page_title: Decline in insect populations
section_title: 2019 Sánchez-Bayo and Wyckhuys review
hierarchical_section_title: Decline in insect populations / Evidence / 2019 Sánchez-Bayo and Wyckhuys review
caption_attribution_description: Sympetrum infuscatum. Libellulidae, Anisoptera.
context_page_description: Several studies report a substantial decline in insect populations. 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. A 2020 meta-analysis published in the journal Science found that globally, terrestrial insects appeared to be declining in abundance at a rate of about 9% per decade, while the abundance of freshwater insects has increased by 11% per decade.
  Some of the insects most affected include bees, butterflies, moths, beetles, dragonflies and damselflies. Anecdotal evidence has been offered of much greater apparent abundance of insects in the 20th century; recollections of the windscreen phenomenon are an example.
  Possible causes of the decline have been identified as habitat destruction, including intensive agriculture, the use of pesticides, urbanization, and industrialization; introduced species; and climate change.
context_section_description: A 2019 review by Francisco Sánchez-Bayo and Kris A. G. Wyckhuys in the journal Biological Conservation analysed 73 long-term insect surveys that had shown decline, most of them in the United States and Western Europe. While noting population increases for certain species of insects in particular areas, the authors reported an annual 2.5% loss of biomass. They wrote that the review "revealed dramatic rates of decline that may lead to the extinction of 40% of the world's insect species over the next few decades", a conclusion that was challenged. They did note the review's limitations, namely that the studies were largely concentrated on popular insect groups (butterflies and moths, bees, dragonflies and beetles); few had been done on groups as Diptera (flies), Orthoptera (which includes grasshoppers and crickets), and Hemiptera (such as aphids); data from the past from which to calculate trends is largely unavailable; and the data that does exist mostly relates to Western Europe and North America, with the tropics and southern hemisphere (major insect habitats) under-represented.
  The methodology and strong language of the review were questioned. The keywords used for a database search of the scientific literature were [insect*] and [declin*] + [survey], which mostly returned studies finding declines, not increases. Sánchez-Bayo responded that two thirds of the reviewed studies had come from outside the database search. David Wagner wrote that many studies have shown "no significant changes in insect numbers or endangerment", despite a reporting bias against "non-significant findings". According to Wagner, the papers' greatest mistake was to equate "40% geographic or population declines from small countries with high human densities and about half or more of their land in agriculture to 'the extinction of 40% of the world's insect species over the next few decades'." He wrote that 40 percent extinction would amount to the loss of around 2.8 million species, while fewer than 100 insect species are known to have become extinct. While it is true that insects are declining, he wrote, the review did not provide evidence to support its conclusion.  Other criticism included that the authors attributed the decline to particular threats based on the studies they reviewed, even when those studies had simply suggested threats rather than clearly identifying them. The British ecologist Georgina Mace agreed that the review lacked detailed information needed to assess the situation, but said it might underestimate the rate of insect decline in the tropics.
  In assessing the study methodology, an editorial in Global Change Biology stated, "An unbiased review of the literature would still find declines, but estimates based on this 'unidirectional' methodology are not credible. Komonen et al. considered the study "alarmist by bad design" due to unsubstantiated claims and methodological issues that undermined credible conservation science. They stated what were called extinctions in the study represented species loss in specific sites or regions, and should not have extrapolated as extinction at a larger geographic scale. They also listed that IUCN Red List categories were misused as insects with no data on a decline trend were classified as having a 30% decline by the study authors. Simmons et al. also had concerns the review's search terms, geographic biases, calculations of extinction rates, and inaccurate assessment of drivers of population change stating while it was "a useful review of insect population declines in North America and Europe, it should not be used as evidence of global insect population trends and threats."
caption: Except for taxa regarded as beneficial or charismatic, such as the pictured dragonfly, there is relatively little population decline data available for specific insect species.
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