Behavioral Differences between Enallagma Species (Odonata) Influencing Differential Vulnerability to Predators

Abstract

In previous research I have shown that fish predation contributes to the exclusion of a group of Enallagma species (Odonata: Zygoptera) from lakes containing fish, whereas dragonfly predation contributes to the exclusion of another group of Enallagma species from fishless lakes. In the study reported in this paper I performed a series of laboratory experiments to identify behavioral differences between the two groups that could contribute to their differential vulnerabilities to the two predators. In one experiment I observed the behaviors of four Enallagma species, two from fishless lakes and two from lakes containing fish, in the presence of no predators, dragonfly larvae (Anax junius), and fish (Lepomis macrochirus) to identify behavioral differences that may cause the species to be differentially detected by the predators. In another experiment, I observed Aeshna dragonflies feeding on Enallagma larvae to evaluate whether species in the two groups differ in how they attempt to avoid attacking dragonflies. The Enallagma species from fishless lakes walked in rapid bursts of 2—3 s and frequently performed a conspicuous behavioral display, while the Enallagma species from lakes containing fish walked very slowly for durations of 20—30 s and almost never performed the conspicuous display. The species from lakes containing fish also swam for longer durations and remained motionless longer following swims than species from fishless lakes. The species from fishless lakes also greatly reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of dragonflies, but only reduced the number and duration of the conspicuous display in the presence of fish. In contrast, the species from lakes containing fish reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of both fish and dragonflies. In videotaped feeding trials, larvae of an Enallagma species from fishless lakes usually swam away from attacking Aeshna dragonflies. Larvae of an Enallagma species from lakes containing fish usually did not respond to attacking dragonflies, and were usually captured without attempting to evade the dragonfly. These behavioral differences are consistent with observed differences in their vulnerabilities to predators, and probably contribute to causing the pattern of habitat separation observed in the field.