A global assessment of endemism and species richness across island and mainland regions reveals that islands have significantly higher endemism richness than mainland regions. Endemism richness, a measure combining both endemism and species richness, was calculated for 90 terrestrial biogeographic regions, including islands, and compared with terrestrial vertebrates. The results show that endemism richness of plants and vertebrates is strongly related, with values on islands exceeding those of mainland regions by a factor of 9.5 and 8.1, respectively. Islands are also more vulnerable to human impact and land cover change, with higher human impact indices and projected future land-use changes. These findings highlight the conservation value of islands, which have high levels of endemism richness, small land areas, and high conservation risks. The study also shows that islands are centers of past and imminent species extinction, emphasizing the need for conservation efforts in these regions. The results indicate that islands may offer particularly high returns for species conservation efforts and should be prioritized in global biodiversity conservation. The study also highlights the importance of considering taxon-specific differences when prioritizing conservation efforts, as well as the need for more detailed, taxon-based data to improve conservation planning. The study underscores the importance of protecting biodiversity hotspots, which are regions with high levels of endemism and species richness. The findings suggest that conservation strategies should focus on protecting these regions, as well as addressing threats such as invasive species and climate change. The study also emphasizes the need for further research to improve the accuracy of global endemism richness data, particularly for vascular plants. Overall, the study highlights the importance of islands in global biodiversity conservation and the need for targeted conservation efforts to protect these regions.A global assessment of endemism and species richness across island and mainland regions reveals that islands have significantly higher endemism richness than mainland regions. Endemism richness, a measure combining both endemism and species richness, was calculated for 90 terrestrial biogeographic regions, including islands, and compared with terrestrial vertebrates. The results show that endemism richness of plants and vertebrates is strongly related, with values on islands exceeding those of mainland regions by a factor of 9.5 and 8.1, respectively. Islands are also more vulnerable to human impact and land cover change, with higher human impact indices and projected future land-use changes. These findings highlight the conservation value of islands, which have high levels of endemism richness, small land areas, and high conservation risks. The study also shows that islands are centers of past and imminent species extinction, emphasizing the need for conservation efforts in these regions. The results indicate that islands may offer particularly high returns for species conservation efforts and should be prioritized in global biodiversity conservation. The study also highlights the importance of considering taxon-specific differences when prioritizing conservation efforts, as well as the need for more detailed, taxon-based data to improve conservation planning. The study underscores the importance of protecting biodiversity hotspots, which are regions with high levels of endemism and species richness. The findings suggest that conservation strategies should focus on protecting these regions, as well as addressing threats such as invasive species and climate change. The study also emphasizes the need for further research to improve the accuracy of global endemism richness data, particularly for vascular plants. Overall, the study highlights the importance of islands in global biodiversity conservation and the need for targeted conservation efforts to protect these regions.