The ongoing biodiversity crisis becomes evident in the widely observed decline in abundance and diversity of species, profound changes in community structure and shifts in the species' phenology. Insects are among the most affected groups, with documented decreases in abundance up to 76% in the last 25–30 years in some terrestrial ecosystems. Identifying the underlying drivers is a major obstacle as most ecosystems are affected by multiple stressors simultaneously and in situ measurements of environmental variables are often missing. In our study, we investigated a headwater stream belonging to the most common stream type in Germany located in a nature reserve with no major anthropogenic impact except climate change. We used the most comprehensive quantitative long‐term dataset on aquatic insects available that includes weekly measurements of species‐level insect abundance, daily water temperature and stream discharge as well as measurements of additional physico‐chemical variables for a 42‐year period (1969‐2010). Overall, water temperature increased by 1.88°C and discharge patterns changed significantly. These changes were accompanied by an 81.6% decline in insect abundance, but an increase in richness (+8.5%), Shannon diversity (+22.7%), evenness (+22.4%) and interannual turnover (+34%). Moreover, the community's trophic structure and phenology changed: the duration of emergence increased by 15.2 days while the peak of emergence moved 13.4 days earlier. Additionally, we observed short‐term fluctuations (<5 years) in almost all metrics as well as complex and non‐linear responses of the community towards climate change that would have been missed by simply using snapshot data or shorter time series. Our results indicate that climate change has already altered biotic communities severely even in protected areas, where no other interacting stressors (pollution, habitat fragmentation etc.) are present. This is a striking example of the scientific value of comprehensive long‐term data in capturing the complex responses of communities towards climate change. Article impact statement: In a nature reserve, climate change can lead to a significant decline in insect abundance and severe restructuring of communities. This article is protected by copyright. All rights reserved

A recent global meta‐analysis reported a decrease in terrestrial but increase in freshwater insect abundance and biomass (van Klink et al., Science 368, p. 417). The authors suggested that water quality has been improving, thereby challenging recent reports documenting drastic global declines in freshwater biodiversity. We raise two major concerns with the meta‐analysis and suggest that these account for the discrepancy with the declines reported elsewhere. First, total abundance and biomass alone are poor indicators of the status of freshwater insect assemblages, and the observed differences may well have been driven by the replacement of sensitive species with tolerant ones. Second, many of the datasets poorly represent global trends and reflect responses to local conditions or nonrandom site selection. We conclude that the results of the meta‐analysis should not be considered indicative of an overall improvement in the condition of freshwater ecosystems.This article is categorized under: Water and Life > Conservation, Management, and Awareness