CONICET | Buscador de Institutos y Recursos Humanos

We examined the life history, annual production, diet, habitat preferences and competition of two species of stoneflies Notoperla fasciata and N. magnaspina in a Patagonian mountain headwater stream. Benthic samples (n=36) and adult collections were taken monthly from July 2004 to June 2005. A habitat selection study was performed concurrently during the high and low water periods at five substrate types (n=30). Although both species were semivoltines (N. fasciata: 20 months and N. magnaspina: 3 years) the life history and temporal dynamics were different. Growth was rapid during summer and early autumn in coincidence with warmer temperatures but emergence period was shorter in N. magnaspina (November to January) than in N. fasciata (January to April). N. fasciata was the dominant Notoperla species, being N. fasciata mean annual density 6 times bigger and secondary production 4.5 higher than those of N. magnaspina. While boulder in riffles and pools, and leaf-pack habitats supported significantly more individuals of N. magnaspina in the low water period (ANOVA, p<0.02) habitat preferences were not observed in the studied species (ANOVA, p>0.26). Seasonal patterns were opposite for both species with N. fasciata dominating in the high water period and N. magnaspina during the low water period. Analysis of gut contents revealed that both species were herbivorous and grazers, as functional feeding group, and consumed the same food types. The interspecific overlap in density, biomass and annual production among habitat types was high in the low water period (PS>0.5). However, inspection of size frequencies per habitat showed that N. fasciata sizes were lower than N. magnaspina. The existence of different life history strategies and the temporal shift of main generations and density peaks were critical to allow species coexistence and to reduce competition

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