How is parasite species diversity maintained? If species are competing for shared hosts, then why is there not just one parasite species that dominates every niche? Maintenance of parasite diversity has historically taken a host-centric view. Yet, most parasites spend at least some portion of their life outside the host in a free-living stage, exposing them to the environment. These environmental gradients affect parasite traits at the between host scale, altering within-host competition that predict species coexistence (fluctuation-dependent mechanism). In contrast, diversity can also be maintained via life-history strategies like dispersal that impacts species aggregation and within-host competition (fluctuation-independent mechanisms). In a year-long, multi-site field survey, we tested what stabilizing mechanisms maintain parasite coexistence at the between host scale using two competing hypotheses in soil-dwelling entomopathogenic nematodes (EPN). Under “spatio-temporal niche partitioning” hypothesis, coexistence occurs when species differ in their competitive abilities based on varying environmental conditions. This mechanism would provide predictive insight into species distribution patterns across space and time. Under “aggregation” hypothesis, we tested if species aggregation within sites can promote stable coexistence. In this model, if a superior parasite competitor is aggregated in a fraction of available patches, the unoccupied patches can allow for the inferior competitor to successfully infect due to decreased interspecific competition. If intraspecific aggregation is greater than interspecific aggregation, greater intraspecific competition can facilitate species coexistence. Results/Conclusions Our findings from a multi-site, year-long survey support that parasite diversity was maintained by stabilizing mechanisms, including niche differentiation and aggregation behavior in a natural parasitic nematode community. At the between site scale, we find that environmental gradients – soil temperature and moisture – allowed for two species: S. costaricense and S. affine to co-occur across sites but competitively excluded a third species: S. texanum in warm and wet patches. At the within-site scale, we find that environmental variation does not explain parasite coexistence. So, what allows S. costaricense and S. affine to stably coexist within a site? We found that difference in aggregation behavior facilitates species coexistence (i.e., stronger intra- than interspecific aggregation) at the within-site scale. Together, our results provide field-based evidence that environmental niche differences and aggregation behavior stabilize parasite community dynamics and contribute to the maintenance of parasite diversity in nature.
