Morphological variability as a buffer against extinction: insights from Hawaiian honeycreepers

Trait variation is essential for population-level responses to novel selective pressures. Because beak morphology is a trait crucial for tasks directly influencing fitness, such as resource acquisition, we hypothesize that species with greater intraspecific variation in beak morphology may access a broader range of food resources, promoting an expanded dietary niche and potentially reducing extinction risk. Hawaiian honeycreepers, once comprising about 60 species, have suffered a high rate of extinction, with less than 35% of species persisting today. We quantified beak morphological variation of extinct and extant Hawaiian honeycreepers collected between 1880 and 1913 and estimated species' niche breadth from stable isotope ratios. While a direct link between morphology and diet variation was not evident at the community level, our analysis revealed that, within guilds, species with lower morphological variation also exhibited narrower isotopic niches. Moreover, extinct species had less morphological variation than those extant, suggesting a link between adaptive potential and extinction vulnerability. Our study highlights the value of historical datasets and museum collections, not only for understanding past ecological patterns but also for providing critical baselines to compare with extant species, allowing us to better interpret present-day biodiversity and extinction risk through the lens of the past.