Summary

Species distribution data are crucial for assessing the conservation status of species (red listing, IUCN) and implementing international conservation targets, such as those set by the International Convention on Biological Diversity. Although there have been a number of efforts aimed at aggregating biodiversity data, information on the distribution of many taxa is still scanty (i.e. the Wallacean Shortfall). In this study, we use a large database, including over 19 million species occurrence records, to identify knowledge gaps in biodiversity inventories for butterfly records at a global level. Bayesian hierarchical spatial models were used to quantify the relationship between gaps in inventory completeness and the density of roads, protected areas and elevational range, the former variable being a proxy for accessibility, the latter two for attractiveness to recorders. Our results show that despite >100 years of butterfly sampling, knowledge of the distribution of butterflies is still limited in tropical areas. The results revealed that gaps in butterfly inventories are largely concentrated in areas of low elevational range, low density of protected areas and low road density. We conclude that the Wallacean Shortfall is a problem even for one of the best studied insect groups. In the light of these data limitations, we discuss prospects for filling gaps in butterfly inventories at the global scale within relatively short time frames. We argue that a combination of citizen science and quantitative tools may help to fill knowledge gaps and inform conservation decisions.