Two threats at once: encounters with predator cues alter host life-history and morphological responses to parasite spores

Parasites and predators are ubiquitous threats in every ecosystem. Host and prey species, respectively, have evolved effective protective mechanisms which are assumed to involve costs. In this study, we analyzed potential interactions between both threats. We exposed waterfleas (Daphnia longicephala) simultaneously to parasite spores (the yeast Metschnikowia) and cues from predatory notonectids (Notonecta glauca). In response to the parasite, D. longicephala had a delayed maturation time and produced less and smaller offspring, even though the parasite developed no spores. This suggests that hosts can successfully fight off the parasite invoking defensive costs. Some of these effects were altered or even reversed by the presence of predator cues. For example, time to maturity was further delayed when the Daphnia were exposed to both threats than under parasite stress alone. In addition, more offspring were produced in the presence of both threats, although parasites alone reduced their number. However, there was no effect of parasite exposure on the expression of morphological defenses. Our results imply that the impact of parasites on host species depends strongly on the presence of further threats. Similar types of experimental approaches may enhance our understanding of the effects of multiple stressors in natural systems.