Weighting functional diversity by extinction risk obscures ecological strategies.

Functional diversity metrics are increasingly used to inform conservation prioritisation, yet they are commonly derived from aggregated trait spaces that implicitly assume commensurability among ecological functions. Here, I show that this assumption masks critical ecological differences with direct consequences for conservation decision-making. Using global bird trait data, I compute functional priority indices across three independent ecological trait spaces—locomotion, diet, and reproduction—and compare Functional Uniqueness–Specialisation (FUS) with its threat-weighted counterpart (FUSE). By explicitly analysing trait spaces separately, I demonstrate that (i) species ranked as functionally important differ substantially among ecological dimensions, (ii) FUS and FUSE identify contrasting sets of priority species within and across trait spaces, and (iii) these differences propagate to community-level patterns, yielding divergent global biogeographical priorities. While FUS highlights widespread intrinsic functional distinctiveness, FUSE concentrates priority in regions where unique ecological strategies coincide with elevated extinction risk. Our results reveal that “functional diversity” is not a single, interchangeable concept, but a composite of distinct ecological dimensions that should be analysed separately. We argue that conservation strategies based on aggregated functional indices risk obscuring key mechanisms and misdirecting prioritisation efforts. Disentangling ecological functional spaces provides a more mechanistic and transparent framework for functional conservation planning.