Predicting the future of the Caatinga endemic Melocactus pachyacanthus under climate and anthropogenic landscape changes

Anthropogenic activities have increasingly pressured biodiversity worldwide. The cactus Melocactus pachyacanthus, an endemic and endangered species of Brazil’s Caatinga biome, is particularly vulnerable. This study assessed the synergistic effects of climate change, land use, and fire on the distribution of M. pachyacanthus across three ecoregions in Bahia, Brazil. To address data sparsity for this range-restricted species, we employed an Ensembles of Small Models (ESMs) framework combined with a repeated cross-validation scheme to evaluate current climatic suitability and project future scenarios (2050), overlaying predictive outputs with land-use and cumulative fire data. The current baseline model identified a high concentration of suitable habitat within the Southern Sertaneja Depression. Future climate projections reveal an alarming contraction of suitable areas across all ecoregions, with the São Francisco Dunes becoming entirely unsuitable. Strikingly, historical land-use conversion combined with fires has already induced a massive reduction in potential habitat availability (~ 44.57% in 2022), approaching the total magnitude of long-term macroclimatic shifts projected for 2050 under the intermediate scenario (~ 51.26% under SSP2-4.5). Our findings underscore the importance of ESM frameworks as robust tools for identifying priority conservation areas and guiding the creation of strictly protected areas. These results highlight the urgent need for proactive land-use policies to safeguard endemic species amid ongoing environmental changes, providing broad insights into how climate and human activities jointly reshape species distributions in dryland ecosystems worldwide.