Background: Despite Sweden's rich legacy in entomology, a significant portion of its insect fauna remains poorly studied. Addressing this and other biodiversity knowledge gaps, the Swedish government unveiled the Swedish Taxonomy Initiative (STI) in 2002, with the ambitious goal of documenting and scientifically describing all multicellular species in the country. One of the largest projects funded by STI is the Swedish Malaise Trap Project (SMTP). The SMTP project, the data resulting from it, and the analyses of that data constitute the core of the current thesis.

Methods and Results: The SMTP deployed 73 Malaise traps across 55 diverse habitats from 2003 to 2009, capturing an estimated 20 million insects. The catch has been sorted to over 300 taxonomic fractions suitable for further processing by taxonomic experts. The sorted material has been studied by over 100 taxonomists, identifying 4,000 species in about 1% of the total material. A third of these were previously unrecorded in Sweden, including nearly 700 potentially new to science. The SMTP represents a significant community effort and we describe the history, organization, logistics and methodology of the SMTP project, with a focus on the lessons learned along the way and the optimized workflows that resulted in the end. The SMTP output was used to estimate the species richness and composition of the Swedish insect fauna. This included expert assessments, analysis of new species discovery rates, and statistical extrapolations from abundance and incidence data, including a novel non-parametric estimator. These methods converged on an estimate of 33,000 species, 26% of which were unknown at the inventory’s start, and 15% of which still await discovery. To improve the speed and accuracy of the analysis of Malaise trap samples, we introduced morphotype barcoding, combining manual sorting into morphospecies with individual DNA barcoding of representative specimens. Morphotype barcoding is shown to offer more accurate abundance estimates than metabarcoding. In contrast to metabarcoding, it also provides material that is directly suitable for enhancing barcode reference libraries. At the same time, it is shown to be significantly cheaper and require less consumables than megabarcoding (specimen-level barcoding of all specimens in the sample).

Conclusion: The SMTP exemplifies the successful application of community science to biodiversity research, leveraging volunteer efforts alongside professional expertise, a model that has proven to be effective in gathering extensive biodiversity data. The thesis thus offers valuable insights into planning and executing large-scale biodiversity inventories. The analyses of SMTP data suggest that a significant portion of the diversity remains undiscovered or undocumented within one of Europe's most well-studied insect faunas. The thesis highlights critical taxonomic and ecological biases in our current understanding, evidenced by the predominance of Hymenoptera and Diptera species, and decomposers and parasitoids, among the newly discovered species. These findings are pivotal in reshaping our understanding of global biodiversity and the specific ecological roles of insects. The study also emphasizes the need for a more inclusive taxonomic scope in biodiversity inventories, a challenge heightened by the urgency suggested by recent reports of alarming global declines in insect populations.