Spatial trends in the sighting dates of British butterflies.

A strong relationship between appearance dates and temperature has been demonstrated over two decades for most British butterflies. Given this relationship over time, this paper tests whether comparable spatial trends in timing are also apparent. A major survey of British butterflies is used to calculate mean sighting dates of adults across the country, and these are compared with geographic patterns in temperature. With the use of regression techniques, we calculated latitudinal (south-north) and longitudinal (east-west) gradients in sighting date and temperature. The majority of butterflies appear later in the east of Britain where temperatures are lower during summer, but not the rest of the year. Most butterflies are also seen later in the cooler north of the country, by upto 3-4 days/100 km. However, no geographical relationship between temperature and timing of appearance was detected for over a third of the species analysed, suggesting their populations may be adapted to their local climates. We suggest possible mechanisms for this and discuss the implications of such adaptation for the ability of butterfly species to respond to rapid climate warming.