Taxonomy and conservation might seem to operate as separate bodies (S. T. Garnett and L. Christidis Nature 546, 25–27; 2017). In fact, they are joined at the hip. Taxonomists provide the language to plead conservation's case. And conservationists could be taxonomy's greatest allies — the record of what lives and what might be lost is the field's strongest justification today.

The authors call for coordination between taxonomy and conservation, which is already happening informally. The International Union for Conservation of Nature (IUCN) has a huge stake in understanding species, with millions of organisms at risk but only 80,000 assessed so far. Its Red List of Threatened Species provides consistency in species' status. The IUCN also sets guidelines for predicting species' responses to climate change and for classifying the impact of invasive alien organisms. This interpretation of complex data underpins both policy and practice.

The Red List is maintained by the IUCN's specialist groups, which include taxonomists. Although a species' taxonomic status is crucial to its conservation status and the data on populations and threats are assessed by strict criteria, no guidelines for species circumscription exist. By formalizing the updating and consistency of its list, the IUCN could provide a certified registry of the life worth conserving.

More species could be 'pre-listed' as extant, valid and potentially under threat using the Red List's Not Evaluated status. This would stimulate conservation thinking in taxonomy and promote formation of specialist groups. Because Red List maintenance relies on volunteer input, new funding mechanisms would be needed to expand its structure.