The decline in both insect populations and in the number of species is well documented, though the evidence is patchy outside Europe and North America. Scientists agree that agriculture has a negative inﬂuence. Both the expansion and intensiﬁcation of farming seem to be to blame.
Compared to plants, mammals, birds and ﬁsh, insects are little researched. Only a small fraction has even been classiﬁed. Particularly little research has been done on the long-term occurrence and population dynamics of insects outside Europe and the US.
Scientists agree that several well-studied species, such as monarch butterﬂies, some groups of moths and butterﬂies, and some species of bees and beetles are in decline – especially in Western Europe and North America. There is also consensus that insect biodiversity is decreasing in many parts of the world, while the numbers and biomass of the animals vary greatly depending on the region, climate change and land use, as well as the adaptability of each species.
There is no scientiﬁcally conﬁrmed ﬁgure for the global decline in insects. A ﬁrst review by the University of Sydney in 2018 compiled information from research studies in various regions. It found that the populations of 41 percent of species are in decline, and one-third of all insect species are threatened by extinction. While cautioning that the available evidence is relatively thin, the researchers estimated that total insect biomass is declining by 2.5 percent a year. Most of the research studies they included in their review came from Europe, some from North America and only a few from Asia, Africa or Latin America.
The existence of these gaps has been met with criticism. Some critics pointed out that the researchers had paid too little attention to studies that showed positive changes in insect numbers. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) says the proportion of insect species worldwide that are endangered is unknown. But based on the available data, this international organization cautiously estimates that 10 percent of species are endangered.
In Europe and North America, research shows that the numbers and diversity of moths, butterﬂies, beetles, wild bees and other insects are clearly dwindling, though at different rates in each region. Individual analyses in other parts of the world reveal the same trend. A study on the Caribbean island of Puerto Rico over a period of 36 years found that the biomass of arthropods in the rainforest fell by between 78 and 98 percent (arthropods include insects along with creatures such as spiders, scorpions and millipedes).
Studies in Madagascar and New Zealand, and the Red List of Threatened Species compiled by the International Union for Conservation of Nature (IUCN), show that insect species are at risk throughout the world. At the same time, research in colder regions has found that insect numbers there are rising. Research in Russia revealed that the population of springtails in the tundra has increased as temperatures there rise.
Insects are disappearing mainly from cultivated land and intensively used pastureland. Since the early 1960s in New Zealand, the population of moths in grasslands has fallen by 60 percent, and in intensively used areas with a high livestock density by as much as 90 percent. The Academy of Sciences Leopoldina, in the city of Halle, states that the frequency of species in agricultural landscapes in Germany has fallen by around 30 percent.
In woodland, marshland and settlements, by contrast, numbers have remained stable or have even risen. The scientiﬁc consensus is that agriculture has a negative inﬂuence on insects. Farmland throughout the world is being used more and more intensively. Applications of fertilizer and pesticides have risen signiﬁcantly in an attempt to squeeze out higher yields per hectare.
Above all, though, the type of land use has been changing. In just 300 years, between about 1700 and 2007, the areas of arable land and pastureland both increased ﬁvefold, with big expansions especially in the 19th and early 20th centuries. Humans cleared forests, drained swamps, and converted steppes and savannas to ﬁelds and pastureland. Wild animal and plant species that require undisturbed habitats declined or disappeared.
Between 1980 and 2000, over half of the new agricultural land in the tropics was created by clearing forests. Between 2000 and 2010, the ﬁgure was 80 percent. Two countries, Indonesia and Brazil, were responsible for over half of this tropical forest loss. But it is precisely in the tropical countries of Latin America and Asia that the numbers and diversity of insects are especially high. The most important reasons for deforestation are to clear pastureland for cattle, establish oilpalm plantations, and opencast mining of minerals.
The demand for farm products is rising across the globe: the Food and Agriculture Organization of the United Nations predicts a 60 percent increase by 2050. That will go hand in hand with an expansion in agricultural land – depending on rising yields per unit area – of up to 100 million hectares. But these developments can be averted. If the developed world were to consume less meat and if agricultural products were no longer used as fuel, the pressure on the land areas could be reduced considerably.