Density‐Dependent Cannibalism in Larval Dragonflies

Cannibalism is a likely mechanism of population regulation in dragonflies that inhabit fish—free temporary ponds. I tested for density—dependent effects of cannibalism on survival and size structure of larvae of the dragonfly Tramea carolina in two short—term experiments in replicated artificial ponds. I was able to distinguish mortality due to cannabalism from other sources of mortality by preventing some populations from engaging in cannibalism. The first experiment included two density levels and the presence or absence of cannabilism in a factorial design. The no—cannibalism treatment was achieved by removing the labial palps from all individuals to prevent them from grasping large prey. T. carolina survived significantly better in the no—cannibalism populations at both densities, and this difference was greater at high density than low density, indicating that the proportion cannibalized was density dependent. A second experiment explored the form of the density—dependent cannibalism curve over a broad range of density. The results were adequately fitby a predator—prey model incorporating size structure, so that not all members of the population were vulnerable to cannibalism. In both experiments the survival of only the smaller instars was reduced, suggesting that cannibalism may reduce variation in the size distributions of dragonfly populations and contribute to emergence synchrony. The results demonstrated that cannibalism was strongly density dependent and may contribute to population regulation of dragonflies in temporary ponds.