1. Heritable symbionts, found within a diverse array of flora and fauna, are often observed at intermediate prevalence within host populations, despite expectations that positive fitness feedbacks should drive beneficial symbionts to fixation. Intermediate prevalence may reflect neutral dynamics of symbionts with weak fitness effects, transient dynamics of symbionts trending toward fixation (or elimination), or a stable intermediate outcome determined by the balance of fitness effects and failed symbiont transmission. Theory suggests these outcomes should depend on symbiont-conferred effects and vertical transmission efficiency, which may both depend on environmental context.

2. We tested these alternatives by tracking temporal change in population-level symbiont prevalence in a three-year field experiment with grasses and vertically transmitted fungal endophytes under the context of manipulated precipitation. Precipitation can mediate endophyte effects on host grasses, and is a relevant context, as global climate models predict changes in precipitation amount and seasonal distribution.

3. We found evidence for each of the three proposed mechanisms for intermediate symbiont prevalence, but the outcome differed qualitatively across years and precipitation treatments. Specifically, within the first study year, all populations under ambient precipitation trended toward symbiont fixation, whereas elevated precipitation literally neutralized the interaction resulting in no long-run equilibrium. The second study year yielded a different result with long-run equilibria at an intermediate prevalence for both ambient and elevated precipitation treatments. We linked these population-level outcomes with individual-level symbiont effects on grasses and showed that the benefits of symbiosis were concentrated at the seedling stage, especially under drought stress during the time period coinciding with recruitment. The drought stress evident during the first year of the study was not present in the second year due to greater ambient precipitation during this time period. Together, these findings provide the first experimental evidence that population-level outcomes of host-symbiont interactions, which arise from the composite outcome of demographic vital rates and transmission, can be dynamically stable and can fluctuate in response to shifts in spatio-temporal context.

4. Synthesis: Overall, these results indicate that intermediate prevalence of heritable microbial symbionts likely reflects a combination of neutral, transient, and stable mechanisms, and context-dependent fluctuations among them.

Data collected from the field and lab. Data presented here is the final version for use with the JAGS parameterization R script for the JAGS model.

The files provide the demographic data collected from the fieldsite and the agrinostic assay data for use in parameterizing the JAGS models.

AGHY is the USDA code for Agrostis hyemalis.

• AGHY_plot_dat.csv   - provides plot-level data

• AGHY_subplot_dat.csv   - provides subplot-level data

• flowering_individuals.csv   - provides flowering data for individual demographic plants

• flowering_plts.csv   - provides flowering data at the plot-level

• flowering_sbplts.csv   - provides flowering data at the subplot-level

• infs_dat.csv   - provides inflorescence count data 

• infs_unk_stacked.csv  - provides inflorescence count data for plants of unknown symbiont status

• linreg_dat.csv   - provides data for linear regression for the conversion from seed counts to seed mass (g)

• rec_final.csv   - provides recruitment data

• rec_plot_final.csv   - provides plot information for the recruitment data

• seedmass_dat.csv   - provides seed mass data

• survival_dat2.csv   - provides survival data in the correct alignment for use in the recruitment model

• survival_known.csv   - provides survival data for plants of known symbiont status

• survival_plot.csv   - provides plot information for the survival model

• survival_unk_sbplt.csv   - provides survival data for plants of unknown symbiont status

• total_plants_infs.csv   - provides data on inflorescences and total plants

• vtrans_dat.csv   - provides data on vertical transmission (number of symbiont positive seeds and total number of seeds scored for the symbiont)