Data from: The costs of ignoring species detectability on functional diversity estimation

Functional diversity (FD) approaches have been increasingly used to
understand ecosystem functioning in bird communities. These typically rely
on the assumption that species are perfectly detected in the field,
despite the fact that imperfect detection represents a ubiquitous source
of bias in biodiversity studies. This may be notably important in FD
studies, because detection may depend on the functional traits used to
compute FD metrics. However, little effort has been devoted to account for
imperfect detection in FD studies, and therefore the degree to which
species traits and detectability affects FD remains poorly understood. We
predict that observed FD metrics may either under- or overestimate
detection-corrected FD, because FD has multiple independent dimensions
with different data properties. We assessed whether detection was related
to bird traits (body mass, diet, and foraging stratum), accounting for
habitat type, season, and phylogeny. We then used a multi-species
occupancy model to obtain detection-corrected FD metrics (functional
richness FRic, functional evenness FEve, and functional divergence FDiv),
and compared observed and detection-corrected FD estimates in bird
communities from east-central Argentina. Some functional types of birds
(raptors and insectivores) were more easily overlooked, whereas others
(seed and leave eaters) were more easily detected. Some observed FD
metrics underestimated detection-corrected FD (FRic and FDiv), whereas
some others (FEve) overestimated detection-corrected FD. Both observed and
detection-corrected FRic revealed differences between seasons, but not
between habitat types. However, detection-corrected FEve and FDiv showed
differences between seasons, contrary to observed estimates. Our results
indicate that failure to account for unequal ease of detecting species can
lead to erroneous estimates of FD because some functional types of birds
are more easily overlooked. We outline some guidelines to help
ornithologists identifying under which circumstances detection may be a
concern and warn against the indiscriminate use of FD metrics without
accounting for species detection.