Seasonal Variation in the Intensity of Competition and Predation Among Dragonfly Larvae

In nature, both similar and disparate sizes of Libellula lydia and L. luctuosa larvae frequently co—occur in time and space. To determine if these larvae interact as competitors, and/or predators and prey, I used artificial ponds to manipulate density, species composition, and size range of co—occurring larvae. Detailed life history data were used to design separate fall and spring experiments. In both experiments, "competition treatments" contained only larvae similar in size, whereas "predation treatments" contained larvae disparate in size. In fall competition treatments there were no density—dependent growth responses. However, in the spring experiment, larvae of both species grew significantly faster in low density than in high density treatments. This seasonal difference in competition was attributed to fluctuations in resource abundance. Competition did not directly affect survivorship. In spring and fall predation treatments, mortality was significantly higher than it was in competition treatments. Inter—odonate predation accounted for 25—45% of total larval mortality in fall, but only 10—15% in spring. In the absence of inter—odonate predation, total mortality was lower for larger larvae than smaller larvae, suggesting the latter are more susceptible to predation by other invertebrate predators. Thus, competition, by decreasing growth rates, should indirectly affect larval survivorship. These data provide evidence that competition and predation will simultaneously affect coexistence between these two dragonfly species. Predation early in larval development should ameliorate the intensity of subsequent competitive interactions at a time when resources are most likely to be limiting. This type of mixed competition/predation interaction is analogous to predator—mediated coexistence, and might explain how such ecologically similar species can coexist at such high densities.