Literature survey and study selection
To identify studies that evaluated the effects of environmental stressors on infectivity and host fitness traits in host-parasite systems, on February 9th of 2021, we conducted a systematic literature search in Web of Science using the search terms: (parasit* OR pathogen* OR disease) AND (environment* OR temperature OR pollution OR resource OR provision* OR toxi* OR contamination) AND (infection OR load OR yield OR resistance) AND (”birth rate” OR ”death rate” OR surviv* OR mortality OR reproduct* OR fecundity). We limited our search to journal articles published in English between 2010 and 2020 and scanned titles and, if relevant, abstracts of all 20,684 hits. This initial screening effort was split and carried out by two experienced independent reviewers (AVS and BW). We identified ten additional studies from references of selected studies. One experienced reviewer or two student reviewers further examined articles documenting effects of environmental stressors on infectivity and host fitness.
We classified stressors into three groups: 1) environmental factors, which can vary naturally but are also subject to human-induced perturbation (hereafter “endogenous environment”); 2) presence or quantity of chemical pollutants (hereafter “chemical pollution”), that lead to negative expected outcomes for hosts; and 3) resource availability for hosts (hereafter “resource limitation”). Although, in natural systems, these stressors often overlap (e.g., increased temperature can alter resource availability), we included studies where only one stressor was evaluated to facilitate the interpretations of our results. We excluded studies if stressful and control environments differed due to additional antagonistic biotic interactions (e.g., presence of predators or competitors) or by the presence of substances purposely used as therapeutic interventions on infected hosts (e.g., chlorine as water treatment). Furthermore, we limited our search to studies with animal hosts and excluded studies on parasitoid infections (Fig. 1).
We included only experimental studies with hosts exposed to or infected by parasites under laboratory conditions. We only included studies if infected hosts were exposed to stressful and control treatments and both host fitness (fecundity and/or survivorship) and pathogen infectivity (prevalence and/or intensity) were reported from the same experiment (i.e., same pool of individuals divided between stressful and control treatments) at matched timepoint(s) (Fig. 1). For example, if a study reported infection intensity at 24 h and 72 h post-infection (hpi), but survivorship was only recorded at 72 hpi, we used 72h data. If a study recorded both fitness and infectivity at multiple time intervals, we included all matched intervals in data collection. We accounted for non-independence of these effects and their sampling errors in the random structure of our statistical models (see sectionsMeta-analyses and Publication bias ). Studies were further excluded for pseudoreplication, missing sample size information, or when estimates were reported without associated errors (Fig. 1).