Butterfly-Hostplant Fidelity, Vagrancy and Measuring Mobility from Distribution Maps

Vagrancy among butterflies, defined as individuals found in mapping units (cells, squares) lacking larval hostplants, offers potential for measuring mobility among species. Herein, we investigate several simple measures of mobility (vagrancy indices) based on the occurrence of vagrants in two independent data sets within the same region. We find that the vagrancy indices do not simply reflect individual abundance, suggesting an innate component to mobility. The vagrancy indices agree closely for the two data sets. They also correlate highly with a migration index, based on distances over which species have been recorded moving during the last 50 yr, indicating that frequency of movement corresponds closely with migration distances of species. The frequency of movement in some species, as measured by the vagrancy indices, is shown to be under-recorded, particularly nymphalids. In other species (e.g. Celastrina argiolus, Anthocharis cardamines, Pyronia tithonus) according to vagrancy indices, migration distances seem to have been previously under-recorded. We suggest how data collection can be improved for obtaining measurement of butterfly mobility.