Physiological and Behavioral Responses to Predators Shape the Growth/Predation Risk Trade-Off in Damselflies

Most organisms must simultaneously find enough food for themselves while trying not to become food for some other organism. Previous field experiments have shown that larvae of Enallagma and Ischnura species are able to coexist in the littoral zones of lakes because they resolve this growth/predation risk trade-off differently: Ischnura species grow more quickly than Enallagma species, but Ischnura species suffer higher mortality rates than Enallagma. We performed a series of laboratory studies to explore the mechanistic basis for the difference in growth between the genera. When held in complete isolation and with unlimited food, larvae of a number of Enallagma species that coexist with fish accumulated mass at much faster rates than Ischnura species. This difference in isolation was due to the fish-lake Enallagma simply ingesting more food. In contrast, when held in the presence of other damselflies or a fish predator, Ischnura had significantly higher growth rates than Enallagma species from fish lakes. All species decreased the amount of food they ingested in the presence of the fish predator as compared to when fish were absent, which resulted in decreased growth in the presence of the predator for all species. However, the interspecific differences in growth rate were due primarily to differences in the abilities of the species to convert ingested food into their own biomass; in the presence of fish, comparably sized larvae ingested nearly identical amounts of food, but Ischnura larvae grew faster because they converted significantly more ingested food into their own biomass than did larvae of Enallagma species from fish lakes. This difference in conversion efficiency between the genera was not apparent when larvae were raised in complete isolation. These results indicate that Enallagma and Ischnura species differ in physiological stress responses to the presence of predators, and this difference may facilitate the coexistence of Enallagma and Ischnura species in the field.