Cooperation destabilizes communities, but competition pays the price.

A classic result in theoretical ecology states that an increase in the proportion of cooperative interactions in unstructured ecological communities leads to a loss of stability to external perturbations. However, the fate and composition of the species that constitute an unstable ecological community following such perturbations remains relatively unexplored. Here, we used an individual-based model to study the population dynamics of unstructured communities following external perturbations to species abundances. We found that while increasing the number of cooperative interactions does indeed increase the probability that a community will experience an extinction following a perturbation, the entire community is rarely wiped out following a perturbation. Instead, only a subset of the ecological community is driven to extinction, and the species that become extinct are more likely to be those engaged in a greater number of competitive interactions. Thus, the resultant community formed after a perturbation has a higher proportion of cooperative interactions than the original community. We showed that this result could be explained by studying the dynamics of the species engaged in the highest number of competitive interactions: After an external perturbation, those species that compete with such a 'top competitor' are more likely to become extinct than expected by chance alone, whereas those that are engaged in cooperative interactions with such a species are less likely to become extinct than expected by chance alone. Our results provide a potential explanation for the ubiquity of cooperative interactions in nature despite the known negative effects of cooperation on community stability.