Dietary niche overlap and mutual predation among coexisting larval Anisoptera

Larval populations of Tetragoneuria cynosura. Celithemis elisa, C. fasciata, and Sympetrum vicinum coexist in allochthonous detritus habitats of Bays Mountain Lake (Sullivan County, Tennessee, U.S.A.). Their diets were described by fecal pellet analysis to determine whether size differences among larval cohorts reduced dietary overlap within seasons. Differences in larval development time within this assemblage resulted in considerable size overlap among co-occurring larvae. Some species-specific differences in diet were found (e.g., T. cynosura tended tn concentrate on insect prey, while S. vicinum took more microcrustaceans); but there was considerable dietary overlap among species within seasons. Most ate. a disproportionate number of medium-sized cladocerans, ostracods, oligochaetes, burrowing and climbing midges, trichopterans and odonates. Fewer copepods, large and small cladocerans, and sprawling midges were eaten than might have been expected from estimates of prey abundance. The similarities out-weighed the differences, such that niche-overlap indices (Hurlbert's were always greater than 1.0 (range 2.4 to 8.7). — A comparison of diets among larvae within species found no consistent reduction of dietary overlap among instars. Larger inslars were often more specialized due to predation on relatively rare large prey (e.g.. odonate and trichopteran larvae), but they did not omit smaller items (e.g.. chydorid cladocerans and ostracods) from their diets. Thus smaller instars experienced relatively high niche overlap with their larger conspecifics, — The incidence of mutual predation among odonate larvae was very high during seasons when very small instars coexisted with large ones. The increase in this ultimate form of "interference" competition may be a more important consequence of seasonal segregation of life histories than reduced "exploitation" competition due to reduction of dietary niche overlap.