Host‐mediated filtering and interspecific associations structure strongyle parasite communities in Sable Island horses

Mixed-species parasite infections are common in wildlife, yet the ecological processes that structure these communities remain poorly understood. We hypothesized that host-mediated and extrinsic environmental filtering, alongside biotic interactions, act concurrently to shape parasite community composition, and that these structuring patterns would depend on parasite traits or evolutionary history. We applied nemabiome DNA metabarcoding to 3147 faecal samples from 916 feral horses with no history of anthelmintic treatment, collected over 10 years as part of a long-term individual-based study on Sable Island, Canada. Using joint species distribution models, we tested whether strongyle parasite communities were structured by host-associated factors, extrinsic environmental variables or interspecific interactions. Host factors, particularly age and individual identity, explained the greatest proportion of variation in both parasite prevalence and infection intensity, indicating strong host-mediated environmental filtering. In contrast, spatial location and year explained minimal variation, suggesting weaker effects of extrinsically mediated environmental filtering. Parasite traits such as body size and gut site preference had limited associations with species' environmental responses. Positive species associations were common, particularly among species occupying the same gut region, suggesting that strong, consistent competitive interactions were not prominently reflected in the residual covariance structure. Negative associations were less frequent but consistently involved Strongylus equinus, indicating potential context-dependent biotic interactions involving this species. By integrating species-level molecular data with hierarchical joint species models, this study quantifies patterns of host-related filtering and species co-variation within a natural parasite community and provides a framework for examining mixed infections in wild systems.