The dynamics of interspecific interactions have strong implications for the communities within which they occur.  While there is growing appreciation that the abiotic environment influences the dynamics of interspecific interactions, the potential of the biotic environment to have similar impacts is often overlooked.   In terrestrial systems, associations between plants, fungal endophytes, and arbuscular mycorrhizal fungi (AMF), are widespread.  The independent effects of these interactions on plant performance have been shown to range from mutualistic to parasitic depending on abiotic environmental conditions.  Multiple simultaneous interactions are also likely to influence the dynamics of an interspecific interaction through the use of a common resource, host plant photosynthate.  In a greenhouse experiment, we examined the biotic context-dependent nature of interspecific interactions by manipulating the presence and nature of interactions between plants, fungal endophytes, and AMF in a fully-factorial manner.  We utilized two populations of Elymus hystrix plants.  Each population harbors a separate strain of the fungal endophyte Epichloë elymi, with each strain considered either relatively mutualistic or parasitic.  Likewise, we inoculated plants with AMF species known to be more (Glomus mosseae) or less (G. claroideum) effective in promoting plant growth.  Endophyte-free individuals and sterile soil were used as controls.   

Results/Conclusions

Analyses of total plant biomass production indicates that across AMF treatments and endophyte strains, endophyte-infected plants had higher biomass than uninfected individuals (p=0.01; t=2.60).  Additionally, while inoculation with G. mosseae did not differ from the sterile control, G. claroideum reduced plant biomass regardless of endophyte treatment (p<0.00; t=14.96).  Within the populations associated with the more parasitic fungal endophyte strain, we found the effect of G. claroideum inoculation and sterile soil treatments to be influenced by endophyte infection status, such that uninfected individuals produced less biomass than their endophyte-infected counterparts (p<0.01; t= 3.77, p=0.01; t=2.86, respectively).  The presence of an endophyte infection did not influence responses of plants inoculated with G. mosseae.  These differences were not detected in population associated with the more mutualistic endophyte association.  Our study furthers understanding of the context-dependent nature of interspecific interactions, and provides a more realistic appreciation of the potential influences these associations have on dynamics of communities in nature.