Understanding the origin and maintenance of microbial diversity is a major challenge. How does a community that contains a mixture of toxin-producing, toxin-sensitive, and toxin-resistant species remain stable? Previous theoretical studies have uncovered salient processes that stabilize diversity in such communities, but have only done so for communities involving few discrete species. Here we explore a microbial community model where species are defined on a continuous trait dimension, which theoretically allows for communities with an unlimited number of species. The continuous spectrum generalizes intra-species relationships of toxin (or antibiotic) production, sensitivity, and resistance that arise randomly in populations through mutation as well as interspecies relationships arising due to genetic diversity. Indirect (tripartite) interactions shape species coexistence as toxin-resistant (antibiotic-degrading) species modify the toxin/antibiotic interactions between two other species. Each model species has three attribute properties (species it can kill, species it is resistant to, and species by which it can be killed), all of which are defined in terms of “relational bands” on the trait axis. Species are drawn at random from the trait axis to populate a habitat (e.g., agar plate) where they can interact with other species located within a specific “interaction radius.” Adjustable model parameters are the width of the relational bands, the size of the interaction radii, growth rate, and mutation rate.
Results/Conclusions
The interplay between the size of the relational bands for killing and inhibitory interactions leads to spontaneous chaotic and cyclic dynamics. Moreover, the size of the killing relational band dictates the chaotic dynamics, independent of the killing interaction radius. Extreme mutation rates direct the community into chaotic or stable states, but intermediate mutation rates lead to a dynamic stability where species’ population sizes are constant but interactions occur at the spatial boundaries, independent of inhibition. Intermediate-sized relational bands also engender dynamic stability, but with cyclic species dominance. Further, having both inhibition and killing capabilities in the model, coupled with mutation, consistently causes strong cyclic patterns in species dominance, cycles that are highly nonlinear with varying periodicity, amplitudes, and plateaus. Collectively, these results demonstrate the rich spatiotemporal dynamics that are possible when large numbers of microbial species with restricted but heterogeneous rules for aggressive, inhibitory, and resistant interactions live in a common spatial arena. These findings of rich dynamics may also be relevant for coral communities and other spatially structured systems featuring diverse types of interspecific interactions.