Survival on the Margins: Niche Partitioning and Density-Dependent Spatial Distribution of Equine Cyathostomins.

Habitat preferences of gastrointestinal nematodes are well-documented in small hosts, but equine cyathostomin diversity has largely been characterized only at farm or regional levels. While organ-level site preferences are known, within-organ spatial distributions remain unexplored. We investigated the longitudinal and radial distribution of adult cyathostomins within the large-intestinal lumen. Intact large intestines from 5 Shetland ponies were frozen post-necropsy and transected into 4-cm sections. Three micro-habitat samples (2.5 cm diameter) were collected per section. We recovered 2,444 intact worms, which were identified individually to species. Data were analyzed using generalized linear mixed models (GLMMs) and Shannon-Wiener Diversity metrics. Fourteen species were identified, dominated by Cylicocyclus nassatus and Cyathostomum catinatum. Spatial modeling revealed a highly structured community hierarchy: dominant species monopolized the central lumen of the anterior ventral colon. Consequently, less abundant species exhibited strict spatial niche partitioning. When aggregated as a guild, "rare" species utilized the cecum and peripheral lumen as significant spatial refugia to avoid luminal competition (P < 0.001). Furthermore, increased host infection intensity did not force spatial expansion into marginal habitats; instead, high worm burdens significantly exacerbated crowding within the anterior central lumen (P < 0.001). These findings support interspecific niche partitioning consistent with Schad's species-flock framework: dominant species occupied the central lumen of the orad ventral colon, whereas less abundant species used peripheral/paramucosal and cecal refugia. However, increased host infection intensity did not produce density-dependent spatial expansion; instead, higher worm burdens intensified crowding within the preferred central/orad ventral-colon habitat.