The ants Formica podzolica, Myrmica fracticornis and M. incompleta commonly build nests in a large peatland complex in N-central Montana. Nests of Formica are much larger than those of Myrmica spp. and occur in a microtopographic mosaic of hummocks and hollows. The large mound nests of F. podzolica and the hummocks are similar in size, and both had elevated levels of K, PO₄⁻, Mg and Na compared to the peatland surface, suggesting that the hummocks are abandoned ant mounds. Ant mounds provide an environment for plants that has better aeration and is warmer as well as nutrient-enriched. A few species of strongly rhizomatous graminoids occur on active mounds, but abandoned nests (hummocks) provide habitat for larger shrubs as well as many species of plants that otherwise could not grow in the cold, nearly saturated peat. Formica workers obtain much of their nutrition by tending aphids that feed on the shrubs growing on hummocks. We hypothesize that this is a positive feedback relationship that has promoted the increase of Formica colonies and allowed them to permanently change the structure and composition of a large portion of the vegetation in this rich fen.