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 (λ) of rare and common species by 18% and 32%, respectively, but only the rare species was predicted to decline (λ < 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.
Poa alsodes demographic transitions
Survival, growth, and reproduction of Poa alsodes individuals in experimental plots 2008-2011. Populations established at the Indiana University Research and Teaching Preserve at Lilly Dickey Woods, Nashville, Indiana, USA (39°14'54''N, –86°13'05''W). Column heading descriptions as follows: "TAG": individual identifier, "PLOT": the plot to which that individual belongs, "Endo": whether plant was endophyte-symbiotic (1) or endophyte-free (0), "Year1": the beginning year of the transition period, "Age1": age of individual at year 1, "Size1": size of individual at year 1 (number of tillers), "Seeds1": projected amount of seed production at year1, "Survto2": whether that individual survives to the next year, "Size2": if it survives, the size of the individual at the next year.
POAL_transitions_R.csv
Poa sylvestris demographic transitions
Survival, growth, and reproduction of Poa sylvestris individuals in experimental plots 2008-2011. Populations established at the Indiana University Research and Teaching Preserve at Lilly Dickey Woods, Nashville, Indiana, USA (39°14'54''N, –86°13'05''W). Column heading descriptions as follows: "TAG": individual identifier, "PLOT": the plot to which that individual belongs, "Endo": whether plant was endophyte-symbiotic (1) or endophyte-free (0), "Year1": the beginning year of the transition period, "Age1": age of individual at year 1, "Size1": size of individual at year 1 (number of tillers), "Seeds1": projected amount of seed production at year1, "Survto2": whether that individual survives to the next year, "Size2": if it survives, the size of the individual at the next year.
POSY_transitions_R.csv
Poa alsodes seedling establishment
Seedling establishment rates of Poa alsodes at the plot level 2008-2011. Column headings as follows: "Plot": plot identifier, "Endo": whether plants in that plot were endophyte-symbiotic (1) or endophyte-free (0), "Year1": beginning year of transition, "Seeds1": total number of seeds produced by plants in that plot , "Recruits2": total number of new recruits to that plot the next year, "P.est": rate of seedling establishment (=Recruits2/Seeds1).
POAL_p.est_R.csv
Poa sylvestris seedling establishment
Seedling establishment rates of Poa sylvestris at the plot level 2008-2011. Column headings as follows: "Plot": plot identifier, "Endo": whether plants in that plot were endophyte-symbiotic (1) or endophyte-free (0), "Year1": beginning year of transition, "Seeds1": total number of seeds produced by plants in that plot , "Recruits2": total number of new recruits to that plot the next year, "P.est": rate of seedling establishment (=Recruits2/Seeds1).
POSY_p.est_R.csv
Growth and survival of Poa alsodes new recruits
Growth and survival of Poa alsodes new recruits in their first year. Column headings are as follows: "TAG": seedling identifier, "PLOT": plot into which seedling recruited, "Endo": the endophyte status of that plot, "Year1": the year seedling first recorded, "Size1": size (number of tillers) of seedling, "Seeds1": reproduction of seedling at first year, "Survto2": seedling survival into the next year, "Size2": if it survived, seedling size in the next year.
POAL_seedlings_R.csv
Growth and survival of Poa sylvestris new recruits
Growth and survival of Poa sylvestris new recruits in their first year. Column headings are as follows: "TAG": seedling identifier, "PLOT": plot into which seedling recruited, "Endo": the endophyte status of that plot, "Year1": the year seedling first recorded, "Size1": size (number of tillers) of seedling, "Seeds1": reproduction of seedling at first year, "Survto2": seedling survival into the next year, "Size2": if it survived, seedling size in the next year.
POSY_seedlings_R.csv
IPM and LTRE code
R code to parameterize integral projection models for each species with transitions between endophyte statuses based on transmission rates. Between-species LTRE modeled to compare the contributions of intrinsic demographic rates and effects of endophyte symbiosis towards the observed difference in population growth.