Mutualistic endophyte may confer resistance to enemies resulting in niche expansion of its grass host

Background/Question/Methods

The study of organisms’ niches has led to important advancements in the study of species coexistence, community assembly, and speciation. Researchers have focused almost exclusively on niche reduction caused by negative species interactions, but the possible effects of positive species interactions (e.g. mutualism and facilitation) have been virtually ignored. 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.

We examine how a fungal endophyte (Neotyphodium sp.) affects the niche of its native grass host, Bromus laevipes, in common gardens throughout northern and central California (representing ~1/2 the species range). We experimentally manipulated endophyte levels (via fungicide) in B. laevipes plants from 11 populations  and planted them into common gardens at five sites – 2 sites with naturally 100% symbiotic plants and 3 with naturally 0% symbiotic plants. Over the last two years, we have collected data on plant fitness, herbivory, and environmental variables in these gardens to determine the impact of endophytes on B. laevipes performance and niche. 

Results/Conclusions

Across all five gardens, plants with high-levels of endophyte experienced higher fitness than plants with low-levels or no endophyte, even at sites where endophyte was absent from the natural population. The fitness increase was large: ~40% more stems and leaves, ~80% more flowering, and 25% lower mortality for plant with endophyte. Plants with high endophyte levels also receive less damage from natural enemies (insect herbivores and fungal pathogens), which suggests that the higher fitness experienced by the symbiotic plants likely results from endophyte-mediated natural enemy deterrence/resistance. This mechanism is further supported by field surveys of natural populations. In 13 naturally mixed symbiosis populations, the percentage of plants with natural enemy damage was significantly less (~50%) on symbiotic plants. Also, across 33 natural populations, the percentage of plants with endophyte decreased as natural enemy damage increased (P<0.0001). Mutualist-conferred protection against natural enemies may be underlying niche expansion of B. laevipes. To fully assess the impact of this mutualist on its host’s niche, we are continuing to monitor our gardens, collecting demographic data needed for the evaluation of lifetime fitness and the likelihood of population persistence. We have also set up an experiment manipulating enemy damage to test this potential mechanism.