Forms of rarity in vascular plants in the world's largest and most diverse seasonally dry tropical forest: challenges and opportunities for conservation in the Brazilian Caatinga

Rare species face greater extinction risk, making their protection crucial for effective conservation planning. We classified the angiosperms of the Caatinga Dominion into Rabinowitz's rarity forms (RF) and assessed the adequacy of Protected Areas (PAs) in safeguarding these species. We also compared the RF of endemic species with the IUCN Red List of Threatened Species. Of 2294 species analyzed, 1734 were rare: 1701 had scarce populations, 920 were habitat specialists, and 910 had narrow geographic distributions. Of the 296 endemic species, most were unassessed on the Red List, and only 29 were classified as threatened. Our ordinal and logistic regression models indicated that Rabinowitz's rarity index is not a good predictor of Red List category, nor do Red List categories predict rarity index values. We found only 228 species protected, 1073 partially protected, and 993 unprotected. Improved sampling inside PAs could reveal more protected species, making our estimates conservative. We recommend urgent measures to enhance Caatinga flora conservation, including expanding the PAs network to better represent formations rich in rare species and increasing targeted sampling efforts to address knowledge gaps that currently limit effective conservation planning.