Lazy ecologist's guide to water beetle diversity: Which sampling methods are the best?

Biodiversity surveys of aquatic macroinvertebrates in standing water rely on various methods, but a thorough comparison of the techniques is lacking. This hampers analyses across surveys and impedes development of efficient sampling schemes. We compare the selectivity and efficiency of four methods commonly used to collect aquatic insects – activity traps (ATs), box trap (BT), handnetting (HN) and light trap (LT) – using a large dataset on water beetles in a site with ～100 species. We propose to use time investment as a natural basis to compare efficiency, since it applies to any method. The results inherently differ from results based on samples or individuals because methods are neither equally demanding nor equally rewarding. Most differences between methods arise from their size selectivity: ATs select for larger species, while HN and BT seem least selective. Attraction to light is taxon-specific and LT yields more depauperate samples than ATs, BT and HN, limiting the use of LT in community studies. To boost the development of cost-effective protocols, we also identify the best designs for rapid bioassesment by simulating short surveys from the data. Combinations of ATs and BT give most species; the results are robust to partitioning of effort between both methods. However, these rapid surveys miss on average more than 40% of all species in our study. Our results therefore emphasize that long-term studies using multiple methods are vital for measuring diversity in species-rich freshwater habitats.