Associate Professor University of Oregon Eugene, United States
Abstract: Although advances in coexistence theory at the turn of the century have spurred increased interest in questions regarding species coexistence, the question of which mechanisms maintain observed biological diversity remains unanswered. Further, the stabilizing role of interactions between competing species and their associated microbial communities has been understudied relative to the role of environmental variability and other types of biotic interactions. To evaluate the potential for plant species-associated microbial communities to influence competitive outcomes, we have paired a common garden experiment with DNA sequences of soil microbial communities. We hypothesized that species with more similar microbial communities would have increased niche overlap and lower probabilities of coexistence. To test this hypothesis, we established a competitive arena experiment in which eight annual plants, native to prairies in the Willamette Valley, were grown across a density gradient of each possible competitor species. We counted the number of individuals within a 19cm radius circular neighborhood and measured the fecundity of focal individuals. This data was used to fit population models and estimate all pairwise competition coefficients between the eight species. These population models enabled us to estimate niche differences and fitness inequalities among species pairs and to predict competitive outcomes. We also collected soil samples from focal individuals grown alone and within their own monoculture competitive conditions. Because this study took place at a local plant nursery, we also had access to plots in which each of our species were grown for at least 3 years in high density monocultures, so we took additional soil samples from these plots. These soil samples were sequenced to characterize fungal communities associated with each of our eight species. We found that species significantly differed in their competitive interactions with most species pairs predicted to result in competitive exclusion. Species also varied in their associated microbial communities. The presence of plant fungal pathogens was associated with stabilizing niche differences, and overall differences in fungal communities stabilized coexistence. The results of this study add further support for the role of species associated microbes in mediating host community dynamics. Future work should employ more controlled experiments to untangle the mechanisms underlying these plant-soil feedbacks, with special attention to variation in the cultivation of species-associated microbial communities.
