Ontogenetic diet shifts and their incidence on ecological processes: a case study using two morphologically similar stoneflies (Plecoptera)

Most ecological studies consider conspecific individuals as ecologically equivalent, assuming that inter-instar variation is weak and has a limited influence on functional processes. To test this assumption, we investigated the life history and trophic basis of production in two predatory stoneflies, Perlodes microcephalus and Isoperla acicularis, in a mountain stream. The variety of prey types increased with predator size. However, larvae started their development with a strictly phytophagous diet (3 months), then ingested both vegetal material and animal prey (2–3 months). Finally, larvae were strictly carnivorous when head width reached 1.1 mm. Young (herbivorous) I. acicularis larvae occurred from June to August. Young P. microcephalus larvae hatched in October, and were herbivorous until December, when I. acicularis larvae were omnivorous. Competition for animal prey was likely to occur in the spring, but P. microcephalus grew faster and emerged earlier. The cohort productions were mostly based on carnivory (78.7–90.7%), because most production occurred in later instars. Ontogenetic diet shifts could play a role in the structuring of species assemblages by adjusting species' requirements to the temporal dynamics of environmental conditions, including food availability and biotic interactions. However, their incidence on global processes is quantitatively limited, large individuals having the greatest impacts on the way energy flows.