Targeting squares far survey: predicting species richness and incidence ofspecies for a butterfly atlas

1. Predictions on species richness and incidence of species are made using data for three scales of mapping from the Greater Manchester butterfly atla s; for the whole of the conurbation (2 km x 2 km scale) and for two sample areas centred on the Mersey Valley (1 km x 1 km scale; 1 ha scale). Predict ions are based on data for recording effort, altitude, biotopes, host-plant s and nectar resources. 2. Data for Greater Manchester indicate that substantial shortfalls may occ ur in recording butterfly species for atlases despite the fact that butterf lies are generally easily identified and well supported with recorders. Sho rtfalls lend to be larger for species with fewer records, indicating that s ome species may be more easily overlooked than others. 3. The results demonstrate that targeting squares for re-survey is necessar y and feasible. The predictions have other valuable research applications, the most important of which is being able to assess the accuracy of distrib ution maps, to correct them, and to make projections of distribution change s. 4. Predictions may be enhanced by improvements to mapping in three ways: (i ) Collecting data on recording effort. Variation in recording effort typica lly accounts for differences in species richness and incidence of species m ore than any other variable; (ii) Collecting data on biotopes and specific resources. The present results are promising and demonstrate that the colle ction of environmental data linked to a suitable sampling Frame could facil itate knowledge of the distribution of species over extensive areas that re main under-recorded; and (iii) Distinguishing between breeding individuals and vagrants. Vagrancy is a problem associated both with species and scale. Although species vary substantially in their capacity to migrate beyond th eir habitats, the effect of vagrancy on distribution maps becomes an increa singly large problem as the grain of mapping (size of recording units) decr eases. It is suggested that over-recording can be a problem, particularly w hen mapping is fine-grained.