Data from: Using functional traits to identify conservation priorities for the world's crocodylians

Over half of all crocodylians are threatened with extinction, with many species considered to be functionally extinct throughout their historical ranges. How such losses impact crocodylian functional diversity (FD) and its conservation is poorly understood, but can be investigated using measurable traits. Where such information is unavailable, phylogenetic diversity (PD) has been proposed as an effective strategy to capture FD, but this assumption is not well tested.

We constructed a global trait database for 28 extant crocodylians [uploaded here], and quantified their functional diversity, distinctiveness and groupings. Functionally distinctive crocodylians include the most and least evolutionarily distinct species, likely because unusual trait values and combinations can be produced by both evolutionary isolation and evolutionarily recent adaptive radiation. We identified four functional groups of crocodylians, within which species have similar ecological roles.  Highly distinctive species, such as the gharial, fit less well into functional groups.

We combined functional distinctiveness and threat ranking scores to prioritise species for conservation. The metric is termed EcoDGE (Ecologically Distinct and Globally Endangered), and we tested how well it operates to preserve crocodylian FD amongst extant Crocodylia. Under current extinction scenarios, 32-38% of crocodylian FD might be lost within 100 years. Losses increase to 48% if all threatened species go extinct, with Asia identified as a hotspot of threatened FD in crocodylians. Highly-threatened distinctive species lead this extinction scenario, exacerbated by clusters of species sharing traits linked to high extinction risk. These traits include habitat specialism and lower reproductive investment; in contrast, species that tolerate climate extremes are more resilient.

Prioritising at-risk species based on functional distinctiveness and threat status appears to be an effective way to conserve present-day crocodylian diversity. Conservation prioritisation based on the EcoDGE metric performed well to minimize projected losses in FD. Among extant crocodylians, FD losses are projected when high diversity overlaps with high extrinsic threats.

We then examined the utility of PD-based measures as surrogates for FD conservation, to better understand the advantages and drawbacks of such substitutions. A PD-based measure such as the EDGE metric, provided an effective tool to capture functional diversity in our tests.