Résumé
Conservation efforts in global biodiversity hotspots often face a common predicament - an urgent need for conservation action hampered by a significant lack of knowledge about that biodiversity.New Caledonia is one of the highest biodiversity hotspots. It is home to over 3,600 plant taxa, three-quarters of which are endemic and nearly half are threatened with extinction. Yet, the inventory of plant diversity remains incomplete. Many undescribed taxa are narrow endemics, making them inherently difficult to detect in existing datasets. Among the most significant anthropogenic threats to New Caledonia's biodiversity is nickel laterite mining. Mining is endangering the unique ultramafic ecosystems at an increasing rate. It is now a race against time to identify the rarest species and protect them before they disappear.In this study, we developed a workflow to identify lesser-known ultramafic taxa of New Caledonia. We aggregated all data available worldwide, and tackled the geographical and taxonomic biases, avoiding the data loss resulting from automated filtering. Every doubtful specimen underwent meticulous taxonomic analysis by international experts. Data georeferencing accuracy was verified through dataset cross-checking, local botanist field knowledge, and herbarium specimen examination. Additionally, targeted field studies clarified unresolved taxa and the extinction risk of the rarest species was assessed using IUCN criteria.By applying this methodology to over 85,000 data records, we doubled the number of known ultramafic narrow endemic taxa to 457. We identified 459 threatened taxa, including 68 putative new taxa. The distribution and range for all rare taxa was mapped, which is a crucial step for their conservation.