Ecological niche position and functional traits jointly shape abundance-occupancy patterns of stream fishes in the Dabie Mountains, China

Abundance-occupancy relationship (AOR) is central to explaining why some species are both locally abundant and regionally widespread, yet the mechanisms underlying this pattern in freshwater fishes remain poorly understood. Here, we examined AOR in stream fish assemblages from the Dabie Mountains, China, based on 40 species surveyed across 230 sites, and evaluated how ecological niche properties and functional traits jointly influence species abundance, occupancy and their coupling. Realized niches and functional strategies were quantified to evaluate the relative contributions of niche position, niche breadth and functional traits to AOR within a hierarchical analytical framework. Fish assemblages exhibited a clear positive AOR (R 2 = 0.844), while retaining a measurable degree of interspecific variation around the overall pattern. Niche position emerged as the primary constraint on both local abundance and regional occupancy, whereas niche breadth showed weak and inconsistent effects. Composite functional traits related to swimming, feeding, and spatial positioning exerted little direct influence on either abundance or occupancy, but significantly modified the AOR, such that species characterized by a strong swimming-related component attained higher occupancy at comparable levels of abundance. Structural equation modelling further indicated that niche position influenced occupancy mainly through its effect on abundance, while this composite functional dimension primarily regulated how variation in abundance was translated into spatial occupancy. Our results demonstrate that abundance-occupancy patterns in mountain stream fishes arise from the combined effects of strong environmental filtering and trait-mediated processes, highlighting complementary roles of niche properties and functional traits in shaping species distributions within spatially constrained river networks.