Defending against predators and parasites, while essential for survival, is costly. Expression of anti-predator and anti-parasite defenses involve changes in behaviour, morphology and physiology that can have non-lethal effects on hosts because those changes often compromise reproduction, nutrient intake, and other fitness-related processes. Non-lethal effects of predation and parasitism have been studied extensively, but in isolation. However, clear overlaps exist in predator-prey and parasite-host dynamics, and the pervasiveness of these types of interactions in natural populations call for an integrated approach for evaluating the role of non-lethal effects in ecology. We present a general framework for modelling non-lethal effects of predator-prey, parasite-host, and other consumer-resource interactions. We also describe results of a meta-analysis, informed by our framework, that compares the relative magnitude of non-lethal effects from predation and parasitism.
Results/Conclusions
Our framework distinguishes between defenses expressed prior to, during and after consumers attack. Doing so highlights how differences in detectability of predators and parasites, combined with differences in the probability of surviving their attack and consumption, can lead to divergent non-lethal effects. For example, the lethality of predator consumption should select for strong pre-attack defenses, while the possibility of surviving consumption by parasites may select for more retroactive responses. Our meta-analysis shows that in comparison to parasites, predators do indeed elicit stronger adjustments in behavioural defenses prior to attack. Defense responses to the combined presence of predators and parasites were similar in magnitude to those of predators in isolation, which may indicate prioritization of anti-predator over anti-parasite defenses. Unlike anti-predator defenses, defense responses against parasites, while generally weak, were made not only prior to attack, but during attack and consumption (i.e. infection) as well. Given the longer time allocation to anti-parasite defenses, the net non-lethal effects of parasites may be similar in magnitude to those of predators despite the higher costs associated with predator consumption. Our framework offers guidance for tests of this and other hypotheses regarding non-lethal consequences of different types of consumers.