Mutualistic symbiosis between hosts and their microbial partners is widespread and can result in new traits and functions for the host. While extensive research has shown the benefits of microbial partners to host individuals, it is not clear how these effects manifest at the population level. Maintenance of symbiosis in host populations depends in part on the local environmental conditions that may or may not favor the symbiosis. Here, we investigate how the symbiotic, fungal endophyte Epichloë contributes to population growth across the geographic range of its host, the native California perennial bunchgrass Bromus laevipes. Previous experiments showing that the endophyte ameliorates drought stress combined with quantitative comparisons of species distribution models with and without endophyte suggest that this symbiosis causes host range expansion into drier habitats. However, it is unclear whether the symbiosis translates to increased host population growth rate in dry, low suitability habitat. To test this, we developed an integral projection model that quantified B. laevipes population growth rates (λ) of 11 source populations grown at five sites varying in mean climatic conditions. Source populations were chosen that either did (E+) or did not (E-) naturally associate with the endophyte. We hypothesized that symbiosis should increase population growth rates in dry, low suitability habitat where the endophyte would ameliorate drought stress, and that the costs of symbiosis to population growth rate should be greatest in wet, high suitability habitat.
Surprisingly, sites with high habitat suitability from the species distribution models generally resulted in higher λ values of E+ populations than E- populations, while those sites with low habitat suitability projected similar λ values for both E+ and E- populations. However, when we considered the suitability advantage gained by associating with the endophyte (i.e. ratio of habitat suitability for E+:E- plants), those sites with an apparent E+ advantage generally had higher λ values for E+ than E- populations, while those sites with an E- advantage showed no difference between E+ and E- λ values. Our results show that climatic conditions predicted to favor symbiosis do in fact lead to higher population growth rates when the host associates with the endophyte. We demonstrate that the benefits of mutualistic symbiosis scale from individuals to populations, with important consequences for the geographic range of the host species.