A Hierarchical Niche Structure, Not Niche Partitioning, Organizes a Seasonally Dynamic Flea Community.

Interactions in single-host-parasite systems provide a tractable framework for understanding the ecological mechanisms that maintain community stability; yet, the link between species' multidimensional niches and their functional roles within these networks remains underexplored. Here, we integrated network topology, multidimensional niche analysis, and functional group delineation to investigate the adaptive strategies and assembly rules of a 12-species flea community on Mongolian gerbils (Meriones unguiculatus). The host-flea network was characterized by a stable, nested structure and exhibited strong seasonal dynamics, with connectivity peaking in summer and modularity increasing in autumn. To understand the underlying mechanisms, we quantified the niche breadth of each species along four identified ecological gradients. Our analysis revealed that the community was organized along a steep hierarchy of generalization. Two hyper-generalist species (Nosopsyllus laeviceps kuzenkoui and Xenopsylla conformis conformis), characterized by near-maximal niche breadth and core network positions, dominated the community. A broad niche was a major determinant of a species' role, showing a significant positive association with a wider range of exploited hosts (Wilcoxon test: p = 0.03, effect size = 0.82). In contrast, specialist species, such as the extreme specialist (Ophthalmopsylla jettmari), were confined to the network's periphery and a narrow subset of ecological conditions. Clustering based on the multidimensional niche profiles identified four distinct functional groups, reflecting a clear hierarchy of ecological strategies. Overall, this study suggests that, within this seasonally dynamic system, a hierarchical niche structure, rather than complex trade-offs, is a primary organizing principle, providing a more nuanced understanding of stability in parasitic systems.