter but food as well. Specially produced food packets are found on the leaves solely for the use of ants. The ants in return defend the plant from all dangers, real or perceived (for example, me).
Bert Holldabler and E.O. Wilson in their very readable treatise on ants, list nearly 100 genera of plants in the tropics that have such an ant defense mechanism (mymecolphily). Although mymecolphily appears to be more common in the tropics, the leaves of our tolerant fern, Pteridium aquilinum (Brachen Fern) produce special nectar glands frequently visited by ants.
Next time you pick peonies and find them crawling with ants attracted by peony-produced sugar, you might just pause to wonder what the ants are doing for the plant.
Some readable sources on ant-plant association:
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