The positive effect of the maternally transmitted fungal endophyte Epichloe festucae on his host grass Festuca eskia is not systematically associated with high transmission rates of the symbiont over short time scales, in particular following an environmental change.
We used a demographic population modeling approach to identify the mechanisms operating in a natural stand of an intermediate population (i.e. 50% of plants symbiotic) of the native alpine grass Festuca eskia. We recorded demographic data in the wild over a period of three years, with manipulation of the soil resources for half the population.
We showed that symbiotic and non-symbiotic plants can coexist in a stable manner (~58 % of endophyte prevalence) within grass populations, due to the combination of a positive effect of the endophyte on host fitness, and imperfect vertical transmission (around 0.63 to 0.87).
In the presence of nutrient supplementation, population growth rates were still significantly higher for symbiotic than for non-symbiotic plants, but endophyte prevalence fell to 0% due to lower transmission rates.
See the full paper in Gibert et al (2015) in PLOS ONE