Positive interactions increase the risk of extinction in changing environments

Positive interactions play crucial roles in shaping the structure and function of many ecological systems. Despite the importance of these interactions, their influence on species’ evolution and adaptation to new environments is still poorly understood. The scenario in which adaptation is perhaps the most crucial is when an environment changes into an inhospitable one, causing a population to head towards extinction. In this scenario, survival requires a quick adaptation to the new environment via the rise of new mutants - a situation termed evolutionary rescue. Here, we elucidate how different types of positive interactions influence the likelihood of evolutionary rescue by conducting numerical and theoretical analyses. We find that the probability of evolutionary rescue is greatly reduced in the case of interspecies mutualism. This is primarily due to the fact that rescue requires the rise of rescuing mutations in both of the mutualistic species, thus necessitating two independent rare mutation events. These results indicate that while positive interactions may be beneficial in steady environments, they can hinder adaptation to changing environments. The results of this research highlight situations in which mutualistic systems have an elevated risk of collapsing. Furthermore, they may hint at the selective pressures that drove co-dependent unicellular species to form more adaptable organisms able to differentiate into multiple phenotypes, including multicellular life.