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Grid square distribution maps have been used widely to measure rates of decline and target conservation resources. However, common species that may have many populations per grid square can decline substantially from within squares without being lost from entire squares. In order to quantify this process, fine scale population and habitat data have been collected for the common blue butterfly Polyommatus icarus in a 35 km2 area of fragmented landscape in north Wales. Present day habitat associations, determined from over 2000 transect walks, combined with data on historical and present day habitat distributions reveal that the species has declined by about 74% since 1901. Similar data concerning the species' major host plant Lotus corniculatus indicate a decline of 46%. Based on 1 km2 grid maps, neither species have been assessed as declining at all. These results suggest that apparently 'common' species may have declined just as much as many of Britain's rare species: using present methods of assessment these declines are undetecte Inglés