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

Host-associated resident microbiota can protect their host from pathogen--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 that 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. Our results state that colonization resistance often does not have a monotonic relationship with the species richness of the resident community. Instead, colonization outcomes are often the consequence of 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). We observe these two underlying 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 provide an explanation for the seemingly contradictory trend in the resistance-diversity relationship in previous reports.