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Decline in insect populations

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An annual decline of 5.2% in flying insect biomass found in nature reserves in Germany – about 75% loss in 26 years.[1]

Several studies report what appears to be a substantial decline in insect populations. Some of the insects most affected include bees, butterflies, moths, beetles, dragonflies and damselflies. The issue has been described anecdotally as the windshield phenomenon[2] and reported in the press with alarming headlines such as the "Insect Apocalypse" or "ecological Armageddon".[3][4][5]

Possible causes of the decline have been identified as habitat destruction, including intensive agriculture, the use of pesticides (particularly insecticides), urbanization, and industralization; introduced species; and climate change.[6] Not all insect orders are affected in the same way; many groups are the subject of limited research, and comparative figures from earlier decades are often not available.

History

A 1902 illustration of a Rocky Mountain locust. These insects were seen in swarms estimated at over 10 trillion members as late as 1875. Soon after, their population rapidly declined, with the last recorded sighting in 1902, and the species formally declared extinct in 2014.

The fossil record concerning insects stretches back for 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. The Permian–Triassic extinction event saw the greatest level of insect extinction, with the Cretaceous–Paleogene being the second highest. Insect diversity has recovered after past mass extinctions, due to periods where new species originate with increased frequency, though the recovery can take millions of years.[7]

The pre-historic extinction events are understood to be caused by natural phenomena such as volcanic activity or meteor impact. Concern over a possibly human caused Holocene extinction has been growing since the late 20th century, though much of the early concern was not focused on insects. Studies finding insect decline have been available for many decades, with one tracking a decline all the way from 1840 to 2013. Yet it was the 2017 re-publication of the German nature reserves study[1] that saw the issue of insect decline receive widespread attention in the media.[3][6] [8]

Causes of decline

Several studies indicate that insect populations are in decline in the late 20th and early 21st centuries, at least in certain regions. 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 easily adapt to the change while others struggle to survive.[9]

The most important cause of decline in insect populations is thought to be habitat destruction, itself caused by agricultural intensification, commercial development, recreation and pollution. Other issues include pesticides, introduced species, and climate change.[10][6][11][12][13] 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. Climate change and the introduction of exotic species that compete with the indigenous ones put the native species under stress, and as a result they are more likely to succumb to pathogens and parasites.[9] While some species such as flies and cockroaches might increase as a result,[10] the total biomass of insects is estimated to be decreasing by about 2.5% per year.[13]

Research

Studies

A 2014 review in Science noted that "Of all insects with IUCN-documented population trends, 33% are declining ... Globally, a compiled index of all invertebrate population declines over the past 40 years shows an overall 45% decline".[14]

Malaise trap in a German nature reserve[1]

In 2013 the Krefeld Entomological Society reported, according to the British entomologist Simon Leather, a "huge reduction in the biomass of insects" caught in malaise traps in 63 nature reserves in Germany.[3][15] A reanalysis published in 2017 in PLOS One suggested that, in the period 1989 to 2016, there had been a "seasonal decline of 76%, and mid-summer decline of 82% in flying insect biomass over the 27 years of study". The decline was "apparent regardless of habitat type" and could not be explained by "changes in weather, land use, and habitat characteristics". The authors suggested that it is not only butterflies, moths and wild bees that appear to be in decline, as previous studies indicated, but "the flying insect community as a whole".[1][3][a]

According to The Economist, the study was the "third most frequently cited scientific study (of all kinds) in the media in 2017".[b] It gave rise to several media reports of "ecological Armageddon" and similar.[3][13][5][17] Leather wrote that the study's authors had not been able to link the decline to climate change or pesticides but had suggested that intensive farming was involved. While agreeing with the study's conclusions, he cautioned that "the data are based on biomass, not species, and the sites were not sampled continuously and are not globally representative".[c] As a result of the Krefeld and other studies, the German government established an "Action Programme for Insect Protection".[18]

A 2018 study of the El Yunque National Forest in Puerto Rico, by Bradford C. Lister and Andres Garcia and published in the Proceedings of the National Academy of Sciences, reported a decline in arthropods, and in lizards, frogs, and birds (insect-eating species), during the period 1976–2012. The researchers reported "biomass losses between 98% and 78% for ground-foraging and canopy-dwelling arthropods over a 36-year period, with respective annual losses between 2.7% and 2.2%". The decline was attributed to a rise in the average temperature; tropical insect species cannot tolerate a wide range of temperatures.[19][6][13][20]

A 2019 systematic review by Francisco Sánchez-Bayo and Kris A. G. Wyckhuys noted that most scientific and public attention focuses on the conservation of larger, charismatic vertebrates, while insect biodiversity has been low on the agenda.[6] The review searched for and analysed 73 studies that had shown decline. In the view of the authors, 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",[6] a conclusion that was challenged.[21] While reporting population increases for certain species of insects in particular areas, the review found that overall there were "dramatic rates of decline":[6][22]

From our compilation of published scientific reports, we estimate the current proportion of insect species in decline (41%) to be twice as high as that of vertebrates, and the pace of local species extinction (10%) eight times higher, confirming previous findings (Dirzo et al., 2014). At present, about a third of all insect species are threatened with extinction in the countries studied ... Moreover, every year about 1% of all insect species are added to the list, with such biodiversity declines resulting in an annual 2.5% loss of biomass worldwide ...[6]

