Microbial mutualists buffer hosts against negative effects of environmental variability

Background/Question/Methods

Microbial symbioses are widespread in nature and notoriously context-dependent, spanning a continuum from mutualism to parasitism depending on environmental conditions. Most symbioses are subject to temporal fluctuations in environmental conditions, and this is increasingly the case under global climate change. This context-dependency means that symbionts may be most beneficial to hosts in harsh years but neutral or even costly in benign years. Therefore, in the long term, symbiosis with microbes may reduce demographic variance in the host. This talk will explore the hypothesis that symbiotic microbes function as mutualists by reducing variability, even if their fitness effects are neutral, on average. Our work leverages a long-term field experiment in which plant populations were established with or without vertically transmitted fungal symbionts, replicated across seven host-symbiont pairs. We built population projections models based on 10 years of data and decomposed the overall fitness effect of symbiosis into contributions from the means and temporal variances of multiple demographic rates integrated across the life cycle.

Results/Conclusions

In our grass-endophyte model system, microbial symbionts strongly buffered host vital rates such that symbiotic hosts exhibited less year-to-year variability than symbiont free hosts. However, we also found that the overall contribution of variance buffering to the fitness outcomes of mutualism was much weaker than that of the overall average benefit of harboring symbionts. Thus, we provide novel evidence for variance buffering through symbiosis but show this is a quantitatively weak component of the overall fitness outcomes of host-symbiont mutualism. Our next steps our to understand whether global climate change, which includes elevated year-to-year variability, will amplify the importance of fitness buffering in this host-symbiont mutualism.