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.