Biological invasions are a major driver of global change, and prevention is the most effective mitigation strategy. Bioclimatic species distribution models (SDMs) are widely used to estimate invasion risk, assuming that species retain their realized native climatic niches after introduction. We tested this assumption for 194 alien mammal species established across 11 zoogeographic realms, examining realized niche changes, their drivers, and significance for invasion projections. We used a robust