Moderators
We examined three factors that could moderate the magnitude of stressor
effects. For Q1, we considered infection status (infected and
uninfected), stressor type, and response trait (fecundity and
survivorship) as moderators. Here, we were specifically interested in
whether infection status amplified any negative fitness consequences of
stressors. As mentioned above, stressors were of three types: 1)
endogenous environmental factors, such as temperature, humidity,
salinity, dissolved oxygen, habitat structural complexity, etc., 2)
chemical pollution by toxins or synthetic compounds typically derived
from pesticides or herbicides, and 3) resource limitation, primarily by
restricting access to food but also included limitation of specific
nutrients in food items, such as nitrogen and phosphorus. For response
traits, fecundity was typically recorded as the total number of
offspring, whereas survivorship was reported as proportion alive, number
alive, and sometimes, time to death.
In Q2, we focused exclusively on infected individuals under the
abovementioned criteria. We investigated stressor type and response
trait as moderators. Here, we aimed to contrast the effects of stress on
fitness vs. infectivity responses. We, therefore, included two
additional response traits as infectivity proxies: infection intensity
and prevalence. Prevalence was always reported as the number or
percentage of infected individuals. Infection intensity was often
quantified in different ways for different types of pathogens, for
example, (log) copy number for viruses, colony-forming units for
bacteria, and spore counts for fungi. To compare the relative
sensitivity of fitness and infectivity, and because prevalence and
infection intensity represents the opposite of host defense, signs of
unbiased standardized mean differences were flipped. By doing so, a
positive effect size reflects greater defense and a beneficial outcome
for hosts, whereas for fitness traits, a positive sign indicates higher
survivorship or fecundity.