Climate Change: Aquatic Insect Numbers Down by 81 Percent

A long-term, high-resolution study reveals obvious changes in streams

Together with his colleague Viktor Baranov of the Ludwig-Maximilians University in Munich and a team of additional researchers, Senckenberg scientist Peter Haase studied changes in the composition of the insect fauna in a German stream. Over a period of 42 years, samples were taken at least once a week from the Breitenbach stream, located in a nature reserve in the State of Hesse. The data show that the number of aquatic insects decreased by 81.6 percent, while the species diversity showed a slight increase. In their study, published today in the scientific journal “Conservation Biology,” the team of scientists reveals that this development can be traced back to the global climate change.

The Breitenbach in the mountains of East Hesse is considered a typical representative of a small lower mountain stream – the most common stream type in Germany and Central Europe. “Nevertheless, the Breitenbach stands out among these streams! Over a period of 42 years, insect samples were taken every week, and the water flow and temperature were measured daily. It stands to reason that no other stream in the world has been subject to such long-term and frequent sampling,” explains Prof. Dr. Peter Haase, Head of the Department of “Stream Ecology and Conservation Research” at the Senckenberg Research Institute and Natural History Museum Frankfurt.

Haase and Dr. Viktor Baranov of the Ludwig-Maximilians University in Munich used this unique dataset to study the development of the insect fauna in correlation with changes in the climate. “Our efforts were aided by the fact that the Breitenbach is located in a nature reserve of approximately 610 hectares and is therefore far removed from direct human impact,” explains Baranov, and he continues, “We were thus able to pursue a central question in ecology research, i.e., what is the effect of climate change on – otherwise largely undisturbed – ecosystems?”

The results are alarming: In the period between January 1969 and December 2010, the average water temperature rose by 1.8 degrees, and the number of insect individuals decreased by 81.6 percent. “Contrary to this ‘loss of abundance,’ we recorded an overall increase in the biodiversity of the Breitenbach – this means that we have fewer individuals, but more species,” adds Haase. The researchers explain this development with a shift in the stream properties: “42 years ago, the sampled section represented a classic upper reach of a stream. Due to the temperature increase, it is now classified as a middle reach, which generally are home to a higher number of species,” explains Baranov. However, the research team’s data also show that this trend is subject to a new turnaround – measurements from the past two decades reveal that the species diversity in the Breitenbach is declining. Again, climate change-related shifts play an important role in this regard: Since 1990, dry years have predominated, with a corresponding effect on the aquatic insects.

“Our study shows that climate change already has a significant impact on our aquatic ecosystems, even in areas that are otherwise mostly unaffected by anthropogenic factors. In addition, it illustrates nature’s complexity, which does not behave in a linear fashion. This can only be fully understood by means of long-term measurements like those in the Breitenbach – short-term studies or mere modeling would undoubtedly have led to clearly different results in our study area,” adds Haase in closing.

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. Early View. doi:10.1111/cobi.13477