Geographical range size is a key ecological variable, but the consequences of measuring range size in different ways are poorly understood. We use high–resolution population data from British butterflies to demonstrate that conventional distribution maps, widely used by conservation biologists, grossly overestimate the areas occupied by species and grossly underestimate decline. The approximate flight areas occupied by 20 out of 45 colonial British species were estimated to cover a median of only 1.44% of the land surface within occupied regions. Common species were found to be declining faster than conventional distribution maps suggest: common and rare species had no significant difference in their population-level rates of extinction. This, combined with the log–normal form of the range–size frequency distribution, implies that species–level extinction rates may accelerate in the medium to long term. Population–level conservation is a matter of great urgency for all species, not just for the rarest.