Creating networks of nature reserves to protect areas rich in biodiversity from the adverse impacts of anthropogenic change is a critical and urgent task. We illustrate the skewed geographical and size distributions of protected areas in the Western Hemisphere. For instance, 811 of 1413 reserves in the Western Hemisphere are smaller than 10 km2, and 35% of the total area of these reserves is in Alaska. We compile ranges for all bats in the continental Western Hemisphere and find that 82% of threatened and small-range species are not protected adequately. Many of the most vulnerable species occur in the areas of highest human density. We provide maps delineating areas where conservation investments may have the greatest impact in preventing biodiversity losses.