The collector practices that shape spatial, temporal, and taxonomic bias in herbaria

Natural history collections (NHCs) are essential for studying biodiversity. Although spatial, temporal, and taxonomic biases in NHCs affect analyses, the influence of collector practices on biases remains largely unexplored. We utilized one million digitized specimens collected in the northeastern United States by c. 10 000 collectors to investigate how collector practices shape spatial, temporal, and taxonomic biases in NHCs; and similarities and differences between practices of more- and less-prolific collectors. We identified six common collector practices, or collection norms: collectors generally collected different species, from multiple locations, from sites sampled by others, during the principal growing season, species identifiable outside peak collecting months, and species from species-poor families and genera. Some norms changed over decades, with different taxa favored during different periods. Collection norms have increased taxonomic coverage in NHCs; however, collectors typically avoided large, taxonomically complex groups, causing their underrepresentation in NHCs. Less-prolific collectors greatly enhanced coverage by collecting during more months and from less-sampled locations. We assert that overall collection biases are shaped by shared predictable collection norms rather than random practices of individual collectors. Predictable biases offer an opportunity to more effectively address biases in future biodiversity models.