Biotic and Abiotic Characteristics Influencing Nest Location and Trophic Relationships of the Fungus-Growing Ant Trachymyrmex septentrionalis (Formicidae: Attini)
Trachymyrmex septentrionalis represents the only species of fungus-growing ant in the southeastern United States. T. septentrionalis cultivates a symbiotic fungus on which the ants feed. Worker ants collect plant and animal debris to feed the fungus. These ants are common in Florida, but increasingly scarce as they near the northern extent of their range in New Jersey. Colonies of T. septentrionalis occur in patches throughout Blackwater Ecological Preserve, a longleaf pine forest in southeastern Virginia. Blackwater Ecologic Preserve is a 129 hectare tract of land, located in Isle of Wight county, that is undergoing a regimen of controlled burning to increase the population of longleaf pines ( Pinus palustris). Over an 8 year period, multiple abiotic and biotic factors were studied to better understand what conditions determine the distribution of T. septentrionaliscolonies within an ecosystem and how those factors may affect the ability of individual colonies to collect sufficient substrate to sustain the fungal symbiont and feed the colony. Soil temperature (23.5 vs. 25.1°C), air temperature (22.3 vs. 26.4°C), and light intensity (302.8 vs. 595.2 lumens) were all found to be significantly higher around inactive nests. Plant cover, which would affect light intensity around the nests, was significantly higher above active nests (71.1%) than over inactive nests (57.9%). Soil samples from T. septentrionalis populated and adjacent, unpopulated sites resulted in soil moisture being significantly lower in the upper 60 cm of soil in populated versus unpopulated areas. No consistent differences were found for soil organic matter, cation exchange capacity, or pH between populated and unpopulated sites. Colony densities, calculated for each nesting site, ranged from 0.072 to 0.145 nests/m 2, and these densities can be correlated to soil and air temperatures, light intensity, and vegetation cover. In an attempt to understand the patchy distribution of T. septentrionalis in Blackwater Ecologic Preserve, the composition of ant and plant communities in areas with and without T. septentrionalis nests were analyzed. A total of 34 ant species was collected, with 27 species from sites with T. septentrionalis and 30 from those without. Only Aphaenogaster treatae displayed significantly different numbers between sites, but there were numerous species that occurred only in one or the other. A total of 15 plant species were identified in areas populated by T. septentrionalis, but 22 were identified in unpopulated areas. As with the ant community, numerous species could be located in one site but not the other. Just as air temperatures affect the overall activity of T. septentrionalis colonies, they also influence foraging rates. Field preference of forage falls into four categories (Berry, Pteridium aquilinum, Quercus sp., Unknown Materials). Carbon and nitrogen concentrations increase as the elements are tracked from the substrate to the fungus then to the ants themselves. During a season of activity, a nest of 356 workers, alates, and pupae is expected to capture approximately 29.58 kcal/m 2 of energy through substrate collection. From this total, the T. septentrionalis/fungus symbiont is estimated to assimilate between 11.1 and 12.79 kcal/nest/yr. T. septentrionalis appears to be highly sensitive to environmental conditions such as temperatue and light intensity both for daily activities and the foundation of new colonies. Soil moisture may also be considered as an important factor, either directly as it influences chamber excavation and stability or indirectly by determining vegetative cover. Overall, ant and plant communities did not affect the presence or absence of T. septentrionalis. T. septentrionalis acquires limited nutrients and energy from the surrounding ecosystem, this may be due to an inability to exploit the environment or the higher energetic demands required to acquire richer sources of nutrients and energy by leaf cutting.
Attini, Formicidae, Fungus-growing ant, Nest location, Trachymyrmex septentrionalis, Biology, Ecology and Evolutionary Biology, Entomology
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