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Wednesday, 6 November 2019

The decline of insect populations over the last decade has been largely underestimated


For several years, biologists have noted a continual decline in insect populations around the world, linked to the disappearance of certain biospheres and the alteration of ecosystems by human activities, particularly the intensification of agriculture and deforestation. . However, a new large-scale study reveals that the magnitude of this decline in insect populations over the last decade has been largely underestimated.

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In a large-scale biodiversity study, an international team of researchers found that insect species in forests and grasslands in Germany had decreased by about a third. And this only in the last decade.

" A decline of this magnitude over a period of just 10 years has completely surprised us, " said Wolfgang Weisser, ecologist at the Technical University of Munich (TUM). " It's scary, but it's the picture in a growing number of studies ."

Between 2008 and 2017, the research team collected more than one million insects from 300 sites in Germany. Of the approximately 2700 species studied, many appear to be declining. In fact, the team said that in recent years, some rare insects have never been found.

A significant drop in insect populations in the grasslands

No matter where the researchers looked, the conclusion was the same. From pastures for sheep to grasslands to forests, the team reported significant losses in insect diversity, and the largest losses were in grasslands, particularly those surrounded by farms.

In this type of environment, insect abundance decreased by 78%, while biomass dropped by 67%. The loss of insects in German meadows has already been demonstrated, but never in such detail. Most of the previous studies focused only on biomass, the total weight of all insects, not the number of species present.

The researchers found a significant decline in the abundance of insect species. Rare species disappear in favor of more common opportunists. Credits: Sebastian Seibold et al. 2019

" The fact that a large part of all insect groups is actually affected has not yet been demonstrated. Before our survey, it was unclear whether and to what extent forests were also affected by the decline of insects, "says TUM ecologist Sebastian Seibold.

Forest environments also affected

In forest areas, biomass has decreased by 40% and the number of species has decreased by a little over a third. According to the team, those who suffered the most were insects that covered long distances. Although this may be due to decimated forests, further research will be needed to determine the cause.

Although the decline in biomass, abundance and number of insect species is higher in grasslands (blue), forests are also affected by these extinctions (orange). Credits: Sebastian Seibold et al. 2019
" Our results show that there is a general decline in biomass, abundance and number of arthropod species across trophic levels. The decline of arthropods in forests shows that loss is not limited to open habitats, "the researchers write.

Rare insects replaced by opportunistic species

Additional research will be needed to obtain a complete picture of these changes in biomass, abundance and diversity - but the data contains some clues. The losses in the German grasslands were the highest among the rarest insects, which could explain to a large extent the alarming numbers.

In the forests, on the other hand, the scenario is different. Here, the biomass of insects has remained relatively constant over 10 years. In fact, the most abundant insects have become even more ubiquitous. This suggests that when insects disappear in forest environments, they are quickly replaced by other, more opportunistic species.

Identify the specific causes of insect decline

In the most catastrophic scenario, some entomologists warned that insects could disappear within a century; others believe that it is more likely that a small number of species will survive by taking advantage of the loss of competitors.

The authors did take climate change into account, but these were beyond the scope of this study. Not everyone is convinced that climate change is the main factor in the loss of insects, but there is good reason to believe that it could help.

" Although the factors of arthropod decline in forests remain unclear, in grasslands these factors are associated with the proportion of agricultural land in the landscape. However, we can not determine whether the observed declines are caused by the effects inherited from historical intensification of land use or by the recent intensification of agriculture at the landscape level.

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