Predator-prey interactions in freshwater systems: The importance of chemical communication

Abstract: Predator released chemical cues provide a valuable source of information to prey organisms that can be used to alter their behaviour in order to reduce predation risk. In this thesis, I have examined under what circumstances chemical cues may be used by prey, and if prey discriminate between different cues. I have also investigated the role of predator experience in prey ability to detect and respond to predator cues and the heritability of this ability. Furthermore, costs and benefits associated with a prey response has been measured, as well as the indirect effect of this response on other trophic levels. The experiments have been performed both in the laboratory and in the field. My results indicate that chemical cues are involved in many predator-prey interactions, sometimes in combination with other sensory stimuli but chemical cues may dominate in environments with poor visibility. The cues seem to be highly specific; prey species were able to discriminate between predators and non-predators. However, the ability differed between prey species. Whereas a water beetle was able to distinguish hungry predators from satiated, dietary cues were of no importance for an amphipod. Hence, the situation (predation pressure) and prey defences (e.g. unpalatability) seem to determine how specific the cue has to be to elicit a response in prey. Prey species with no experience with fish did not respond to fish cues unless the cue was combined with familiar alarmsubstances from conspecifics. The ability or inability to respond was heritable; prey with experienced parents responded irrespective of their own predator experience, and vice versa. To an experienced prey, a behavioural response confered a fitness benefit in terms of increased survival whereas associated costs (decreased foraging) seem to be only short-term. Predator induced changes in prey behaviour had a positive indirect effect on prey food sources. This effect was as strong as density-mediated predator effects. However, I also found that the presence of fish cues can have a positive indirect effect on primary consumers by reducing the number of secondary consumers colonising a habitat. Taken together, chemical cues from predators may affect not only the prey itself but also food chain interactions through both non-lethal and lethal effects.