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Insect populations on German stream have declined by 81% since 1960s due to climate change

February 14, 2020
The blue-winged olive (Serratella ignita), one of the aquatic insects monitored in the new study, which found that insect populations declined by 81.6% between 1969 and 2010 on the Breitenbach stream, Germany. Image: Francisco Welter-Schultes, Wikipedia Creative Commons

Insect populations are in big trouble. A number of recent scientific studies have documented insect declines across the world, prompting a 2019 meta-analysis of the evidence by Dr. Francisco Sánchez-Bayo and Dr. Kris Wyckhuys to conclude that “almost half of [insect] species are rapidly declining and a third are being threatened with extinction.”

Aquatic insects – which number more than 55,000 species – are important in many freshwater food webs, with species found everywhere from tiny puddles to large rivers and lakes. However, like their terrestrial counterparts, aquatic insect populations are in decline, largely due to the effects of intensive agriculture, water pollution and, increasingly, climate change.

However, researchers have often found it hard to disentangle the effects of climate change on insect populations from those of other stressors. In this context, a new study uses a unique long-term dataset from a German stream located in a nature reserve to isolate the effects of a changing climate on the stream’s insect communities since the 1960s.

Their findings are startling. Writing in Conservation Biology, the research team state that insect abundance on the Breitenbach stream declined by 81.6% between 1969 and 2010. Over this period, water temperature in the stream increased by 1.88°C, and there was a general shift towards low water flows in increasingly dry years. Their study suggests that climate change has already significantly impacted freshwater ecosystems, even in protected areas where agricultural and urban stressors are minimal.

Historical images of the Schlitz river station and Breitenbach river in flood (bottom left) and summer drought (bottom right). Images provided by Dr. Viktor Baranov.

Lead author Dr. Viktor Baranov explains, “Our analysis is based on a unique long term dataset collected by the scientists at the Schlitz river station on the Breitenbach stream from early 1960 – though our particular dataset only starts in 1969 – until 2010. This dataset is uniquely detailed and based on the weekly collection of the aquatic insects from the large, greenhouse like emergence-traps installed over the stream and parts of the riparian zone.

“The Breitenbach is a tiny stream: 6.3km in length, with a catchment area of 8.3 km². It flows to the Fulda river, with entirety of the catchment sitting in the Breitenbach valley nature conservancy area. The site was therefore sheltered from direct anthropogenic impacts such as agricultural development and industrial pollution during the period of observation. Therefore, the dataset offered us a disentangled view of the climate change impacts on this freshwater ecosystem.”

The study is the result of a collaboration at the Department of River Ecology and Conservation, Senckenberg Research Institute and Natural History Museum Frankfurt, Germany between Dr. Baranov, Dr. Jonas Jourdan, Dr. Francesca Pilotto and Dr. Peter Haase, alongside Prof. Rüdiger Wagner from the University of Kassel, Germany.

Historical images of insect collection at the Schlitz river station. Images provided by Dr. Viktor Baranov.

Dr. Baranov outlines the importance of studying insects as a means of understanding wider environmental change, “Aquatic insects are providers of crucial ecosystem services such as river water purification, fertilisation of flood plains, and food sources for many other animals. They are therefore generally an ‘engine’ of lowland and riverine ecosystems. We can thus learn a lot about the state and health of the environment by observing changes in insect diversity and numbers.

“Environmental changes have led to the decrease of the aquatic insects by 81.6% in the Breitenbach stream. But we also observed an increase in richness (+8.5%), Shannon diversity (+22.7%), evenness (+22.4%) and inter-annual turnover (+34%). Moreover, the community’s trophic structure and phenology has changed: the duration of emergence increased by 15.2 days while the peak of emergence moved 13.4 days earlier.

Additionally, the trophic structure of the community has altered drastically: grazer, scraper and gatherer–collector species have decreased significantly in their relative abundance, while passive filter feeders and predators have increased. Increased temperatures and increasingly dry years appear to be the chief drivers of change in the insect community.”

Insects from the Breitenbach stream in a collection jar. Image: Dr. Viktor Baranov.

Decades of intensive study at the Schlitz river station generated the rich datasets which allowed the researchers to analyse the environmental effects of climate change in the stream over time.

Dr. Baranov emphasises the importance of such ongoing long-term studies, stating, “Our findings would have been missed in shorter datasets, as most of the observed effects span decades. This study illustrates the value of the long term datasets in the study of the climate change effects on the freshwater communities, and the complex – and often non-linear – nature of community responses to the global climate crisis.”

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Baranov, V., Jourdan, J., Pilotto, F., Wagner, R. and Haase, P. (2020), Complex and nonlinear climate‐driven changes in freshwater insect communities over 42 years. Conservation Biology. Accepted Author Manuscript. doi:10.1111/cobi.13477

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