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.

Size-dependent life-history traits promote catastrophic collapses of top predators

“…In agreement with observations on stunted populations with limited maximum individual length (26,27), our results suggest that size-selective predation may relieve competition within a population and reduce its stuntedness. However, this effect comes at the price of introducing a breakpoint, where the system exhibits a catastrophic collapse from an equilibrium with predators to an equilibrium without.…”

Differential dispersal propensities between individuals in maleLeucorrhinia intacta(Odonata: Libellulidae)

“…In the summer prior to this study, during the larval period of these males, Crane Pond had greater than lOx the larval density of L. intacta than East Marsh (19.llarvae m 1 in Crane and 1.8 larvae m 1 in East Marsh; E. Werner unpubl.). Nutritional constraints can result in smaller size at emergence in odonates (Rolff et al 2004;De Block & Stoks 2005) and larval densities in anisopterans have been negatively associated with body size in final instar larvae under some conditions (Van Buskirk 1993). Therefore, if the adult males captured at Crane are principally developing in Crane, their smaller size may be the result of high levels of competition during the larval period relative to that experienced by males at East Marsh.…”

Emergent impacts of cannibalism and size refuges in prey on intraguild predation systems

“…Such substitutive designs are commonly used to examine indirect interactions among multiple predators and do not confound predator identity with predator density (Sih et al 1998). Each treatment received 16 small A. umbrosa, which falls within the range of its natural densities (Van Buskirk 1993). To mimic the natural diversity of the dragonflies' prey community, we added a variety of invertebrate and vertebrate species at natural densities (60 Ephemeroptera larva, 60 one-week-old Rana catesbeiana tadpoles, 32 large Corixidae, and 14 small Corixidae) (Tolonen et al 2003) to reflect the most common prey species of both dragonflies found in the local ponds.…”