Unpacking fitness differences between invaders in a multispecies context

Abstract

Ecosystems are constantly exposed to new strains of a given species. Which strain will be able to invade a resident multi-species community depends on the strain's relative fitness. Here we develop an approach to quantify fitness differences between two co-invading strains based on the replicator equation. By assuming that the resident species' frequencies remain constant during the initial phase of invasion, we are able to determine the invasion fitness differential between the two strains, which drives growth rate differences post-invasion. We then apply our approach to a critical current global problem: invasion of the gut microbiota by antibiotic resistant strains of the pathobiont Escherichia coli. Our results underscore the context-dependent nature of fitness and demonstrate how species frequencies in a host environment can explicitly modulate the selection coefficient between two strains. This framework can predict relative fitness in new environments and may contribute to design strategies to lower resistance levels in microbiomes.