Fluctuating positive and negative feedbacks can foster plant species coexistence in diverse communities

Background/Question/Methods

The Janzen-Connell hypothesis predicts that enemy-mediated negative feedbacks can foster species coexistence in diverse ecosystems but empirical evidence remains mixed. Disparity between theoretical expectations and empirical results may arise from complex effects of mycorrhizal fungi on plants and their natural enemies. Surprisingly, the role of fungal-mediated feedbacks on plant species coexistence remains unexplored both theoretically and empirically. Evidence suggests that fungal-mediated feedbacks range between negative and positive effects depending on the difference between the nutrients benefits and the carbon costs fungi convey to the plants. Additionally, scholars have shown the importance of mycorrhizal fungi in plant defense against natural enemies. Building upon this existing knowledge, we developed a multi-species Lotka-Volterra model combined with a bi-directional resource exchange system. This model also incorporates an interaction between the enemy and mycorrhizal fungal populations. Using this mathematical model, we investigated how fluctuations in the strength and direction of mycorrhizal effects and enemy impacts on plant communities can foster coexistence.

Results/Conclusions

Our model demonstrates complex dynamics between mycorrhizal fungi and plant natural enemies that lead to plant species coexistence under two conditions: (1) the per capita rate of increase in a plant population must be greater than the sum of the negative per capita effects of predation, interspecific competition, and mycorrhizal association; and (2) the per capita numerical response of enemies to plants protected by mycorrhizal fungi must exceed the magnitude of the per capita effect competition among enemies.  We reviewed 67 empirical studies of the mycorrhizal fungal and enemy impacts on plant populations and compared them with the patterns predicted by our model. All of the 27 studies that tracked mycorrhizal effects over time found evidence for fluctuations in strength and direction. Out of 33 studies that investigated interactions between enemy and mycorrhizal fungal abundances, 12 supported negative interactions, 4 showed positive interactions, 2 found no correlation, and 15 found mixed results. Four studies out of 12 studies found mycorrhizal effect increases with plant abundance, 3 found the opposite, and 5 found mixed results. These results support our model and offer an existing avenue of research combining the interactions between plants, enemies, and fungi to study plant species coexistence.