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).