The impact of climatic change on butterfly geography: does climatic change produce coincident trends in populations, distributions and ranges?

With climatic warming is the expectation of coincident changes in distributions and range limits driven by population changes. An outstanding question is whether such coincident changes (positive or negative) occur, especially in smaller regions in which management for change tends to be conducted. Using atlas and survey monitoring data (BMS) we studied population and distribution changes in 31 butterfly species in North West England over three recording periods (1940-1994, 1995-2001, 2001-2007). We found that since the first recording period many more species have shifted their centres of gravity significantly northwards and uphill than have increased in population abundance (density) and distribution cover. At the same time, far fewer species have effectively shifted southwards and downhill than have decreased in density and distribution cover. A significant association between change in distribution cover and density is divorced from the weaker association between shifts in altitude and northing; whereas many species are gaining northwards and at higher altitude, they are doing so from a failing base at lower altitude and at the core of their distributions. Usually losses at lower altitudes are ascribed to loss of biotopes. But, declines in some species, such as Lasiommata megera, are occurring at a much faster rate than physical changes to the landscape and in land uses. The findings of this study indicate that changes in populations, distributions and ranges are a great deal more complex than hitherto considered. Currently, the basis for such rapid changes is not being supported by detailed autecology on species.