It is still unclear how large the regional differences in the worldwide decline in insects are and which groups are most affected by the decline. The data on this can be described as sketchy at best. But where there is data, in the vast majority of cases it shows that the insect kingdom is shrinking, both in terms of species diversity and biomass. It is also largely certain that the insects suffer from many stresses at the same time: loss of habitat, pesticides, environmental pollution, climate change.
But what is the role of each of these factors, and what are the interactions? That has not been finally clarified for a long time. In a recent study in the science journal Nature Researchers led by Charlotte Outhwaite from University College London are now trying to read the answer from a large amount of data. In total, they looked at studies from 6,000 locations and almost 18,000 species of insects, including beetles, flies, bees, butterflies and grasshoppers. They focused on agriculture and climate change, which are generally considered key pressures in the biodiversity crisis. The result: where intensive agriculture is practiced and global warming begins to exceed the range of natural fluctuations, the average insect biomass is reduced by almost half and species diversity by around a quarter compared to near-natural landscapes without warming .
However, it is notoriously difficult to make such statements because there are few long-term studies on insect diversity. One of the rare exceptions is the 2017 in Plos One published Krefeld study, which has documented a decline in the biomass of flying insects of around 80 percent in several German protected areas since 1989. Such data, however, is available from very few places, and certainly not from outside of Europe.
In near-natural landscapes, insect diversity remained more or less intact
The researchers tried to circumvent this problem by combining data from different locations: Although most of the studies used only ran for a maximum of two years, they covered a period from 1992 to 2012. On the one hand, Outhwaite’s team was able to compare relatively untouched areas with intensively used agricultural landscapes. On the other hand, surveys in one place from the early 1990s, when global warming was barely noticeable, could be compared with surveys made elsewhere two decades later, when warming was already significant. By combining the large amount of data, it is hoped that the relative influences of land use and warming can be determined.
This approach has weaknesses; Of course, the diversity of insects in the tropical rain forest is different from that in the steppes. The forms of agriculture in Asia, Africa or the USA are hardly comparable. “Such meta-studies are academically interesting, but it is difficult to draw practical conclusions from them,” says Josef Settele from the Helmholtz Center for Environmental Research in Halle, co-chair of the latest global biodiversity report. “Even the categorization is difficult, what counts as intensive agriculture, what as natural landscape?”
Nevertheless, the researchers were able to identify some plausible trends. It was shown, for example, that intensive agriculture always had significant negative effects, while insect diversity in near-natural landscapes was maintained to some extent despite warming. However, it is questionable whether this will remain the case for long, especially since only data up to 2012 were considered – since then the warming has already progressed significantly. “Until now, agriculture has actually been more important as a factor in biodiversity decline,” says Settele. “But with increasing warming, climate change will catch up, you can already see this trend today.”
Natural landscapes in the vicinity of fields can act as a buffer
In some cases, the abundance of insects even seems to increase with warming, at least in semi-natural areas outside the tropics. However, the researchers warn against interpreting this as the all-clear: Many species that are particularly sensitive to warming could have disappeared there a long time ago. In addition, while climate change is allowing some species to invade more northerly areas, this cannot compensate for the losses in other regions where it has become too hot for them.
What is also evident is that natural landscapes in the vicinity of fields can act as a buffer to compensate for other pressures. At least where agriculture is not monoculture and does not use significant pesticides, Outhwaite and her colleagues found that lush retreats with natural vegetation could almost completely offset the effect of global warming. In areas with 75 percent near-natural areas, the warming caused only a minimal decline in insects. If, on the other hand, there were only 25 percent natural areas in the area, the insect biomass collapsed by more than 60 percent with the warming.
For him, this buffer effect is the most exciting thing about the study, says Settele. “That goes well with other results: if you have a structurally rich landscape with semi-natural areas, this can significantly increase the chances of species surviving.”