Significance Various sources have reported insect decline in total biomass, numbers, and species diversity. With German data on a species-rich hoverfly community over 25 y and a theoretical model, we show how these decline rates are interrelated. The relationship between biomass and diversity losses depends on whether common or rarer species are most affected. Our analyses show stronger declines of common than rare hoverfly species. Strong reductions (up to −80%) in total abundance and biomass correspond with observed species richness declines of −20% to −40% on a seasonal basis. On a daily basis, however, hoverfly diversity declined in proportion to biomass loss, with important consequences for the functioning of ecosystems.