New Study Gives a More Complex Picture of Insect Declines

The researchers gathered data from 166 surveys of insect abundance around the world, mostly conducted since the 1980s.

Hawk moth
A new analysis of insect abundance surveys finds the decline may not be as dire as previously thought. (Photo by Charles J Sharp via Wikimedia Commons under CC BY-SA 4.0)
By Theresa Machemer, SmithsonianMag.Com

Since 2018, news of an impending insect apocalypse has circulated widely and faced criticism. A report in Germany found that the abundance of flying insects had fallen by three quarters in less than three decades. The paper inspired entomologist Roel van Klink, of the German Center for Integrative Biodiversity Research, to conduct a wider analysis, Science News’ Susan Milius reports.

The new study, published last week in the journal Science, brings together the results of 166 surveys of 1,676 sites around the world to give a more nuanced view of insects’ decline. While the findings are less severe than previous studies, the results are still harrowing. Van Klink and his colleagues found that on average, land-dwelling insects have faced a one percent loss every year.

“This is not even something you would notice from year to year, because the insect population varies so much,” Van Klink tells Science News. “But after 30 years you will have lost a quarter of your insects.”

To the BBC’s Matt McGrath, van Klink adds, “And because it’s a mean, there are places where it is much worse than that.”

While terrestrial insects have seen their populations fall, the analysis found that freshwater insects have seen their abundances rise at about one percent-per-year, or 11 percent in a decade. But since freshwater insects make up a small proportion of the six-legged critters overall—freshwater only covers about 2.5 percent of Earth’s land—their population growth won’t offset the decline facing other insects.

Van Klink and his colleagues focused on surveys that lasted more than ten years, using data from as far back as 1928, though most came from the 1980s. The studies are primarily from North America and Europe, with only two datasets from African countries, none from India, and “shockingly” few from Australia, van Klink tells Science News. The new analysis of the surveys found that protected areas like nature preserves show less decline than areas that are not protected, but the researchers also point out that nature preserves are over-represented in the survey data.

The analysis doesn’t point to only one threat that has caused insect abundances to decline. Insects are affected by habitat destruction, climate change, pesticides and light pollution, and halting one factor alone won’t make a sudden difference.

“But we know from our results that the expansion of cities is bad for insects because every place used to be more natural habitat – it is not rocket science,” van Klink tells the Guardians Damian Carrington. “This is happening in east Asia and Africa at a rapid rate. In South America, there is the destruction of the Amazon. There’s absolutely no question this is bad for insects and all the other animals there. But we just don’t have the data.”

To Science News, van Klink speculates that freshwater insects’ rising populations may be a result of environmental laws in the U.S. that improved water quality and recovered damaged habitat. The positive trend among aquatic bugs was also strong in northern Europe and, since the 1990s, in Russia.

The new paper’s methodology seems “much more thorough and analytically thoughtful” than previous studies on the so-called insect apocalypse, Cornell University quantitative ecologist Alison Johnston tells Science News. But she also emphasizes the outsized proportion of data from North America—when surveys from the continent are removed from the analysis, the decline is halved.

Individual surveys collected in the analysis show that the picture is complex, with struggling insects living geographically next door to successful populations of the same species. A study of wild pollinators in the U.K. conducted between 1980 and 2013 found that a tenth of species studied—mostly those that specialized in pollinating crops—increased in abundance, while 30 percent—that relied on plants being pushed out by expanding farmland—saw population declines.

“Knowing where and why certain species are struggling is as important as trying to fix it. Insects are in trouble, but each bug faces its own battle.” Stuart Reynolds, biologist and former president of the Royal Entomologist Society, writes for the Conversation. “It’s a complicated picture, but the sheer number of records collected under different conditions from diverse sources in this new study gives grim confirmation that something is very wrong.”

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