Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ(15)N values from colonies fed an animal-based diet had δ(15)N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ(15)N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ(15)N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ(15)N were 1.6-2.4‰ for L. humile and 1.8-2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ(15)N varied from 1.2 to 2.5‰ depending on the site, with δ(15)N values for queens ≥ workersbrood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages.