Phenotypic shifts caused by predation: selection or life-history shifts?

Abstract

Predators can impose both selection and life-history shifts in prey populations. Because both processes may affect phenotypic distributions, the estimates of selection differentials may be biased. We carried out two field experiments to disentangle these separate effects. We studied whether dragonfly predation by Aeshna cyanea changes the distributions in body size and lamellae morphology in the damselfly Lestes sponsa. Damselflies have caudal lamellae which are used in escapes by swimming. In a first experiment, we manipulated predator presence (No Aeshna, Encaged Aeshna or Free-ranging Aeshna) and stopped the experiment when all larvae had moulted once. In a second experiment, larvae were confronted with a Free-ranging Aeshna but collected before moulting, and survivors were compared with a control sample taken at the start of the experiment. The presence of Aeshna largely reduced the survival probabilities of the Lestes larvae at a very similar rate in both experiments. Daily survival probabilities did not differ between the No Aeshna and Encaged Aeshna treatments. In the Free-ranging Aeshna treatment of the first experiment, size was reduced compared to the other two treatments, creating a significant apparent selection differential. This was probably mainly due to predator-induced reduced growth because in the second experiment, where growth effects were excluded, size of the survivors did not differ from the control sample. In both experiments there was a significant selection pressure for larger lamellae. Standardized directional selection differentials were similar in both experiments (0.57 and 0.28 phenotypic standard deviation units). No survival selection on lamellae shape was detected. These results are in agreement with previous findings that lamellae size, but not lamellae shape, enhances swimming performance and thereby predator escape in this species.