Insect decline more extensive than suspected

Causes of insect decline and biodiversity loss to be found at the landscape level

2019/10/31

Compared to a decade ago, today the number of insect species on many areas has decreased by about one third. This is the result of a survey of an international research team with contributions by scientists from the Technical University of Darmstadt. The loss of species mainly affects grasslands in the vicinity of intensively farmed land – but also applies to forests and protected areas.

Professor Nico Blüthgen and Dr. Nadja Simons measure body characteristics of insects.

Various studies have already demonstrated that there are far fewer creatures chirping, buzzing, creeping and fluttering in German meadows today than 25 years ago. “Previous studies, however, either focused exclusively on biomass, i.e. the total weight of all insects, or on individual species or species groups. The fact that a large part of all insect groups is actually affected has not been clear so far,” says Dr. Sebastian Seibold, a scientist with the Terrestrial Ecology Research Group at the Technical University of Munich (TUM).

In a large-scale biodiversity study, an international research team headed by scientists at TUM surveyed a large number of insect groups in Brandenburg, Thuringia and Baden-Württemberg between 2008 and 2017. Now the team has published its analysis in the scientific journal “nature”.

Insects affected both in grasslands and forests

The researchers collected more than one million insects at 300 sites. They were able to prove that many of the nearly 2700 investigated species are in decline. In recent years, certain rare species could no longer be found in some of the regions studied. Both in forested areas and grasslands, the scientists counted about one third fewer insect species after 10 years.

“So far, we have been able to compare the 300 areas and thus demonstrate how land use affects different insect groups in several detailed studies. Frequent mowing of meadows with rotary mowers, for example, causes a significant insect decline, while cow and sheep pastures maintain a much higher insect diversity,” says Nico Blüthgen, Professor of Ecological Networks at the TU Darmstadt. Together with Professor Wolfgang Weisser from TUM, he leads these projects on insects in the framework of the DFG-funded collaborative research project Biodiversity Exploratories. “These meticulously collected data now show for the first time the decline over the years – and to a similar extent in meadows, pastures and forests. In addition to land use within each individual area, e.g. mowing or wood use, changes in entire landscapes are also at play here,” adds Prof. Blüthgen.

The research team found that the biomass of insects in the forests studied had declined by around 40 per cent since 2008. In grassland, the results were even more alarming: at the end of the study period, the insect biomass had decreased to only one third of its previous level. The very sharp decline in grassland fits into the picture painted by an increasing number of studies.

Selected specimens of the bug species Calocoris roseomaculatus caught on the study sites.

The decisive factor: the immediate surroundings

Every type of forest and grassland site studied by the team was affected: sheep pastures, meadows that are mowed and fertilized three to four times per year, forestry dominated coniferous forests and even unused forests in protected areas. The researchers identified the biggest losses in grasslands surrounded by intensively farmed land. “On these grasslands, the most heavily impacted species were those unable to travel far. Hence, repopulation of those grasslands by those species after disturbance is very slow or impossible”, said Dr. Nadja Simons, previously involved in the study at TUM and now working at TU Darmstadt.

In the forested areas, by contrast, the hardest-hit insect groups were those that cover long distances. “It might be that more mobile forest-dwelling species have more contact with agriculture”, explains Dr. Simons. “The decline in insects could however also be driven by the living conditions in the forests. Here, further study will be needed”.

Standalone initiatives have little prospect of success

“Current initiatives to address insect losses are overly concerned with the cultivation of individual plots of land and operate independently of one another for the most part,” says Dr. Seibold. “To stop the decline, however, our results indicate that more coordination is needed at the regional and national levels.”

Publication

Seibold, S., Gossner, M.M., Simons, N.K., Blüthgen, N., Müller, J., Ambarli, D., Ammer, C., Bauhus, J., Fischer, M., Habel, J.C., Linsenmair, K.E., Nauss, T., Penone, C., Prati, D., Schall, P., Schulze, E.-D., Vogt, J., Wöllauer, S. & Weisser, W.W. Arthropod decline in grasslands and forests is associated with drivers at landscape level. Nature
DOI:https://doi.org/10.1038/s41586-019-1684-3

More information

The study was carried out under a Germany-wide cooperative project, the Biodiversity Exploratories. This open research platform is financed by the German Research Foundation (DFG). Researchers from TU Munich and TU Darmstadt, the Max Planck Institute for Biogeochemistry (Jena, Germany), the Swiss Federal Research Institute WSL (Birmensdorf, Switzerland), the Universities of Bern (Switzerland), Düzce (Turkey), Salzburg (Austria), as well as Freiburg, Göttingen, Marburg, and Würzburg were involved.

The scientists in the project are exploring such issues as the possible interactions between different elements of biodiversity – for example between plant and insect diversity. The project is also exploring the effects of different forms of land use on biodiversity and the processes within an ecosystem.