US Counties and Amphibians
Within the USA, county boundaries were primarily used, with only 2% of the area over 500m from county borders containing amphibian range limits, whereas 20% of county boundaries have >10 amphibian species range boundaries recorded, and only 31% of all US county borders do not show overlap (Figure S2a). Almost 60% of amphibian species in the US had all their range boundaries on county borders, and the 40% that show some of their ranges off county borders include invasive alien species like Xenopus laevis . In addition, many of these species were range-restricted, with 20% showing a range <100km2, 6% <10km2 and 3% <1km2. Notably, similarly low-level administrative areas were not used in other regions.