POPULATION TRENDS OF COMMON BRITISH BUTTERFLIES AT MONITORED SITES

1. In recent decades, many of the common and widespread butterflies in Britain which do not already occupy the whole of the country have expanded in range. 2. Populations of the species which have expanded in range, monitored for varying periods during 1974-92 have, overall, shown significantly more increases than declines in abundance, as have the species with ranges which already occupy most of Britain. 3. Nearly all of the common species have increased in abundance more in the east of Britain than in the west. 4. Although the reasons for recent range expansions and increases in abundance of common butterflies are not known, it seems likely that changes in weather have played a role. 5. It is argued that the geographical differences in population trends suggest that a factor (or factors) with a similar eastern bias is also implicated in the changes in abundance and perhaps of range. Some possible factors are discussed briefly.