Trait Flexibility as a Buffer Against Extinction: Intraspecific Variation Modulates Biodiversity Loss in Island Ecosystems

Habitat fragmentation often causes delayed extinctions, yet the mechanisms buffering biodiversity loss remain elusive. Using decade-long demographic data from 29 land-bridge islands in China’s Thousand Island Lake and trait data for 71 woody species, we test how intraspecific trait variation (ITV) modulates extinction dynamics. At the species level, species abundance and island area govern immediate mortality, while ITV in acquisitive traits (specific leaf area) reduced mortality, while variation in conservative traits (leaf dry matter content) prolonged extinction debt. At the community level, ITV in wood density increased unpaid extinction debt, revealing a persistence–turnover trade-off. Island area and species abundance indirectly shaped extinction risk via ITV structure. These findings identify trait flexibility as a key buffer against biodiversity loss, integrating ITV into island biogeography theory.