Long-term intraspecific trait change amplifies changes in community functional

There has been rapid intraspecific trait change in many species over the past few centuries, while the impacts of such long-term changes on community functional structure remain unclear, yet is vital to evaluate and predict responses of biodiversity and ecosystem function to global change. We examined intraspecific trait changes of 547 bird species from 86,966 specimens collected over the past 140 years and species composition data for 840 bird assemblages surveyed in 1969-2021 in North America. We found that rapid intraspecific trait changes have substantially affected trait composition and functional diversity of communities, even reversing the community-wide patterns driven solely by species composition change. Intraspecific changes of < 3% across four traits have doubled rates of temporal changes in functional richness over the last 50 years. Our findings indicate that intraspecific trait change may introduce a second wave of loss in community functionality beyond that due to species loss.