Niche expansion and partitioning mediated by mutualism in a native grass-fungal endophyte symbiosis

Background/Question/Methods The study of organisms’ niches has been touted as central to our understanding of ecology and evolution, driving advances in coexistence, community assembly, and speciation. Niche research has focused almost exclusively on abiotic limitations of organisms and negative impacts of competition and predation, but a potentially equally important component of niche theory has been virtually ignored: niche expansion and partitioning resulting from POSITIVE species interactions. In contrast to negative interactions, mutualism and facilitation can expand the realized niche of a species, possibly beyond the fundamental niche, by conferring benefits that ameliorate biotic or abiotic stresses. Environmental context-dependency of mutualisms could also lead to niche partitioning within a species, if individuals associating with partners have different niches from those that do not.

We examined how mutualistic symbionts – fungal endophytes – affect the niche of their native grass host, Bromus laevipes. We surveyed ~100 naturally occurring populations of the grass throughout California to determine frequency of endophytes. These data combined with climate data were used to generate two ecological niche models with the program MAXENT – one for symbiotic and one for non-symbiotic B. laevipes. We compared these models to determine the impact of mutualism on the geographic and climatic niches of B. laevipes.

Results/Conclusions Our results strongly suggest both geographic and climatic niche expansion of B. laevipes via mutualistic symbiosis and niche partitioning between symbiotic and non-symbiotic plants. Consistent with niche expansion, the niche models detected high suitability for symbiotic B. laevipes (E+) in California’s coastal regions that were unsuitable for non-symbiotic plants (E-). Conversely, some regions (e.g. northern Sierra Nevada foothills) were highly suitable for non-symbiotic but not symbiotic plants. Taken together, these data also suggest niche partitioning between E+ and E- B. laevipes. Mutualist-mediated niche expansion is further supported by the significantly larger niche breadth of symbiotic B. laevipes compared to non-symbiotic plants across all climatic and geographic niche axes examined (~20-65% larger niche breadth of E+ plants). Niche overlap between E+ and E- plants ranged from ~60-70%, indicating that they differ substantially in their niches (consistent with both niche expansion and partitioning). Currently, we are testing model quality by sampling endophyte status of new populations in the species range and via transplant studies that will also provide an experimental test of mutualist-mediated niche expansion/partitioning at a large geographic scale.