Fungal symbionts maintain a rare plant population but demographic advantage drives the dominance of a common host

1. A potential driver of species abundance that remains understudied is the interaction between host species and their microbial symbionts. Beneficial symbionts could promote the dominance of common host species by increasing their population growth rates more than they do for rare species, and symbiont benefits could be important for maintaining rare species in communities. Alternatively, intrinsic differences in demography, independent of interactions with symbionts, could be the main driver of species’ relative abundances. 2. Here, we used demographic modelling with 5 years of data from experimental host populations to compare how symbiotic fungal endophytes, which are vertically transmitted from parent plant to offspring, influenced the population dynamics of one pair of co-occurring, congeneric rare versus common host grasses (genus Poa). 3. The common plant species achieved higher population growth than the rare species. Endophyte symbiosis increased the geometric population growth rate (k) of rare and common species by 18% and 32%, respectively, but only the rare species was predicted to decline (k < 1) in the absence of the endophyte, demonstrating that symbiosis was essential to maintain this species in the community. 4. Endophyte symbiosis differentially affected the demographic transitions of the two hosts, increasing survival and growth for the common host, Poa sylvestris, and increasing survival but decreasing the probability of flowering for the rare host, Poa alsodes. The total contribution of the endophyte effects on host demographic rates to the overall difference in population growth between host species was small compared to the plants’ intrinsic differences in demography. However, low rates of vertical transmission in P. sylvestris lessened its advantage in intrinsic demography over P. alsodes and thus decreased the projected difference in population growth between host plants. 5. Synthesis. Our results highlight the importance of plant–symbiont interactions in the persistence of a rare plant population, as well as the utility of demographic models in teasing apart the relative importance of plant demographic rates versus host–symbiont interactions on the regional abundance of rare and common host plant species.