Decline in insect populations
s are the most numerous and widespread class in the animal kingdom, accounting for up to 90% of all animal species. In the 2010s, reports emerged about the widespread decline in populations across multiple insect orders. The reported severity shocked many observers, even though there had been earlier findings of pollinator decline. There have also been anecdotal reports of greater insect abundance earlier in the 20th century. Many car drivers know this anecdotal evidence through the windscreen phenomenon, for example. Causes for the decline in insect population are similar to those driving other biodiversity loss. They include habitat destruction, such as intensive agriculture, the use of pesticides, introduced species, and – to a lesser degree and only for some regions – the effects of climate change. An additional cause that may be specific to insects is light pollution.
Most commonly, the declines involve reductions in abundance, though in some cases entire species are going extinct. The declines are far from uniform. In some localities, there have been reports of increases in overall insect population, and some types of insects appear to be increasing in abundance across the world. Not all insect orders are affected in the same way; most affected are bees, butterflies, moths, beetles, dragonflies and damselflies. Many of the remaining insect groups have received less research to date. Also, comparative figures from earlier decades are often not available. In the few major global studies, estimates of the total number of insect species at risk of extinction range between 10% and 40%, though all of these estimates have been fraught with controversy.
Studies concur that in areas where insects are declining, their abundance had been diminishing for decades. Yet, those trends had not been spotted earlier, as there has historically been much less interest in studying insects in comparison to mammals, birds and other vertebrates. One reason is the comparative lack of charismatic species of insects. In 2016, it was observed that while 30,000 insect species are known to inhabit Central Europe, there are practically no specialists in the region devoted to full-time monitoring. This issue of insufficient research is even more acute in the developing countries. As of 2021, nearly all of the studies on regional insect population trends come from Europe and the United States, even though they account for less than 20% of insect species worldwide. In Africa, Asia and South America there are hardly any observations of insects that span several decades. Such studies would be required to draw conclusions about population trends on a large scale.
To respond to these declines, various governments have introduced conservation measures to help insects. For example, the German government started an Action Programme for Insect Protection in 2018. The goals of this program include promoting insect habitats in the agricultural landscape, and reducing pesticide use, light pollution, and pollutants in soil and water.
Context
The fossil record concerning insects stretches back hundreds of millions of years. It suggests there are ongoing background levels of both new species appearing and extinctions. Very occasionally, the record also appears to show mass extinctions of insects, understood to be caused by natural phenomena such as volcanic activity or meteor impact. The Permian–Triassic extinction event saw the greatest level of insect extinction, and the Cretaceous–Paleogene the second highest. Insect diversity has recovered after mass extinctions, as a result of periods in which new species originate with increased frequency, although the recovery can take millions of years.Concern about a human-caused Holocene extinction has been growing since the late 20th century, although much of the early concern was not focused on insects. In a report on the world's invertebrates, the Zoological Society of London suggested in 2012 that insect populations were in decline globally, affecting pollination and food supplies for other animals. It estimated that about 20 percent of all invertebrate species were threatened with extinction, and that species with the least mobility and smallest ranges were most at risk.
Studies finding insect decline have been available for decades—one study tracked a decline from 1840 to 2013—but it was the 2017 re-publication of the [|German nature reserves study] that saw the issue receive widespread attention in the media. The press reported the decline with alarming headlines, including "Insect Apocalypse". Ecologist Dave Goulson told The Guardian in 2017: "We appear to be making vast tracts of land inhospitable to most forms of life, and are currently on course for ecological Armageddon." For many studies, factors such as abundance, biomass, and species richness are often found to be declining for some, but not all locations; some species are in decline while others are not. The insects studied have mostly been butterflies and moths, bees, beetles, dragonflies, damselflies and stoneflies. Every species is affected in different ways by changes in the environment, and it cannot be inferred that there is a consistent decrease across different insect groups. When conditions change, some species adapt easily to the change while others struggle to survive.
A March 2019 statement by the Entomological Society of America said there was not yet sufficient data to predict an imminent mass extinction of insects and that some of the extrapolated predictions might "have been extended well past the limits of the data or have been otherwise over-hyped". For some insect groups such as some butterflies, moths, bees, and beetles, declines in abundance and diversity have been documented in European studies. These have generally led to an overall pattern of decline, but there are variable trends for individual species within groups. For instance, a minority of British moths are becoming more common. Other areas have shown increases in some insect species, although trends in most regions are currently unknown. It is difficult to assess long-term trends in insect abundance or diversity because historical measurements are generally not known for many species. Robust data to assess at-risk areas or species is especially lacking for arctic and tropical regions and a majority of the southern hemisphere.
In March 2019 Chris D. Thomas and other scientists wrote in response to the apocalyptic "Insectageddon" predictions of Sánchez-Bayo, "we respectfully suggest that accounts of the demise of insects may be slightly exaggerated". They called for "joined-up thinking" in responding to insect declines, backed up by more robust data than were currently available.
Global estimates
A 2020 meta-analysis found that globally terrestrial insects appear to be declining in abundance at a rate of about 9% per decade, while the abundance of freshwater insects appears to be increasing by 11% per decade. The study analysed 166 long-term studies, involving 1676 different sites across the world. It found considerable variations in insect decline depending on locality – the authors considered this a hopeful sign, as it suggests local factors, including conservation efforts, can make a big difference. The article stated that the increase in freshwater insects may in part be due to efforts to clean up lakes and rivers, and may also relate to global warming and enhanced primary productivity driven by increased nutrient inputs. However, the data selection and methodology of the article were criticised in several publications and the article now has an editorial expression of concern.In 2022, 66 researchers conducted a survey of 3331 biodiversity experts. This included 629 experts in terrestrial and freshwater invertebrates: on average, they believed that around 30% of these species are or have been threatened with extinction. As insects account for the vast majority of the world's invertebrates, this figure by extension applies to them as well.
A 2019 survey of 24 entomologists working on six continents found that on a scale of 0 to 10, with 10 being the worst, all the scientists rated the severity of the insect decline crisis as being between 8–10.
''Global Assessment Report on Biodiversity and Ecosystem Services''
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services reported its assessment of global biodiversity in 2019. Its summary for insect life was that "Global trends in insect populations are not known but rapid declines have been well documented in some places. ... Local declines of insect populations such as wild bees and butterflies have often been reported, and insect abundance has declined very rapidly in some places even without large-scale land-use change, but the global extent of such declines is not known. ... The proportion of insect species threatened with extinction is a key uncertainty, but available evidence supports a tentative estimate of 10 percent." In 2022, some researchers had expressed concern about the apparent mismatch between this tentative IPBES estimate, and that of 629 experts on invertebrate biodiversity they surveyed, which was closer to 30%. They argued that more investigations into insects and other "hyperdiverse and understudied taxa" are urgently required to clarify the matter.Causes
Destruction and pollution
The causes of the declines in insect populations, and their relative importance, are not fully understood. They are likely to vary between different insect groups and geographical regions. A study placed these causes in the order of importance as follows: " pollution, mainly that by synthetic pesticides and fertilisers; iii) biological factors, including pathogens and introduced species; and iv) climate change."Light pollution also plays a role. Other factors that are thought to be important are anthropogenic noise, introduced species, and eutrophication from fertilisers.
The use of increased quantities of insecticides and herbicides on crops have affected not only non-target insect species, but also the plants on which they feed.
For example, a review in 2017 commented on a large study in Germany about insect population decline as follows: "The authors of the German study were not able to link the observed decline to climate change or pesticide use; although agricultural intensification and the practices associated with it, were, however, suggested as likely to be involved in some way."