Ants: those other horticulturalists

By Gary Raham
Nature Writer and Illustrator

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Humans take great pride in their ability to raise and nurture plants. We pamper orchids and roses. Like Johnny Appleseed, we leave a trail of trees wherever we go. We've vastly improved the ranges of corn, wheat and rice, our favorite food grains.

Nevertheless, we shouldn't overlook our formidable competition in the field of plant management: the ants.

According to entomologists Bert Hölldobler and Edward O. Wilson, the total global biomass of all species of ants equals total human biomass, and ants have been practicing their skills far longer.

"Ants have lived on Earth for more than ten million of their generations; we have existed for no more than a hundred thousand generations," according to the scientists.

Ants go about their business without notice unless we happen to see a trail of them hauling off donut crumbs from the kitchen or we watch with mild alarm as a swarm of winged ants boil out of a crack in the front sidewalk. We may also pause to watch their comings and goings when we notice their nest mound along a trail or in a field.

Like us, ants use plants, but plants get perks out of the interactions, too. Our arid, prairie habitat supports some of the most avid ant horticulturists: the harvester ants, members of the genus Pogonomyrmex.

Harvester ants have long served as role models because of their industry in collecting and storing plant seeds. Solomon wrote: "Go to the ant, thou sluggard; consider her ways, and be wise: Which, having no guide, overseer, or ruler, provideth her meat in the summer, and gathereth her food in the harvest."

David F. Costello in "The Prairie World" estimates that grasslands support about 500,000 ants per acre. A large colony can collect a pint of seeds per day from plants such as pigweed, goosefoot, squirrel tail, prickly poppy, needle and thread grass, hairy goldaster and rubber rabbitbrush.

All this harvesting produces extensive effects. Hölldobler and Wilson contend, "Ants are keystone species, deciding by their presence alone which of the plants flourish and which fail."

They cite an Arizona study where researchers removed ants from certain desert plots. Without ants decreasing seed yields, annual plants grew back at twice their normal density within two seasons. Ants also provide a service to these plants: As the insects are jogging their 528-minute miles, a few seeds fall by the wayside and germinate in pristine territory.

Normally, harvesters insure that the seeds they collect don't germinate in their nest by nipping off the radicle of each seed – the primary root that first emerges from the plant embryo. Like good farmers, they store each seed type in separate bins or chambers within the nest granary area.

A harvester's nest is a comfortable domicile that can be anywhere from a few feet to 10 yards across and serve a colony for at least 20 years. The ants excavate galleries that may extend 9 to 12 feet below ground. They regulate humidity by carrying in droplets of moisture as necessary. The queen, eggs, larvae, pupae, immature workers and adults live in the upper galleries during summer. The queen and adult workers head for the basement in winter and hibernate. Ants share space with various guests and intruders, including smaller ant species, beetles, parasites, crickets, termites and worms.

Fortunately, harvester ants don't unduly pester their human neighbors. Now and then they may decide to perform their mating swarm orgy over a chimney and fall inside, or they may accidentally swarm near a house foundation and find their way inside through cracks, but they don't take up permanent residence. Some kinds of carpenter ants and pharaoh ants, however, can decide to nest indoors and may require the services of an exterminator.

The tropics are where ants and plants share the most intimate associations. Ants often receive free room and board and defend their plant hosts to the death. Tropical biologists learn which plants not to brush up against so they don't invite stinging attacks by the local ant landlords.

Various species of acacias, for example, provide ants with nectar at the base of their feathery compound leaves, offer housing space within stems, and grow nutritious snack buttons called Beltian bodies at the tip of leaflets. No wonder ants protect such a renter's paradise. Acacias without their ant police force suffer predation from a host of beetles and other insect Visigoths.

Hölldobler and Wilson wrote, "At the present time we know hundreds of plant species in more than 40 families that possess special structures to house ants." Many also produce nectaries and food bodies. These plants include legumes (the pea family of which the acacias are members), euphorbs, madders, melastomes and orchids. Hundreds of species of ants participate in these associations.

So, when your green thumb throbs with accomplishment, reflect on those thumbless red harvesters in the mound near the sidewalk that have been engineering plant growth and placement since the days of the dinosaurs