Bird extinctions and introductions are causing taxonomic and functional homogenization in oceanic islands.

Humans are quickly reshaping species assemblages through the loss and gain of species at multiple scales. Extinctions and introductions are non‐random events known to be contributing to taxonomic homogenization. However, it is not yet clear if they also promote functional homogenization. Here, we assess whether extinctions and introductions are leading to taxonomic and functional homogenization of 64 oceanic island bird assemblages, belonging to 11 archipelagos.Based on island lists of extinct and extant, native and introduced species and on species traits, we use probabilistic hypervolumes in trait space to calculate functional beta‐diversity before and after extinctions and introductions. Bird extinctions and introductions promoted taxonomic and functional homogenization on most oceanic islands. These results follow our expectations, considering previous studies on taxonomic homogenization, the predictable link between taxonomic and functional diversity, and the trait similarity of many introduced species, often adapted to anthropogenic habitats, linked to the non‐randomness of bird introductions on islands.Taxonomic homogenization was more common across than within archipelagos, also corroborating previous studies describing stronger homogenization on islands that are further apart and thus had distinctive native assemblages. Surprisingly, the widespread loss of species with similar traits, namely large flightless birds, often led to functional differentiation across archipelagos. However, this differentiation effect tended to be offset by the homogenizing effect of introductions.Functional homogenization increases the vulnerability to global changes, by reducing the variability of responses to disturbance and thus the resilience of ecosystem services, posing a threat to human societies on islands. Our results highlight subtle variations in taxonomic and functional beta‐diversity of bird assemblages in oceanic islands, providing important insights to allow a better assessment of how anthropogenic changes might alter ecosystem functioning, which is vital to develop effective long‐term conservation strategies. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]