Do microbial effects on hosts vary across life stage and vital rate?

Interactions with microbial symbionts are widespread and can influence host fitness through effects on survival, growth, and reproduction. Demographic and evolutionary theory suggests that symbiont effects on components of host fitness should often vary in strength and even in direction as host and symbiont allocate limited resources. Through a meta-analysis of 1,736 host-symbiont effect size measurements, we evaluated how commonly the direction of symbiont effects varies across host life history stages and vital rates, and whether these effects across stages and vital rates differ across taxa. Contrary to our prediction, we found that, compared to a null expectation, published effects on host vital rates more often occurred consistently in the same direction, rather than in opposition. This finding suggests that symbiosis often provides benefits to host performance that manifest across vital rates. While the direction of symbiotic effect was often consistent, the magnitude of effects differed across fitness components and across host taxonomic groups, highlighting diverse demographic mechanisms by which symbiosis can influence host fitness. For hosts in the Poaceae family, we also found that effects of symbiosis varied across symbiont type: vertically-transmitted Epichlöe fungi most strongly benefited seed germination, while the benefits of mycorrhizal fungi and bacterial endophytes were stronger for adult growth and reproduction. Importantly, this literature survey also highlights gaps in the current literature on host-microbe symbiosis. Most studies measure growth during juvenile stages for taxa from iconic, well-studied symbioses. Meta-analytic estimates of symbiotic effects on host life history provide a path towards quantitatively modeling host-symbiont demography across the tree of life.