Natural enemies undoubtedly have significant negative effects on their victims through consumption. However, as has been demonstrated for predators, non-consumptive effects such as life history and behavioral alterations could have consequences that are even more ecologically significant for prey than those resulting from consumption. Aside from predators, parasites represent another ubiquitous threat whose effects have also largely been considered in the context of victim consumption. Importantly, fear of either enemy type can lead to avoidance behaviors that may conflict with one another, but also adversely affect critical activities including foraging and mating. Using a model system, we have investigated avoidance behavior by larval amphibians in response to threat by parasites (flatworms) and predators (larval odonates) to explore their relative influences, as well as whether these hosts respond to variation in their risk of infection in a manner similar to that of predation.
Results/Conclusions
When present as individual threats, tadpoles avoided foraging in areas associated with risk of parasitism or predation, indicating a common fear. Yet as simultaneous hazards, predators had a stronger negative effect on foraging, suggesting that the likely more lethal threat (predation) has greater influence. Previous tadpole exposure to predation cues resulted in behaviors maladaptive to a current threat by parasites, and less-developed (more susceptible) tadpoles did not exhibit greater spatial avoidance of parasites. This indicates a lack of plasticity in their fear response that is inconsistent with the threat-sensitive avoidance hypothesis for predation risk. However, larval amphibians responded to parasite threat in an intensity-dependent manner through greater avoidance of strong versus relatively weak parasite cues, and were also more likely to forgo foraging in the presence of a tadpole-infecting species of parasite compared to one utilizing arthropod hosts. As such, hosts have some capacity to match their fear response to the degree of parasite threat, but this may differ from that to predators. It is critical to consider the complexities related to risk that are inherent in natural systems, such as threat intensity and spatial or temporal overlap, to unify natural enemy theory and achieve a better understanding of the possible joint and separate non-consumptive effects of parasites and predators on their victims.