Reports of declines in biomass of flying insects have alarmed the world in recent years. However, how biomass declines reflect biodiversity loss is still an open question. Here, we analyze the abundance (19,604 individuals) of 162 hoverfly species (Diptera: Syrphidae), at six locations in German nature reserves in 1989 and 2014, and generalize the results with a model varying decline rates of common vs. rare species. We show isometric decline rates between total insect biomass and total hoverfly abundance and a scale-dependent decline in hoverfly species richness, ranging between −23% over the season to −82% at the daily level. We constructed a theoretical null model to explore how strong declines in total abundance translate to changing rank-abundance curves, species persistence, and diversity measures. Observed persistence rates were disproportionately lower than expected for species of intermediate abundance, while the rarest species showed decline and appearance rates consistent with random expectation. Our results suggest that large insect biomass declines are predictive of insect diversity declines. Under current threats, even the more common species are in peril, calling for a reevaluation of hazards and conservation strategies that traditionally target already rare and endangered species only.

近年来，有关飞行昆虫生物量下降的报道震惊了世界。然而，生物量下降如何反映生物多样性丧失仍然是一个悬而未决的问题。在这里，我们分析了 1989 年和 2014 年德国自然保护区六个地点的 162 种食蚜蝇物种（双翅目：食蚜蝇科）的丰度（19,604 只），并用不同的常见物种和稀有物种的下降率模型概括了结果。我们显示了昆虫总生物量和食蚜蝇总丰度之间的等距下降率，以及食蚜蝇物种丰富度的规模依赖性下降，范围从本赛季的 -23% 到每日水平的 -82%。我们构建了一个理论空模型，以探索总丰度的强烈下降如何转化为不断变化的秩-丰度曲线、物种持久性和多样性测量。对于中等丰度的物种，观察到的持续率不成比例地低于预期，而最稀有物种的下降和出现率与随机预期一致。我们的结果表明，昆虫生物量的大幅下降预示着昆虫多样性的下降。在当前的威胁下，即使是更常见的物种也处于危险之中，要求重新评估传统上仅针对已经稀有和濒临灭绝的物种的危害和保护策略。