Life History and Size Structure of Larval Dragonfly Populations

Comparative data on emergence, adult activity, and larval development were used to interpret patterns of phenology, voltinism, and developmental synchrony in a 14-species assemblage of dragonflies (Odonata:Anisoptera). Interspecific differences in these characteristics provided an opportunity to interrelate life history, population size structure, and the potential for intra- and interspecific interactions. Larval populations of most species comprised an extraordinary size range of individuals at any one time. In many species, this intraspecific size variability was primarily due to developmental asynchrony within generations (e.g., Libellula lydia, Celithemis elisa, Perithemis tenera). Oviposition in these species occurred throughout most of the summer so that the timing of instar development varied considerably among broods. By fall, the largest larvae in these populations were 5 to 10 times the size of smaller conspecifics. In other asynchronous breeders, an even wider size range of co-occurring conspecifics resulted from overlap between generations. In Erythemis simplicicollis and Pachydiplax longipennis this generation overlap was due to the presence of mixed uni- and bivoltine cohorts, whereas in Tramea lacerata and Anax junius, it was due to phenologically separate but overlapping migratory and resident cohorts. In the semivoltine Epitheca princeps, size variability among conspecifics was due mainly to overlap between year classes. Most larvae were similar in size at the same point in time in those species that bred synchronously and were univoltine (Epitheca cynosura, Leucorrhinia intacta). Size ratios of the smallest to largest co-occurring conspecifics in these populations were frequently larger than those commonly thought to facilitate interspecific coexistence. Thus, intraspecific resource partitioning should play an important role in population regulation, as should cannibalism. This intraspecific size variability should also create the potential for mixed competition--predation interactions between the same species. A phenological tradeoff between competition and predation should minimize the efficacy of seaonal segregation as a mechanism for minimizing the intensity of interspecific interaction in this assemblage.