Geographical Factors Influencing the Probability of Hipparchia semele (L.) (Lepidoptera: Satyrinae) Occurring on British and Irish Off-Shore Islands

The presence or absence of the grayling butterfly, Hipparchia semele (L.), on British and Irish islands has been studied using logistic regression and discriminant analysis. Two subgroups of islands, one classed as having records of vagrant H. semele and the second occurring beyond the range margin for the species in northern Britain, appear as outliers in plots for the three variables island area, isolation and source population size. Although inclusion of the subgroups does not affect the signi cance of parameter estimates, they do substantially influence explained variances and the classification of individual islands. On the basis of the three variables listed, resident populations of the butterfly on both subgroups of islands are shown to