Mutualism & Stability in Lotka-Volterra communities

Which processes govern the stability of ecological communities in nature? Can the particular types of interactions present in a community affect its stability? If a community experiences species loss, does the probability of a species going extinct depend on how many mutualistic interactions it was involved in? How does all this affect how much mutualism we should expect to see in nature anyway?



A classic mathematical result in ecology states that mutualistic interactions destabilize ecological communities. Theoretical studies using more complex methods also consistently show that communities bearing more mutualistic interactions are more likely to experience species loss following an external perturbation to population densities. Based on theory alone, one may thus naively expect that mutualism should not be present in nature, since those communities which are mutualistic are predicted to lose species more often. However, this naïve expectation is in stark contrast with empirical results showing that positive interactions such as mutualism are prevalent in ecological communities. How may one resolve this apparent disconnect? In 2021-23, I worked with Suryadeepto Nag, my batchmate at IISER Pune, in Dr. Sutirth Dey's lab, where I used individual-based models to try and understand how the distribution of interaction types (mutualism, competition, and exploitation) affect the dynamics of Lotka-Volterra communities. While we recapitulate previous studies in showing that communities with a greater number of mutualistic interactions are more unstable to external perturbations, we find that not all species go extinct following a perturbation. Instead, the community loses some species and then attains a stable configuration (with smaller species richness) that is stable to any subsequent perturbations. Importantly, we find that species engaged in a larger number of competitive interactions are more likely to go extinct in unstable communities than those engaged in a larger number of mutualistic interactions. Thus, in our models, following a perturbation, species richness decreases but the proportion of mutualistic interactions in the community actually increases. Our results thus provide a potential mechanism for the existence of mutualistic interactions in nature despite the destabilizing effect they have at the community level. In particular, our work suggests that mutualism could be prevalent in nature due to non-random extinction processes in the initial community assembly process. This work has now been written up as a preprint that is currently available on biorxiv (Bhat et al. 2023).