Role of ecological context in trait–rarity relationships inferred from a complete single‐island endemic angiosperm flora

Abstract

Understanding drivers of plant rarity, especially for highly threatened and understudied tropical oceanic island flora, can improve conservation prioritization. Studies exploring trait–rarity relationships have the potential to identify species that may be more vulnerable, but inference is often limited by incomplete taxon sampling and failure to account for local ecological context. We used the complete Kauaʻi single‐island endemic angiosperm flora (240 taxa), combining known population sizes and robust population estimates determined for International Union for Conservation of Nature (IUCN) Red List of Threatened Species assessments, and compiled data for 42 plant traits while accounting for elevation and habitat type to explore trait–rarity relationships. We tested for phylogenetic signals, accounted for correlation and log ₁₀ ‐transformed continuous trait data, and implemented phylogenetic linear models with a continuous response (log ₁₀ ( n  + 1), where n is the population size) in the phylolm R package. Discrete variables were treated as fixed effects. We also tested multitrait models and individual confounding traits. We found that shorter species, species with shorter leaves, species with lower seed mass, and species with lower initial seed germinability were significant predictors of rarity. Species occurring at lower elevations and across narrower elevational ranges also tended to have smaller populations. Importantly, elevation influenced trait–rarity relationships, with plant height showing different relationships with rarity across elevation classes. Our findings demonstrated that traits associated with rarity are not universal predictors but context‐dependent indicators that may aid conservation prioritization. More broadly, our study highlights the value of more complete sampling and incorporation of local ecological context for trait‐based assessments of rarity, particularly for threatened oceanic island floras.