We may think of insects as pests, but they play an important role in maintaining the natural order of the world. They provide food for plenty of other species. They pollinate plants. They recycle nutrients.

All of this is why a scientific review of global insect populations published in Biological Conservation is so troubling. More than 40 percent of the world's insect populations are in decline, and they're declining fast.

"The trends confirm that the sixth major extinction event is profoundly impacting life forms on our planet," the researchers wrote in their conclusion.

A widespread drop in bugs

The writing has been on the wall about insects' demise. A German research team announced in October 2018 that the country's insect population had fallen by 77 percent between 1989 and 2016. A researcher in Puerto Rico reported similar declines in insect biomass when he revisited research sites, comparing data from the 1970s to what he found in the the 2010s.

Local can often be global, however, and the review published in Biological Conservation points to that.

In addition to the 40 percent decline, a third of insect species are endangered. Chain these facts together with the finding that insect biomass — the mass of organisms living in an area — is declining by 2.5 percent a year and the researchers report that there could be widespread insect extinctions by the end of the century.

"It is very rapid," lead author and University of Sydney professor Francisco Sanchez-Bayo told The Guardian. "In 10 years you will have a quarter less, in 50 years only half left and in 100 years you will have none."

Sanchez-Bayo, writing with co-author co-author Kris A.G. Wyckhuys from the University of Queensland, found real reason to worry:

Because insects constitute the world's most abundant and (species-diverse) animal group and provide critical services within ecosystems, such events cannot be ignored and should prompt decisive action to avert a catastrophic collapse of nature's ecosystems.

To judge the decline of insects, Sanchez-Bayo and Wyckhuys collected 73 of the best studies done so far about insect population drops. Most were centered around European and American insect populations, but Sanchez-Bayo and Wyckhuys also included studies from Australia, China, Brazil and across South America.

Butterflies and moths are the insect canaries

A blue morpho butterfly on a leaf Butterfly populations are dropping in many places around the world. (Photo: Cristian Gusa/Shutterstock)

According to the review, butterflies and moths are among the worst hit, with bees and beetles not far behind. Butterfly populations have fallen by 58 percent on farmed land in England between 2000 and 2009. California's monarch butterfly population reportedly dropped by 86 percent between 2017 and 2018.

Other species, like ants, flies and crickets are difficult to measure, but there's little reason to believe that they're doing any better.

As for the reasons behind the drops, Sanchez-Bayo and Wyckhuys point to our current agricultural practices as the culprit.

"The main cause of the decline is agricultural intensification," Sánchez-Bayo told The Guardian. "That means the elimination of all trees and shrubs that normally surround the fields, so there are plain, bare fields that are treated with synthetic fertilizers and pesticides."

Stronger insecticides that harm the bugs and the soil around crops aren't helping matters, either, he added.

Where heavy agricultural practices aren't present, climate change and its rising temperatures are wiping out other populations, particularly in the tropics.

Both researchers recommend drastic changes in our agricultural methods, "in particular a serious reduction in pesticide usage and its substitution with more sustainable, ecologically based practices."

Such a reduction could help save the food web that we rely on for sustenance.

"The conclusion is clear: unless we change our ways of producing food, insects as a whole will go down the path of extinction in a few decades," they wrote.

Insect 'extinction event' will transform nature
More than 40 percent of the world's insect species are threatened with extinction, and their loss could upend nature as we know it.