Loss rates of butterfly species with urban development. A test of atlas data and sampling artefacts at a fine scale

Data for the United Kingdom (UK) Manchester Butterfly Atlas produced confli cting species loss rates for increased urban development. In particular, a very low rate of loss was recorded (0.19 species for every 10% increase in urban cover) for the Mersey Valley mapped at a high resolution of 1 ha unit s. It was suggested that sampling artefacts (uneven recording) or failure t o distinguish vagrant individuals from breeding populations cause this. Her ein, results are reported for 30 sample squares, within the Mersey Valley, surveyed uniformly throughout 1999. It is shown that loss rates are as high as areas mapped at lower resolution over wider areas (0.67-0.68 species fo r every 10% increase in urban cover) and that increasingly stringent defini tions of urban cover result in higher loss rates. Comparison with the data from the Atlas, but for the same 30 sample squares, indicate that the low r ates at a fine scale for the complete Atlas data are more likely to be caus ed by uneven recording than from failure to record species status. However, progressive sampling of squares, despite uniform recording, will inevitabl y cause a reduction in loss rates of total species for increases in urban d evelopment.