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The new information suggests that the global health of insect populations is much more complicated than previously thought.
Previous research indicated an alarming decline in numbers in all parts of the world, with losses of up to 25% per decade.
This new study, the largest to date, says the image is more complex and varied.
Insects that live on land are definitely decreasing, the authors say, while insects that live in fresh water are increasing.
Reports of rapid and widespread decline in insects globally have caused scientists great concern.
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May flies have benefited from cleaner water legislation
Creatures are among the most abundant and diverse species on the planet and play key roles, from aerating the soil to pollination and nutrient recycling.
The case studies, as one of the nature reserves in western Germany, indicated a dramatic drop, with a decrease of around 75% in 27 years.
Many other similar reports have followed.
But many of these were region or species specific.
This new study, the largest on insect change to date, aims to provide a more complete understanding of what is really happening to insects worldwide.
Drawing on data from 166 long-term surveys at 1,676 sites, it paints a highly nuanced and variable picture of the state of insect health.
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Grasshoppers are one of the species that have suffered significant declines.
The compilation indicates that insects such as butterflies, ants, and grasshoppers are declining by 0.92% per year, equivalent to 9% per decade, lower than many published rates.
This is not as bad as previous reports, but the authors emphasize that it is still substantial.
“That is extremely serious, more than 30 years means a quarter fewer insects,” said lead author Dr. Roel Van Klink of the German Center for Integrative Biodiversity Research.
“And because it is a medium, there are places where it is much worse than that.”
Many people have an instinctive perception that insects are on the decline, often reported by the so-called “windshield phenomenon,” where fewer dead insects are found splashed in cars. The researchers say it is real.
“Many insects can fly, and they are the ones that break the windshields of cars,” said Professor Jonathan Chase, another author at the German Center for Integrative Biodiversity Research.
“Our analysis shows that flying insects have declined on average.”
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Oliver Thier
Water-based insects, like common mosquitoes, have increased in number
“However, most insects are less visible and live out of sight, on the ground, in the treetops, or in the water.”
Losses were strongest in the United States West and Midwest and in Europe, especially in Germany.
Trends in Europe have turned more negative in recent years, with the biggest declines since 2005.
However, while many terrestrial species are declining, the new study shows that freshwater insects, such as mosquitoes and May flies, grow 1.08% per year.
This positive trend was strong in northern Europe, in the western USA. USA And since the 1990s in Russia.
The researchers believe this is due to legislation that has cleaned up polluted rivers and lakes.
However, increasing water-based insects will not compensate for soil losses.
“They are only a fraction of land insects, not more than 10%,” said Dr. Van Klink
“The freshwater area we have on earth is only a small percentage of the total land mass, so the number of freshwater insects can never compensate for terrestrial insects.”
Scientists say there is no smoking gun on insect decline, but they consider the destruction of natural habitats due to urbanization to be key.
This finding on habitat destruction has been echoed in other important research on biodiversity, including last year’s IPBES Global Assessment.
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A European bee that helps pollinate some flowers
The big picture is complex: Even in nearby geographic areas, some insects may be well next to members of the same species that are fighting.
Ann Swengel, another author of the article, has spent more than 30 years studying butterflies in parts of the United States.
“We have seen a lot of decline, even at many protected sites. But we have also seen some sites where butterflies continue to grow well,” he said.
“It takes many years and a lot of data to understand both failures and successes, species by species and site by site.”
While the findings are complicated, the authors believe they offer hope for the future.
“We believe that because we see these increases in freshwater insects, which are related to the legislation that is being implemented, we are hopeful that if we implement the right types of legislation for land insects, we can also make them recover ” Dr. Van Klink said.
“The good thing about insects is that most have an incredibly large number of young, so if you change the habitat the right way, we’ll see that they recover very quickly.”
The study has been published in the journal Science.
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