Symbiosis and plant rarity: Effects of fungal endophytes on rare and common native grasses

Background/Question/Methods

Plant species fundamentally vary in their relative abundance, spanning a range from invasive to rare.  Several hypotheses have been proposed to explain this variation, but the literature is replete with conflicting results, and currently proposed mechanisms do not sufficiently account for the observed variation.  Investigations of symbiosis may help resolve the lack of empirical consistency in research on factors contributing to plant rarity and commonness.  In particular, beneficial symbionts can increase host plant resistance to biotic and abiotic stress and may thereby enhance the ecological dominance of host species. 

Fungal endophyte symbioses are very widespread and occur in nearly all plants sampled to date, from mosses and liverworts to conifers and angiosperms.  Most endophytes occur as small, localized infections in plants, but some occur systemically throughout all aboveground plant tissues.  Of the systemic endophytes, those in grasses have attracted the most interest, in part due to their economic and ecological importance in ecosystems worldwide.  These include the genera Epichloë and Neotyphodium (Ascomycota: Clavicipitaceae).  Approximately 20-30% of all grass species are estimated to host systemic, epichloid fungal endophytes, and these associations are estimated to be 40 million years old.

I examined a set of nine native grass species that varied in their relative abundances -- from locally common to locally rare.  Using a combination of field and greenhouse experiments, I tested the extent to which fungal endophytes affected host fitness and examined the ecological mechanisms underlying endophyte effects. Field experiments consisted of replicated populations either naturally colonized by an endophyte or with the endophyte experimentally removed from all plants.  Populations were followed for 3-4 years.  Greenhouse experiments examined the effects of endophytes under altered biotic and abiotic stresses, including herbivory, drought, and shade.

Results/Conclusions

Association with epichloid endophytes increased plant resistance to herbivory in seven of nine species examined, enhanced drought resistance in 60% of the species tested, and improved shade tolerance in one species.  Endophyte symbiosis significantly increased reproduction, survival, or recruitment in all but two plant species.  Both plant species for which the fitness benefits of symbiosis were negligible also showed no enhancement of herbivore resistance.  Results thus far suggest that, despite widespread detection of endophyte benefits in native grasses, there are not strong associations between plant commonness and the strength of endophyte benefits to hosts.