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.

中文翻译：

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淡水昆虫群落复杂和非线性气候驱动的变化超过 42 年

持续的生物多样性危机在广泛观察到的物种丰度和多样性下降、群落结构的深刻变化和物种物候变化中变得明显。昆虫是受影响最严重的群体之一，在过去的 25-30 年中，一些陆地生态系统中的昆虫数量减少了 76%。确定潜在的驱动因素是一个主要障碍，因为大多数生态系统同时受到多种压力因素的影响，并且经常缺少对环境变量的原位测量。在我们的研究中，我们调查了德国最常见河流类型的源头河流，位于自然保护区内，除气候变化外，没有重大人为影响。我们使用了现有的最全面的水生昆虫定量长期数据集，其中包括每周对物种水平昆虫丰度、每日水温和河流流量的测量，以及 42 年（1969 年至2010）。总体而言，水温上升了 1.88°C，排放模式发生了显着变化。这些变化伴随着昆虫丰度下降 81.6%，但丰富度（+8.5%）、香农多样性（+22.7%）、均匀度（+22.4%）和年际更新（+34%）增加。此外，群落的营养结构和物候发生了变化：出苗持续时间增加了 15.2 天，而出苗高峰提前了 13.4 天。此外，我们观察到短期波动（< 5 年）在几乎所有指标以及社区对气候变化的复杂和非线性响应中，仅使用快照数据或较短的时间序列就会遗漏这些。我们的结果表明，即使在没有其他相互作用压力源（污染、栖息地破碎等）的保护区内，气候变化也已经严重改变了生物群落。这是综合长期数据在捕捉社区对气候变化的复杂反应方面具有科学价值的一个突出例子。文章影响声明：在自然保护区，气候变化会导致昆虫数量的显着下降和群落的严重重组。本文受版权保护。版权所有。