The world is running out of insects

In 1995 the English countryside experienced the most intense drought so far this century.

Among its victims was the cabbage butterfly, whose populations fell by 66%.

But the decline came close to collapse in the areas most fragmented by agriculture.

An investigation published twenty years later also showed that in less altered habitats these lepidoptera had recovered, but not in cultivated areas.

Now, the study of 800,000 records of almost 18,000 species obtained from 6,095 parts of the planet raises the drama of these butterflies to a global level: where intensive agriculture coincides with greater local climate change, the abundance of insects is almost half less than in the areas least damaged by human actions.

Entomologists have been warning for years about the decline in both the abundance (density of populations) and the diversity (number of species) of insects.

The decline has been confirmed for pollinators such as bumblebees and honeybees.

The causes are many, most of them human, as is the case with the impact of light pollution on fireflies or that of fertilizers on butterflies.

But there is also the advance of urbanization, deforestation and, especially, the change in land use, from natural to cultivated, and climate change.

Now researchers at University College London led by entomologist Charlie Outhwaite have combined an impressive database on animal biodiversity (PREDICTS) with the evolution of maximum and average temperatures over the last century and the dominant land use in the areas collected in that database: whether natural habitat, whether extensive agriculture or livestock or whether it is intensive farming, understood as those that use high amounts of chemicals, large areas of monoculture or mechanization.

The results of this work, which have just been published in the journal

, show that areas with intensive agriculture and that have also suffered greater historical warming have seen how the abundance of insects is 49% lower than in areas where there are still there are no crops and local climate change is relatively minor.

In terms of diversity, in the most altered areas the number of species is 27% lower.

The degree of soil change also plays a role.

“With the same level of climate change, we see larger reductions in intensive agriculture compared to low-intensity agriculture”

Charlie Outhwaite, entomologist at University College London

“Thus, intensive agriculture sites that have also experienced substantial climate change have about 50% fewer insects than primary vegetation sites that have not experienced significant climate change,” Outhwaite says.

The synergies between type of land use and heating is the great novelty of this work.

“The percentage of the reduction is the result of the interaction of the two factors, we do not measure how much of this change is the responsibility of each one separately.

The important thing is that both [climate disruption and crops] work together to cause a greater decline than if they worked alone, she adds.

And they know this because "with the same level of climate change, we see larger reductions in intensive agriculture compared to low-intensity agriculture," she concludes.

In fact,

Insects bear climate change worse than other animals.

An investigation two years ago showed how hundreds of species were advancing their flight to match it with the emergence of flowers.

From the point of view of physiology, many of the species are ectotherms, so they have to adjust their body temperature to the ambient temperature.

Others modulate their behavior according to how hot it is.

"So rising temperatures will directly affect insects and also the availability of cooler temperatures, like in shady spots," Outhwaite says.

For its part, the change in land use "can also affect the availability of shade, which means that it is more likely that insects are more exposed to high temperatures," adds the British entomologist.

Two years ago, the journal

published one of the most ambitious works on the decline of insects to date.

That research showed that, although with great variations between one place and another, the abundance of these arthropods has been declining on average by around 10% per decade at least since the 1980s. Roel van Klink, of the German Center for Research Biodiversity Integrative (iDiv) Halle-Jena-Leipzig, was the main author of this research.

Regarding the new study published in

, he appreciates that its authors have been able to test the impact of the interaction between agriculture and climate change.

"They show that agriculture is bad for insects and that climate change has made it worse," he says in an email.

However, van Klink's work also found that the insects found in rivers, lakes and reservoirs were not only in decline, but had increased their populations by 11% per decade.

Although aquatic species are a minority, they represent a tenth of the total, which is good news.

And this time, humans are responsible.

Van Klink tells us: “It is very likely that the increases, at least in Europe and North America, are due to improvements in water quality since the 1970s. We often forget how bad the quality of the water was.

Thanks to the legislation [the first environmental regulation that protected water resources dates back to 1970 in the United States] and the construction of treatment plants, a large part of the waste stopped ending up in the rivers”.

Another thing that would have helped is climate change,

But the new work shows that while insects thrive in temperate natural areas, they don't in the tropics.

Here the species were accustomed to smaller thermal oscillations and their elasticity to heating seems less.

In fact, it is in Southeast Asia and South America where the synergy between agriculture and climate change is most pronounced and where the reductions in abundance and diversity are greatest.

"Since insects constitute a large proportion of the biomass, they also have a basic role in supporting other species in the pyramid of life"

Roel van Klink of the German Center for Integrative Biodiversity Research

What consequences can have a general decline in insect populations.

Beyond the pollinating function, they are also pest control agents: many species are insectivorous, they eat other bugs that, in their absence, would devour the plants without restraint.

The professor of applied ecology at the University of Reading (United Kingdom), author of the study of the butterflies from the beginning, recalls other functions: “They are key to the decomposition of waste and the cycle of nutrients.

Since insects make up a large proportion of the biomass, they also have a basic supporting role for other species in the pyramid of life, such as providing food for birds, bats and small mammals.”

The most paradoxical thing is that agriculture is destroying some insects that it needs.

As research in 2019 showed, the more insects, the better harvests.

The main author of that work, the biologist Matteo Dainese, from Eurac Research (Italy), recalls that it is not only about pollinators, but also about other important insects for agriculture.

"In particular, I would like to highlight the key role of natural enemies of pests, such as predatory ladybugs, ground beetles or parasitoid insects that feed on pests that would otherwise damage or even destroy crops."

The researcher at the Doñana Biological Station (EBD-CSIC) Ignasi Bartomeus was co-author of the Dainese study on insects and agriculture.

He does not believe that we are facing a collapse of insect populations, there is no data, he says, to affirm something like that.

"But we are facing a very clear warning signal that it could happen within a few decades."

For Bartomeus, in addition to the threats that surround these animals, there is also an attitude problem: "Insects are not protected as we do with vertebrates," he says.

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