Population Consequences of Larval Crowding in the Dragonfly Aeshna Juncea

Abstract

I studied sources of density dependence in a dragonfly population to reveal how population size is controlled. Larvae of the dragonfly Aeshna juncea, inhabiting rock pools on the Lake Superior shoreline at Isle Royale, Michigan, USA, showed clear evidence of competition when natural densities were high. After 2 yr of larval growth, individuals in crowded pools were developmentally delayed and survived less well than individuals in sparsely populated pools. The stunted larvae caught up with individuals in less crowded pools and reached the final instar in sufficient time to emerge in the summer of their 3rd yr, but even so their final body size remained relatively small. A 5—wk field experiment in natural pools demonstrated the causal connection between density and fitness: feeding activity and growth rates were significantly reduced under crowded conditions, but survival showed no response to density. In unmanipulated rock pools, however, natural increases in density from one year to the next were met with slightly overcompensatory decreases in annual survival. The results illustrate that density dependence operates in a way that could contribute to population regulation in A. juncea, and a 6—yr time series from one of the two study areas showed significant evidence for density—dependent dynamics.