Completing Linnaeus’s Inventory of the Swedish Insect Fauna: Only 5000 Species Left

Despite more than 250 years of taxonomic research, we still have only a vague idea about the true size and composition of the faunas and floras of the planet [–]. Many biodiversity inventories provide limited insight because they focus on a small taxonomic subsample or a tiny geographic area [, ]. Here, we report on the size and composition of the Swedish insect fauna, representing roughly half the macroscopic diversity of one of the largest European countries, based on more than a decade of data from the Swedish Taxonomy Initiative and a massive inventory of the country’s insect fauna [, ]. The fauna is considered one of the best known in the world, but the inventory has nevertheless revealed a surprising amount of hidden diversity: more than 3,000 new species (301 new to science) have been documented so far. We show that three independent extrapolation methods converge on similar estimates of the true size and composition of the true fauna, suggesting that it comprises around 33,000 species. Of those, 8,600 (26%) were unknown at the start of the inventory and 5,500 (17%) still await discovery. Most of the new species belong to Hymenoptera and Diptera groups that are decomposers or parasitoids. Thus, current knowledge of the Swedish insect fauna is strongly biased taxonomically and ecologically, and we argue that this is likely true for most insect faunas. Addressing these biases is critical in understanding insect biomes and the ecosystem services they provide.