Revisiting the invasion paradox: Resistance-richness relationship is driven by augmentation and displacement trends.

Host-associated resident microbiota can protect their host from pathogens—a community-level trait called colonization resistance. The effect of the diversity of the resident community in previous studies has shown contradictory results, with higher diversity either strengthening or weakening colonization resistance. To control the confounding factors that may lead to such contradictions, we use mathematical simulations with a focus on species interactions and their impact on colonization resistance. We use a mediator-explicit model that accounts for metabolite-mediated interactions to perform in silico invasion experiments. We show that the relationship between colonization resistance and species richness of the resident community is not monotonic because it depends on two underlying trends as the richness of the resident community increases: a decrease in instances of augmentation (invader species added, without driving out resident species) and an increase in instances of displacement (invader species added, driving out some of the resident species). These trends hold consistently under different parameters, regardless of the number of compounds that mediate interactions between species or the proportion of the facilitative versus inhibitory interactions among species. Our results show a positive correlation between resistance and diversity in low-richness communities and a negative correlation in high-richness communities, offering an explanation for the seemingly contradictory trend in the resistance-diversity relationship in previous reports. Author summary: Empirically, different trends have been observed between the richness of a resident community and how resistant that community is against the introduction of new members: resident communities with higher richness can be more resistant against invaders in some cases and less resistant in other cases. To explain what can cause such seemingly contradictory trends, we used simulations of the invasion process using a simple model of microbial interactions through metabolites and metabolic byproducts. We found that two underlying trends consistently exist: the chance of augmentation (invader species added, without driving out resident species) and the chance of displacement (invader species added, driving out some of the resident species) decreases, in communities with higher richness. The combination of these two trends can lead to a not monotonic relationship between resistance against invaders and the species richness of the resident community. Our model predicts that in low-richness (versus high-richness) communities, resistance-diversity has a positive (versus negative) correlation, explaining the previously observed trends in the resistance-diversity relationship. [ABSTRACT FROM AUTHOR]