Northern Prairie Wildlife Research Center
The sheer abundance of ants indicates their importance in the ecology of the prairie. All prairie ants can be characterized as meat-eaters (invertebrate flesh) with a sweet-tooth (honeydew and extrafloral nectar), but they fall into several discernible guilds. The largest guild comprises the generalized predators, which hunt and kill other invertebrates. Another guild raises sap-feeding insects -- aphids and related insects -- on plant roots, in a manner analogous to humans raising livestock. A third group of species raids the nests of other ants for slaves and/or food. Finally, while all ants scavenge opportunistically, there are some which specialize in this life style.
Within their size range, ants are among the foremost predators, in part because of their ability to hunt in groups. They patrol the ground and plant surfaces, killing other insects and spiders for food, collectively taking huge numbers of the prey animals. In the tropics, many ant species specialize on just one type of prey, but prairie ants are generalists, taking whatever they can subdue. Major prey items are smooth- and soft-bodied immature insects. Hairy, fetid or hard-bodied insects, e.g. fuzzy caterpillars, stink bugs, or beetles, are less likely to be captured. The protein-rich prey is mostly cut up and fed to the ants' growing larvae.
Ants satisfy their taste for sweets by gathering naturally-occurring sugary substances. The sweets are used mainly as food for the adult worker ants. Many plants, e.g. sunflowers and partridge pea, secrete extrafloral nectar from glands on leaves, stems or buds. The nectar is licked up by ants, which spend long periods of relative inactivity on the plants, awaiting the slow secretion of the sweet substance. When approached by other insects, the apparently idle ants become aggressive and attack or chase them away, and will even climb on and bite the fingers of nosey entomologists investigating this behavior. Through the protection the ants afford them from leaf-eating insects, plants clearly benefit by providing a little nectar to the ants.
Ants linger in a similar manner on plants infested with leaf- or stem-sucking aphids (or scales or plant hoppers). These insects excrete excess sugars from their sap diet as a clear fluid called honeydew. Plants with aphid-tending ants on them derive a mixed benefit. They suffer from the loss of sap and increased likelihood of disease transmission through the large populations of aphids, which develop under the ants' protection. However, while visiting aphids, ants do chase off any insects which attempt to chew on the plant. These interactions, inspired by the sugar-gathering tendency of ants, can affect seed production and energy storage of ant-tended plants, and thus ants may affect species composition and relative abundance in the prairie plant community.
An ant guild with its origins in open-air aphid-tending comprises Acanthomyops and certain Lasius species, which have given up hunting and foraging above ground for subterranean livestock-rearing. These ants live in association with root-feeding aphids, harboring them in subterranean chambers among plant roots. The ants tend the aphids constantly, softly tapping them with their antennae and gathering honeydew as it is excreted in response to this antennation. To meet their protein requirements, the ants regularly slaughter some of the young aphids for meat. Aphid-rearing ants remain permanently underground save for a few days each year when they send off their winged brothers and sisters (males and queens-to-be) on mating flights.
Two groups of ant species are specialized to hunt other ants. One group includes all Polyergus and some species of the related genus Formica. These parasite/predators raid nests of certain neighboring Formica species to steal their pupae and larvae. Some of the stolen brood is eaten, but a good number of the pupae mature and become incorporated into a mixed-species colony. The stolen Formica ants ultimately do most of the work of foraging, nest-maintenance, and brood-care for their kidnapers.
Another group of ants, which hunt ants, includes army ants of the genus Neivamyrmex. These are true specialized predators on the brood of other ants, especially that of Aphaenogaster and Pheidole. Neivamyrmex are found commonly in the southern prairies, and are close relatives of the fabled tropical army ants.
Many smaller ant species in the prairie, including most of the Myrmicinae and Dolichoderinae listed in Table 1, are primarily scavengers, gathering bits of dead insects or lapping up honeydew or fruit on the ground. Solenopsis molesta, the smallest prairie ant, nests within the mounds of larger ants, feeding off their scraps and possibly occasionally snatching larvae or eggs. Other ants, such as Tapinoma, Forelius, Leptothorax and Lasius species run about singly, collecting edible fragments, and honeydew droplets that have fallen to the ground. The ubiquitous little black ant, Monomorium minimum, swarms to invertebrates killed by fire, disease, the hooves of vertebrates, or other factors. One often sees large gatherings of these small ants feasting on larger insects or on the corpses of small vertebrates killed by mowing or other causes, on hiking trails in public prairies.
Pheidole species are scavenger/predators which also regularly gather and crack open small seeds and place their larvae directly on the exposed contents to eat the nutritious inner parts. Indeed, most ants gather plant fragments, seeds and other parts which they eat, or incorporate into their nests. Certain plants rely on this behavior for seed dispersal. Violets, some sedges, trout lily and trillium take advantage of ants by producing nutritious and highly attractive fleshy appendages, or arils, on their seeds. The arils induce ants, especially Aphaenogaster, to carry the seeds home, where the soft parts are then eaten. The aril-less but otherwise intact seeds are then discarded in the ants' underground trash heaps, and there find ideal conditions for germination.
All ground-nesting ant species move prodigious amounts of soil as part of their normal nesting activity. In northern prairies, where there are no native earthworms, ants are the main movers and aerators of soil. Even further south, ants exceed earthworms in importance as earth-mixers in prairies. Waste products, discarded remains of their dead, and inedible parts of their food enrich the soil in and around ant mounds. Some prairie plants, especially annuals and the young of longer-lived plants, characteristically grow upon the loose, rich soil of abandoned mounds. Ant mounds are also the preferred habitat of a variety of small animals, especially commensal and parasitic arthropods which spend part or all of their existence in ant nests.