Mutualist-mediated niche expansion and differentiation in a grass-fungal endophyte symbiosis: Linking an experimental test of drought tolerance to rangewide patterns of mutualist-mediated niche effects

Background/Question/Methods

The study of organisms’ niches has advanced our knowledge of species coexistence, community assembly, and speciation. Research has focused on niche reductions caused by negative species interactions while affording the possible effects of positive species interactions (e.g. mutualism and facilitation) much less attention. In contrast to competition/predation, positive interactions can expand the realized niche of a species, possibly beyond the fundamental niche, by conferring benefits that ameliorate (a)biotic stresses. Partner-generated niche shifts could also lead to niche differentiation within a species if individuals that associate with partners have different niches from those that do not.

Here, we examine how a fungal endophyte (Neotyphodium sp.) affects the niche of its California-native grass host, Bromus laevipes, through a manipulative drought experiment. In previous work, climate-based ecological niche modeling found that the endophyte could be causing substantial niche expansion/differentiation across the host range and that E+ plants (with endophyte) occurred in drier habitats, suggesting that endophyte-conferred drought tolerance could be affecting the niche. Building on these results, we conducted a drought experiment with plants from 3 E+ and 4 E- populations. After manipulating endophyte-level within the plants (via fungicide), they were assigned to 10 watering treatments and monitored for a 6-month growing season. 

Results/Conclusions

Plants from E+ populations had more aboveground biomass at low water levels than plants from E- populations, but the reverse was true at high water levels (P=0.045). This tradeoff is consistent with the niche modeling findings that E+ plants occupied drier habitats, and indicates that drought tolerance could be a mechanism causing both endophyte-mediated niche expansion and differentiation. However, since the experimental reduction of endophyte in fungicide-treated E+ populations did not significantly reduce aboveground biomass at low water levels and increase biomass at high water levels (i.e. the pattern observed in the E- populations), an additional experiment is needed to determine whether endophyte presence or difference in plant genotypes between E+ and E- populations explains this result. We are in the process of conducting an experiment manipulating endophyte presence (rather than level) to decouple these factors. We did find that endophyte-level impacted belowground biomass. Control treatment plants from E+ populations (i.e. the highest level of endophyte hyphae) had significantly more belowground biomass than plants from the other treatments over all 10 watering levels (P=0.005). While these data indicate high endophyte level increases belowground fitness, we did not observe a tradeoff between high and low water availability.