From cryptic herbivore to predator: stable isotopes reveal consistent variability in trophic levels in an ant population

Populations may collectively exhibit a broad diet because individuals have large diet breadths and/or because subpopulations of specialists co-occur. In social insect populations, the diet of the genetic individual, the colony, may similarly arise because workers are diet generalists or castes of specialists. We used elemental and isotopic methods to explore how the invasive red imported fire ant, Solenopsis invicta, achieves its status as a trophic generalist. In one 0.5-ha old field, 31 S. invicta colonies ranged from 1 degrees-consumer to 2 degrees-predator (delta N-15's 0.35-7.38 parts per thousand), a range comparable to that shown in sampled ant communities. Moreover, a colony's trophic rank was stable despite delta N-15 fluctuating 2.98 parts per thousand over the year. Colonies that fed at higher trophic levels were not larger, but consumed more C-3-based resources. Individual worker mass, however, did increase with delta N-15 (r(2) = 0.29, P < 0.001). The ninefold variation in worker mass within a colony generated trophic variance approximately 15% of the population of colonies. Combined, we show how intraspecific trait variation contributes to the trophic breadth of S. invicta, and suggest mechanisms that further explain how their trophic signature varies across space, but remains stable over time.