Pre- and postinfection priority effects have contrasting outcomes for parasite prevalence in host populations

Parasite species frequently co-occur more or less than expected by chance. These nonrandom co-occurrence patterns can be driven by pre- or postinfection priority effects: parasites are more or less likely to attempt infection in a host already infected by another parasite species or may have higher or lower establishment and survival in hosts already infected by another parasite species. How these two types of priority effects differentially affect parasite distributions at the host population level remains unexplored. Using a probabilistic state transition diagram parameterized with field data and analyzed with two simulation methods, I show that the two types of priority effects lead to contrasting outcomes at the host population level. As preinfection priority effects transition from facilitation to inhibition, uninfected hosts decrease by up to 39%, and hosts infected by both parasites decrease by up to 84%, with concomitant increases in single infections by either parasite species. In contrast, as postinfection priority effects transition from facilitation to inhibition, the proportion of uninfected hosts remains unchanged, but the proportion of hosts infected by both parasites decreases by up to 89%, with increases in hosts singly infected by the first-arriving parasite. Interactions between parasites at the within-host level and the specific nature of those interactions alter infection patterns at the host population level.