Functional Responses and Interference within and between Year Classes of a Dragonfly Population

In a laboratory study of the dragonfly Tetragoneuria cynosura (Say) (Odonata: Corduliidae), we measured the feeding rates of second-year-class larvae (Tc2) as a function of Tc2 density and of the density of their first-year-class conspecific prey (Tc1). The experiments were conducted for 24 hr in small, structurally simple, cylindrical plastic aquaria within a controlled environment chamber under a 14L:10D photoperiod. The resulting functional response to prey density followed the decelerated curve (type 2) typical of many predators. Strong feeding interference among Tc2 larvae was indicated by an inverse relationship between feeding rate and predator density. Since we detected no effects of Tc1 or Tc2 densities on movement by larvae within aquaria, density-specific differences in movement probably cannot account for the observed interference. Both a distraction model (in which prey and predators "compete" for the predator's attention) and a pre-emption model (in which interference takes precedence over feeding) fit the data reasonably well. Our laboratory results suggest that the structural complexity and alternative prey present in a previous field study greatly reduced the rate of predation by Tc2 on Tc1 larvae but did not reduce the intensity of interference among Tc2 predators.