Colony Productivity of the Fungus-Gardening Ant Trachymyrmex septentrionalis (Hymenoptera: Formicidae) in a Florida Pine Forest

The ecology of the fungus-gardening ant Trachymyrmex septentrionalis McCook (Hymenoptera: Formicidae) was investigated in a northern Florida longleaf pine, Pinus palustris Mill., forest. This ant is extremely abundant in pine sandhill in the Apalachicola National Forest, in north Florida; a hectare contains on average >1,000 nests, 235,000 T. septentrionalis workers, and 3.5 kg of fungus garden. When colony size and performance were estimated from excavations and the weight of sand in the tumulus, nests were larger and produced the most offspring in open, treeless habitats, whereas the smallest and least productive nests occurred in wooded areas. Our data suggest that the warm soils of open sites stimulate worker activity and colony growth, and cool soils of shaded, wooded sites depress performance. Moreover soils with extremely warm temperatures may have an inhibitory effect because the ants and fungus are susceptible to desiccation and excessive warmth, respectively. This suggests that T. septentrionalis is an indicator ant species of habitat quality in longleaf pine sandhills because its performance seems to be positively related to the natural disturbance regime—frequent fires occurring in the summer. Soil displacement is possibly a major effect that this ant has on the forest ecosystems: colonies may excavate over 1 metric ton of soil each year in a typical hectare of pine forest. The effect of this soil turnover is probably considerable in these nutrient-poor, sandy soils. Experiments are needed to determine the role of this highly abundant ant in longleaf pine forests.