12 May 2023
For years the number of insects has been steadily, worryingly decreasing. PHOTO: Pixabay

A Swiss-German study investigated the main factors behind the constant decline of insects and their biodiversity. Invasive varieties and intensive agriculture are also of concern. 40% of all species will be at risk of extinction in the coming decades

by Emanuele Isonio

The number of insects, especially the very precious pollinators, has been steadily declining for decades. And at the same time their biodiversity also decreases. The main causes? Intensification of land use for agricultural and building purposes, poor management practices, related climate change, spread of invasive alien species. This is highlighted by a study, part of a special edition of Biology Letters, edited by researchers from the Gutenberg University of Mainz, the Technical University of Darmstadt and the Swiss Federal Institute for Forest, Snow and Landscape Research.

“Evidence of an ongoing global collapse in insect populations has increased in recent years. We therefore decided it was time to edit and publish this analysis. Our goal was not to document the decline of insect populations, but to better understand its causes and consequences,” explains Florian Menzel, biologist who leads the research group together with colleagues Nadja Simons and Martin Gossner.

A worrying photograph

The special edition of Biology Letters contains two opinion papers, 10 time series analyzes spanning 10 to 120 years, and two studies applying space-by-time substitution methodology involving terrestrial and freshwater insects in five biomes. A worrying picture emerges from all the studies, not only due to the decrease in insects but also because communities are becoming more homogeneous. “This loss of diversity is likely to have drastic consequences for ecosystem functioning and stability,” the study reads.

Cause e conseguenze della diminuzione del numero e della biodiversità di insetti. FONTE: Biology Letters - Martin M. Gossner, Florian Menzel, Nadja K. Simons, 2023.

Causes and consequences of the decrease in the number and biodiversity of insects. SOURCE: Biology Letters – Martin M. Gossner, Florian Menzel, Nadja K. Simons, 2023.

Three factors that interact with each other

The analysis also highlighted that, in addition to climate change, land use and alien species are three causal factors that interact with each other. In this way – we read – “the combined effects on insect populations and communities can be more serious than the sum of the individual factors”. Some examples: the decrease in vegetation cover caused by intensive land use can reduce the ability of a habitat to mitigate drought linked to climate change. Or climate modifications can facilitate the invasions of alien species, facilitating the presence of those more tolerant to heat.

“It appears that specialized insect species suffer the most,” Menzel explains. “In contrast, the more generalized species tend to survive. That’s why we are now finding more insects able to live almost everywhere while those species that need specific habitats are in decline.” Appealing to the rules of Darwinism does not appear, in the eyes of researchers, a good choice. In fact, the loss of biodiversity and specialized species produces harmful consequences for the territories affected by them. “For example, the loss of bumblebee diversity has resulted in a concomitant decline in plants that rely on certain bumblebee species for pollination.”

Interconnected reserves to encourage migration

In general, a decline in insect diversity threatens the stability of ecosystems: “fewer species means fewer insects capable of pollinating plants and keeping pests in check. And of course, this also means that there is less food available for birds and other insect-eating animals.” Hence the researchers’ appeal: “In addition to population trends, we should simultaneously monitor how they affect insect-mediated ecosystem functions such as pollination, decomposition, food for higher trophic levels, and biocontrol.”

The authors then propose to create a network of interconnected nature reserves. “In this way, species could move from one habitat to another. Less heat-tolerant insects would thus be able to migrate from areas where temperatures are rising to higher altitudes or colder regions.” It would also be necessary to introduce regulations to reduce the migration of invasive plant and animal species caused by the globalization of trade and tourism”.

The largest insect extinction since the Cretaceous

How pressing the problem of the decrease in insect species is was already highlighted 3 years ago in a study published in Biological Conservation by two professors from the universities of Sydney and Queensland. Through a review of 72 studies on the global decline of insects developed over nearly three decades, the work had shown that the biomass of insects has decreased by 2.5% every year in the last three decades. A trend which, transferred over the next few decades, provided a disheartening picture: 40% of species will end up on the endangered lists. The largest extinction since the Late Permian and Cretaceous times (between 300 and 270 million years ago and between 145 and 65 million years ago, respectively).