Incorporating context-dependency into microbe-mediated plant coexistence

Background/Question/Methods

It is well established that plant-soil feedbacks (PSF’s) may alter plant communities, even changing dynamics from competitive exclusion to coexistence, or vice versa. Recent attempts to integrate PSF theory with modern coexistence theory have expanded our understanding of the role of soil microbes in altering either the niche differences or fitness inequalities that determine the outcome of plant competition. However, these models of PSF-mediated species coexistence have thus-far ignored feedbacks in plant-microbe interactions under different environmental conditions. To investigate how context-dependent PSF’s might alter the outcome of plant competition, we built upon previous models (Ke and Wan 2020) of PSF-mediated competition by allowing plant-microbe interactions to vary with environmental conditions. We used these models to simulate how context-dependencies may alter PSF-mediated competitive outcomes. We also applied these models to explore how context-dependent PSF’s may alter the spatial and temporal dynamics of an invasive species, Alliaria petiolata. We focus on A. petiolata as recent work has shown that its long-term population dynamics might be dependent on the accumulation of soil pathogens.

Results/Conclusions

Our simulations clearly demonstrated that not only can microbes and environmental variation independently influence the outcome of plant competition, but the interaction between the two can result in a variety of competitive outcomes along an environmental gradient. Notably, even linear responses to the environment can yield nonlinear trends in niche differences and fitness inequalities. Simulations of a competitively dominant nonnative species suggested the potential for spatial or temporal environmental gradients to influence PSF-mediated invasion dynamics. These results show that soil moisture levels have the ability to alter the predicted outcome of competition between an invader and a native competitor due to shifts in how microbes affect competition among plants. Increasing the complexity of these simulations by adding mutualists and decomposers expanded the range of possible outcomes, including the potential for microbes to shift the system into priority effects. Overall, these results demonstrate the potential for context-dependent PSF’s to alter invasion dynamics across space and time. Further, we demonstrate the importance of considering the context-dependency of plant-microbe interactions and the interplay of environment and species interactions in jointly determining coexistence, competitive exclusion, or priority effects.