Biologists have counted beetles, butterflies and bees all over Bavaria: the biodiversity is particularly low in fields, and urbanization also contributes to the decline. Climate change does not seem to play a role so far.
The death of insects continues, but science is still divided as to what is the main driver: the destruction of habitats? Pesticides? Or is it climate change? A Bavarian research team led by Johannes Uhler from the Department of Animal Ecology and Tropical Biology at the University of Würzburg has now looked at all of Bavaria to answer these questions – from Lower Franconia to Upper Bavaria, from the lowlands to the Alps. This made it possible for the first time to consider the effects of climate and land use in Central Europe separately.
In spring 2019, an army of scientific assistants drove every two weeks into forests, meadows and fields as well as into cities to empty so-called malaise traps in a total of 179 locations. The insects were then counted in the laboratory and identified using DNA sequencing. The result is now in the journal Nature Communications published: According to this, there were the fewest insects in the cities – at least in terms of the number of animals. There was 42 percent less biomass than in near-natural habitats such as forests. The authors therefore declare urbanization to be a key factor in insect decline.
“The higher temperatures had a rather positive effect on the insects”
On the other hand, the agricultural areas, which had as much insect biomass as the near-natural areas, were comparatively good. “The agricultural areas are designed to maximize the number of plants, so more insects can live on them, at least the generalists,” says Uhler.
A completely different picture emerged with the diversity of insect species. The agricultural areas performed worst here. They showed a 29 percent lower biodiversity and even 56 percent less endangered species than the near-natural areas. The cities with their parks, cemeteries and gardens, on the other hand, favor the coexistence of many different species. “The heterogeneous environment of the cities supports a broader range of species,” explains Uhler.
According to the study, however, climate change does not yet seem to play a major role in insect death. Because diversity and biomass turned out to be greater, the warmer it was in an area, across all habitats. “The higher temperatures had a positive effect on the insects,” says Uhler. If it gets warmer, the metabolism of the insect’s body goes up and that could promote their reproduction, the authors write. At least until a certain threshold is reached, above which the insects can no longer bear the heat.
The research year 2019 was a good test for this because it was particularly dry and warm two years ago. “The thermal limit of the vast majority of insects does not seem to have been reached in this country,” says Uhler. In the Mediterranean or the tropics, on the other hand, climate change could have a negative impact.
On the basis of the new results, the authors recommend expanding parks in cities and sealing less areas. Because, according to the study, the insects still feel most at home in the forests, they should be protected and new trees planted. The researchers also recommend planting trees for agricultural areas on which agroforestry can then be carried out.