Influence of mound building and selective seed predation by the red imported fire ant (Solenopsis invicta) on an old-field plant assemblage

We examined how selective seed predation by the introduced red imported fire ant (Solenopsis invicta Buren) influenced an old field plant assemblage by observing the ants' effects on ragweed (Ambrosia artemisiifolia) distribution and growth. Ragweed had higher cover on ant mounds than off mounds, and plants growing on the mounds had higher aboveground biomass than did plants growing away from mounds. Nitrate concentrations were higher on ant mound soil, while concentrations of organic matter, phosphorus, magnesium, copper, and zinc were higher in soil off ant mounds. In seed predation experiments, ragweed seeds were selected less often than those of four other early successional species: pigweed (Amaranthus retroflexus), bluegrass (Poa annua), common lamb's quarters (Chenopodium album), and goldenrod (Solidago altissima). Moreover, fire ants selected both larger and smaller seeds than those of ragweed indicating that selection was not driven by relative seed size. There was an increase in the number of ragweed seeds removed by fire ants as the summer progressed, suggesting that more ragweed seeds were incorporated into the diet of fire ants as other species of seeds became less abundant. Lastly, nearly all seeds of pigweed (a preferred species) placed 1, 2, 3 and 4 meters from ant mounds were consumed, which suggests that fire ants may effectively forage the entire field. Both fire ants and ragweed are early successional species that rapidly invade disturbed areas created by humans and natural events. Fire ants significantly contributed to the success of ragweed plants growing in an old field community.