The following is information on ants in landscapes or turf.
BLACK PAVEMENT ANTS (Tetramorium caespitum) are the most likely culprits if the mounds are relatively small, measuring 2-4" across. BLACK FIELD ANTS (Formica subsericea) are the architects if the mounds are large and relatively flat, measuring 1-3' across and only a few inches tall. Pyramidal mounds that are 1-3' across and 1-2' tall are engineered by ALLEGHENY MOUND ANTS (F. excectoides).
Black pavement ants are small and have pale legs, a brown to black thorax, and a black abdomen. Their habit of locating their underground colonies beside or beneath sidewalks gives rise to their common name. The ants scavenge for a wide variety of food including live and dead insects, honeydew from aphids, grease, etc. The ants are very protective of their feeding territory, and very intolerant of nearby colonies. They are well-known for their bare-tarsal brawls with battles sometime spilling across the entire width of sidewalks.
Black field ants and Allegheny mound ants are both relatively large with foraging workers measuring around 1/4" in length. Both are also associated with aphids, or other plant-sucking insects. The ants protect the sucking insects by fending off predators. In return, the sucking insects provide the ants with high energy carbohydrates in the form of honeydew.
Allegheny mound ants are one of the true mound-builders of the ant world. Their pyramidal mounds rise conspicuously above the surrounding landscape. Mound ants range in color from chestnut red, to black, to a combination of red and black. The ants are very aggressive, and they can use their powerful mandibles to deliver a noticeable pinching bite to a probing finger. However, their threat to people is inconsequential compared to their impact on plants. Mound ants are capable of killing small bushes and trees by injecting formic acid into wounds created by their mandibles. They use this capability to clear plants that shade their mounds. All vegetation, with the exception of large trees, may be killed within 40-50' of large, well-established mounds.
Black field ants are much less accomplished mound-builders. They form large, low-profile mounds of loose soil. These ants do not inject formic acid into plants, so they are not direct plant killers. However, they often heap soil over low-growing plants and they will pile soil high onto plant stems. Their plant-smothering mounds have been known to cause plants to decline and die. In particular, they are sometimes considered a serious pest of turfgrass. These large black ants are also very aggressive, and they will attempt to bite using their powerful, well developed mandibles.
Although both Allegheny mound ants and black field ants feed on honeydew produced by plant-sucking insects, they are also very effective scavengers and predators. The same is true of black pavement ants. The ants may be observed dragging live or dead cuisine back to their mounds to be surgically dismantled for protein. Since these ants are considered beneficial, management efforts should focus on dissuasion. For example, the pavement ants and field ants loathe thick, high-cut turfgrass. Repeated destruction of the mound ants abode can persuade them to locate elsewhere.
Information from the Ohio State Unibersity Buckeye Yard and Garden On-Line