Progressive bias in species status is symptomatic of fine-grained mapping units subject to repeated sampling

Atlas maps of butterflies invariably fail to distinguish the status of records, that is, whether they are observations of breeding populations or vagrant individuals. Yet, for conservation purposes, it is clearly important to know whether records relate to breeding populations in suitable habitats or not. The high mobility of butterfly adults carries two expectations. First, vagrants will frequently be recorded within grid squares, particularly when mapping is fine grained. Second, the frequency of vagrants is likely to be higher in mapping units comprising biotopes with few butterfly habitats, than in those rich in butterfly habitats, if only because there is a greater regional pool of potential vagrants for biotopes depleted in butterfly habitats. It follows that with repeated sampling of squares or biotopes differences in species breeding status will be cumulative between those rich and poor in butterfly habitats. In this paper, these predictions have been tested using data from 30, 1 ha grid squares on Alderley Edge Site of Special Scientific Interest (SSSI) (UK), half of which are in woodland (few butterfly habitats) and half with open habitats (non-woodland; rich in butterfly habitats). Butterflies have been recorded during 38 visits over 4 years. Within squares, vagrants have been distinguished from breeding species on the basis of the presence of larval hostplants and other suitable resources for seasonal maintenance. Owing to the occurrence of vagrants, number of breeding species was found to be significantly fewer than total species in mapping units. Vagrants were found to be significantly more abundant in woodland squares, which contain few species larval hostplants, than in squares dominated by open biotopes with many species larval hostplants. Repeated sampling resulted in a cumulative increase in the frequency of vagrant species. Furthermore, the relative frequency of vagrants increased at a greater rate in woodland squares compared to non-woodland squares. Suggestions are made as to how recording for atlases may be improved.