When functional margins erode: threatened boundary species and functional consequences in Chinese waterbirds

Trait-based approaches reveal how biodiversity loss erodes ecological strategies beyond mere species loss, but whether extinction risk is distributed non-randomly across the functional space of regional waterbird assemblages remains unclear. Here, we tested whether national and global Red List frameworks reveal different patterns of functional vulnerability. We integrated morphological, life-history and dietary traits for 296 waterbird species recorded in China with threat categories from the China Red List and the IUCN Red List of Threatened Species. We constructed a multidimensional functional trait space, quantified functional specialization, local functional isolation and composite functional uniqueness, and simulated functional-richness loss under threat-ordered and random extinction scenarios. Chinese waterbirds occupied an uneven functional space shaped primarily by ecomorphological differentiation between wading and swimming strategies. Threatened species were disproportionately concentrated at functional margins, particularly in trait combinations of smaller body size, longer tarsi and slenderer bills, indicating elevated vulnerability of shallow-water and intertidal foraging strategies. Threatened species also exhibited higher functional specialization and composite uniqueness than non-threatened ones, and threat-ordered removals caused faster functional-richness loss than random removals. These patterns were stronger under the China Red List than under the IUCN framework, suggesting that regional assessments uncover functional vulnerabilities that are less apparent under global threat categories. Our findings suggest that species-based threat assessments should be complemented by trait-based information to identify functionally distinctive species that maintain the margins of ecological strategy space.