The distribution and decline of a widespread butterfly Lycaena phlaeas in a pastoral landscape

1. Ecological specialists are often regarded as most likely to be threatened by anthropogenic habitat changes but few relevant data are available on changes in the status of widespread species. 2. Grid square distribution maps have been used widely to measure rates of decline and target conservation resources but it is known that coarse grain mapping is not appropriate to identify declines in widespread species that initially contain numerous local populations per grid cell. Changes in the status of widespread species need to be quantified. 3. Present-day habitat associations, determined from over 2000 transect counts, combined with data on historical and present-day habitat distributions, reveal that the area of occupancy and population-level rate of decline of the Small Copper butterfly Lycaena phlaeas is likely to have been of the order of 92 and 89% respectively, in 35 km(2) of North Wales. Similar data on the species' major host plants Rumex acetosa and R. acetosella indicate possible declines in area occupied of 48 and 91%. If a 1-km(2) grid was applied to the landscape, and if L. phlaeas, R. acetosa, and R. acetosella had occupied all 1-km(2) cells in the study area in 1901 (non-limestone cells for R. acetosella only), their declines would only have been recorded as 15, 9, and 35% respectively. 4. Many declining ecological specialists are threatened with extinction because of their initial rarity. At a population level, however, they may or may not be declining faster than less specialised species. The results presented here illustrate that some widespread species may have declined as much as many of Britain's rarities.