Emergence of motifs in plant-mycorrhizal networks under nutrient limitation

Background/Question/Methods

Recent years have seen an intense interest in linking the topological structure of mutualistic communities with their dynamical and stability properties. Many studies have found plant-pollinator/seed disperser communities to exhibit a nested structure in which specialists tend to interact strongly with generalists and only weakly with other specialists. However, these results are not unequivocal for all mutualistic communities. In particular, the structure of plant-mycorrhizal associations remains poorly understood. The unique biology of these systems is hypothesized to preclude the formation of specialist-generalist interactions, the key precondition for nestedness. Despite the crucial role they play in supporting virtually all known plant communities, theory on the topological structure of below-ground mutualisticcommunities has lagged behind that for their above-ground counterparts. Here we develop a theoretical framework to investigate the assembly of plant-mycorrhizal communities under minimal conditions of a closed system with constant nutrient input, a single niche axis, no external inputs of nutrients or species, no self-limitation other than that induced by nutrient limitation, and no spatial heterogeneity. Starting from the very basal level — nutrient uptake by plant species — we investigate the emergent topological structure of these plant-mycorrhizal communities.

Results/Conclusions

We investigated the emergence of stable motifs (recurrent patterns of species interactions) through a combination of analytical and numerical methods. We combined mathematical invasion analyses with numerical simulations of the model parameterized with empirical data from the literature. We find that nestedness arises infrequently, with specialist-specialist chains being the most frequent motif to emerge. Invasion of plant species along with their mycorrhizal partners can increase the frequency of nestedness, but emergence of specialist chains or community collapse due to cascading Allee effects are the more frequent outcomes. In a closed system with no heterogeneity but temporal variation, competitive exclusion at both plant and mutualist levels preclude the emergence of nested motifs. We attribute these findings to the fact that fungal symbionts can confer competitive superiority on their hosts, thus reducing plant diversity and the opportunity for generalist-specialist interactions. We are currently expanding the model to incorporate multiple nutrients (e.g., Nitrogen and Phosphorous) to test whether greater coexistence within plant and mycorrhizal communities could lead to the emergence of more complex motifs.