Coexistence of Enallagma divagans and Enallagma traviatum (Zygoptera: Coenagrionidae) in Bays Mountain Lake, Tennessee: an in situ enclosure experiment

Enallagma divagans and E. traviatum coexist as larvae in Bays Mountain Lake (City of Kingsport, Sullivan County, Tennessee) where they are among the most abundant odonate populations in allochthonous detritus habitats. An in situ enclosure experiment, designed to detect both intraspecific and interspecific competition among larvae at approximately natural densities and size-distributions, was conducted during April 1982. E. divagans larvae experienced significantly better survival and biomass increases than E. traviatum during the one-month experiment, but neither response was density-dependent. The mean “condition” (the ratio of individual larval mass to the mass predicted by a regression of In (mass) on In (head width) for all individuals of each species measured during the experiment) of individual E. divagans larvae was density-dependent, with interspecific effects at least as important as intraspecific ones. Fecal pellet analyses detected considerable dietary overlap and little evidence of resource partitioning among the surviving larvae; yet there was no evidence of exploitation competition. Only one of the principal prey populations (oligochaetes) was significantly reduced in the presence of damselflies compared to control enclosures. The density-dependent “condition” of individual E. divagans may be attributable to some form of interference competition.