Effects of body size, trophic mode and larval habitat on Diptera stoichiometry: a regional comparison

Ecological stoichiometry has emerged as a tool for exploring nutrient demand and evolutionary responses to nutrient limitation. Previous studies of insects have found predictable variability in stoichiometry, both in relation to body size and trophic mode, at ordinal levels or higher. Our study further examines the evolutionary and ecological lability in these traits by comparing the effects of body size, trophic mode (larval and adult) and larval habitat on the stoichiometry of insects within one order (Diptera). The study also expands on previous work by analyzing trophic mode both at coarse (detritivore, herbivore, predator) and fine (high- vs low- nutrient quality resources within trophic categories) scales and by comparing nutrient stoichiometry in two geographical regions, Sweden and Arizona. As predicted, adults feeding on nectar or pollen had the lowest body N content in the dataset. Additionally, for Diptera with predatory larvae, species low N diets had lower body N content than those with high N diets. However, body N content was not consistently lower for all species with low N resources, as species feeding on plant material were indistinguishable in stoichiometry from predators with high N diets. We suggest that these results emerge because larval resource exploitation is poorly understood in herbivorous Diptera species. Body P content for Swedish Diptera decreased with body size for all trophic modes, and the only difference among trophic modes was that blood feeders had higher P content than other groups. The regional comparison further showed that the allometry of body P content is a labile trait that may vary at regional scales, as there was no allometric scaling of body P content in the Arizona data set, in contrast to the Swedish data set. These results are not easily explained by existing theoretical frameworks, but instead point to a general context-dependence of P stoichiometry, which should now be a focus for future work.