Probability of site occupancy in the large heath butterfly Coenonympha tullia determined from geographical and ecological data

For threatened species that prove intractable to direct survey, there is a premium on being able to predict site occupancy using more readily available site data. Herein, we use geographical and ecological data for Coenonympha tullia from 148 sites surveyed in 1995-1996 to determine the probability of site occupancy in a further 33 sites surveyed in 1997. Discriminant function analysis using site-specific data on habitat quality and habitat area produces a 91% correct classification of the 148 sites surveyed in 1995-1996 and a 79% correct classification for the 33 surveyed in suitable conditions during 1997. Data on site isolation does not improve the classification of sites. The difference in classifications between the year groups is significant. It is caused by the unexpected (probability < 1%) absence of the butterfly from three large, well-resourced sites at high altitude (> 400 m). The inference is that there is an altitudinal limit or effect (i.e. biennial emergence pattern) to C. tullia in England. These results suggest that occupancy of Eriophorum-Erica mosses by C. tullia can be determined successfully from habitat data for sites. However, in areas where site extinction has progressed further, isolation is likely to be a more prominent factor. (C) 1998 Elsevier Science Ltd. All rights reserved.