Among the most widely used information underpinning conservation is the concept of Red-Listing species according to criteria developed by the IUCN. The Red List designates species extinction risk based on geographic range, population size, or declines in either. However, it has frequently been questioned whether Red List criteria are appropriate for terrestrial arthropods, which comprise the bulk of animal diversity. Due to their small size, difficulty in identification, and inherent rarity, many invertebrates are hard to study, making Red List criteria hard to apply. We assess this criticism using empirical evidence from one of the largest terrestrial arthropod surveys to date, documenting the abundance and distribution of over 13,000 species in Sweden. Of these taxa, 13% have been found at a single site, and 11% of species are found only in a single weekly sample. Using these data we demonstrate that estimates of trends based on low sample sizes are associated with major uncertainty and a major risk of misclassification under IUCN criteria. We argue that even the most ambitious monitoring efforts are unlikely to produce enough observations to reliably estimate population sizes and ranges for more than a fraction of species. Thus, there is likely to be substantial uncertainty in classifying most species according to current criteria. In response, we discuss the adaptation of IUCN criteria to more accurately capture the conservation needs of invertebrates, and to adequately assess the future of the majority of global animal diversity.