Intraspecific trait variation mediates realized extinction and unpaid extinction debt in island ecosystems

The Equilibrium Theory of Island Biogeography (ETIB) explains species diversity through colonization-extinction balance yet assumes species are functionally uniform. We extend ETIB by incorporating intraspecific trait variation (ITV) and species abundance into extinction dynamics in 66 woody plant species on 29 land-bridge islands in China's Thousand Island Lake. Using demographic censuses from 2009 to 2019 and five traits measured in 3990 individuals, we quantified species-level mortality, realized extinction rate, and unpaid extinction debt. Generalized linear mixed models and double-response piecewise structural equation models evaluated spatial, demographic, and trait-mediated pathways. Species abundance and island area dominate species-level mortality. Community-level ITV in leaf area and leaf thickness reduced realized extinction rate; ITV in specific leaf area reduced unpaid extinction debt, and ITV in leaf dry matter content amplified realized extinction rate. Extinction risk therefore depends not only on which species and traits occur on islands but also on how much coordinated functional variation is retained within species. ITV and community-weighted mean traits capture complementary components of extinction dynamics, extending island biogeography theory and clarifying biodiversity persistence in fragmented landscapes.