Resetting our expectations for parasites and their effects on species interactions: a meta‐analysis

Abstract

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host–species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta‐analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non‐parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host–species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically‐transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non‐parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.

Effects of thermal fluctuations on biological processes: a meta-analysis of experiments manipulating thermal variability

“…The use of effect sizes is not intended to fully capture the biological consequences of a response variable but instead to compare responses across different experiments. As such, we attempted to capture some of the likely biological impacts by converting effect sizes associated with detrimental effects to a negative value by multiplying these SMDs by −1 [ 34 ].…”

A meta‐analysis of how parasites affect host consumption rates

“…Specifically, infected hosts may be more likely to adapt their foraging and feeding behaviour to satisfy their nutritional needs. The meta‐analysis of variance could, therefore, reveal hidden effects of parasites and broaden our understanding of biological processes (Nakagawa et al 2015, Sánchez‐Tójar et al 2020, Hasik et al 2023).…”

“…To address these issues, we performed a meta‐analysis of studies that experimentally tested consumption rate of infected versus uninfected hosts. We not only investigated mean differences, but also performed a meta‐analysis of variance (Nakagawa et al 2015, Sánchez‐Tójar et al 2020, Hasik et al 2023), as different parasites may have variable effects on host foraging behaviour. Our analyses of infected animal hosts across taxonomic groups addressed the following questions: 1) how strong is the effect of parasitic infection on host consumption rate and on the variability in host foraging behaviour?…”

Interspecific host competition and parasite virulence evolution

“…Of further interest is how virulence evolution can then feed back into community dynamics (double arrow in figure 1). Parasites not only decrease host fitness [61], but can also affect the strength of host interspecific host competition [37]. Virulence evolution mediated by interspecific host competition may drive a subsequent coevolutionary response in hosts (due to parasite-mediated effects on host fitness) and/or modify virulence evolution (due to parasite-mediated effects on interspecific host competition).…”