Maintenance of parasite species diversity: Spatiotemporal niche partitioning and aggregation facilitate species coexistence

The maintenance of parasite diversity has historically taken a host-centric approach. Yet, many parasites are host generalists, and most parasites spend at least some time outside of their hosts. So, what mechanisms besides host-associated niche partitioning allow parasites to coexist? Using a year-long field survey and lab mesocosms, we examined whether environmental niche partitioning or aggregation could enable coexistence among soil-dwelling entomopathogenic nematodes. Field patterns along an elevational gradient reveal that species abundances differentiate with soil structure and moisture levels. Yet, most species strongly overlap within-sites throughout the year. Thus, niche partitioning alone is not sufficient to explain the coexistence of these species and other mechanisms are necessary to explain their coexistence. Aggregation at the within-site scale provides evidence for such a mechanism. Each species showed significant intraspecific clumping and largely random associations with other species. A mesocosm test of the consequences of intraspecific aggregation found that parasites at low or high densities limit their own population growth. Aggregation can promote negative feedback facilitating species coexistence. Our findings offer field-based evidence that spatiotemporal niche partitioning and aggregation both play a critical role in maintaining parasite species diversity, illustrating the importance of extending out view of parasites beyond their hosts.