Comparing species accumulation within taxonomically well‐ and poorly resolved insect groups over 37 years

While long‐term datasets are invaluable for tracking changes in abundance and species richness, they may still fail to fully capture the diversity of taxonomically unresolved groups, such as many insect taxa. Here, we used a unique 37‐year (1969–2006) freshwater macroinvertebrate time series, collected over seven sites in central Germany, to study biodiversity patterns and species accumulation over time. Focusing on taxonomically well‐resolved insect groups (Ephemeroptera, Plecoptera, Trichoptera [EPT]) versus poorly resolved “dark taxa” (Diptera), we hypothesized that accumulation curves would saturate only in well‐resolved groups. Over the study period, Diptera showed a continuous rise in both abundance and species richness, with accumulation curves not yet reaching a plateau. In contrast, EPT richness increased with no overall abundance changes, and their accumulation curves reached a plateau despite considerably more sampled individuals compared to Diptera. Our findings reveal a critical blind spot in biodiversity monitoring: highly diverse yet poorly resolved groups like Diptera remain underrepresented, even in very long, high‐resolution time‐series.