Sánchez-Bayo and Wyckhuys noted that few studies had been done on such insect groups as Diptera, Orthoptera and Hemiptera. Data from the past from which to calculate trends was largely unavailable, and what did exist mostly related to Western Europe and North America, with the tropics and southern hemisphere (which are the major habitats of a large amount of insects) under-represented.[6] The insect studies that had been undertaken were largely concentrated on the more popular insect groups: butterflies and moths, bees, dragonflies and beetles.[6]

The methodology of the Sánchez-Bayo and Wyckhuys study was questioned. The keywords used to search the scientific literature were [insect*] and [declin*] + [survey]. This meant that the authors mostly identified studies finding insect declines but not those showing increases.[21][22][23][24] The American entomologist David L. Wagner wrote that many studies have shown "no significant changes in insect numbers or endangerment, despite a significant reporting bias against researchers writing articles about non-significant findings and journals to publish them".[21] Several entomologists agreed that the data on insect declines are too piecemeal and unrepresentative of the general population to support some of the hyperbolic extrapolations.[22][24] 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 Sánchez-Bayo and Wyckhuys review did not provide evidence to support its conclusion.[21]

Limitations

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.

One reason that studies are limited is that the science of studying insects – entomology – is itself in decline.[25] Similarly in decline is the profession of taxonomy, necessary to record the diversity of species.[22] General biology courses in college give less attention to insects and the number of biologists specialising in entomology is reducing as other specialities such as genetics expand.[26][27][28] Also, these studies tend to be done by collecting the insects and killing them in traps which is an ethical problem for conservationists.[29][30]

A March 2019 statement by the Entomological Society of America said there is 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". [31] Leather suggested that reports of an "Ecological Armageddon" may be exaggerated and advocated for more funding to allow better collection of long-term data on the decline.[3]

Conservation measures

Citing a 2010 warning from Lord May, the authors of the 2019 worldwide review suggest decisive action is needed "to avert a catastrophic collapse of nature's ecosystems."[6] Following several studies, including the 2017 Krefeld Entomological Society study, the German environment ministry, the BMU, started its Action Programme for Insect Protection (Aktionsprogramm Insektenschutz).[18] This initially contained nine goals:[32]

  1. "Promote insect habitats and structural diversity in the agricultural landscape"
  2. "Restore and connect habitats for insects in other landscape areas"
  3. "Strengthen protected areas as habitats for insects"
  4. "Reduce the use of pesticides"
  5. "Reduce inputs of nutrients and pollutants in soil and water"
  6. "Reduce light pollution"
  7. "Deepening research - multiplying knowledge - closing gaps"
  8. "Improve funding - create incentives"
  9. "Promote the commitment of society"

In a 2019 paper, scientists Olivier Dangles and Jérôme Casas suggested that previous attempts to arrest the repeatedly reported "massive insect decline" have been largely ineffective. They list 100 studies and other references showing how insects can assist with meeting the Sustainable Development Goals (SDG) set by the United Nations, and argue that the best approach may be for the global policy-making community to transition from the current "insects as providers of ecosystem-services" view to one that see "insect as solutions for SDGs".[33]

Chris D. Thomas and other scientists warned of the need for "joined‐up thinking" in responding to the decline, ideally backed up by more robust data than is available so far. In particular, they warn that excessive focus on reducing pesticide use could be counter productive. They state that pests already cause a 35% yield loss for crops, which sometimes rises to 70% when pesticides are not used. If the crop shortfall is compensated for by expanding agricultural land with deforestation and other habitat destruction, it could exacerbate insect decline.[34]

Citizen science contributes to the monitoring of insect populations. People have noticed a decline in the number of insects splattered on car windshields when they drive across their country – the windshield phenomenon. In 2004, the Royal Society for the Protection of Birds (RSPB) organised a Big Bug Count, issuing "splatometers" to about 40,000 volunteers so that they could count the number of insects colliding with their number plate. They found an average of one insect per 5 miles (8 km), which was less than expected.[35]

The Entomological Society of America advise that measures regular people can take include maintaining plant diversity in their gardens, and leaving "natural habitat, like leaf litter and dead wood." [31] Phone apps such as iNaturalist can be used to photograph and identify specimens; these are being used in programs such as the City Nature Challenge. Activities and projects may focus upon a particular type of insect, such as National Moth Week and monarch butterfly conservation in California.[36]

See also

Notes

  1. ^ Francisco Sánchez-Bayo and Kris A. G. Wyckhuys (Biological Conservation, April 2019): "In 2017, a 27-year long population monitoring study revealed a shocking 76% decline in flying insect biomass at several of Germany's protected areas (Hallmann et al., 2017). This represents an average 2.8% loss in insect biomass per year in habitats subject to rather low levels of human disturbance, which could either be undetectable or regarded statistically non-significant if measurements were carried out over shorter time frames. Worryingly, the study shows a steady declining trend over nearly three decades."[6]
  2. ^ The Economist (21 March 2019): "The study [Hallmann et al. 2017] was the third most frequently cited scientific study (of all kinds) in the media in 2017 and pushed the governments of Germany and the Netherlands into setting up programmes to protect insect diversity."[16]
  3. ^ Simon Leather (Annals of Applied Biology, 20 December 2017): "Four years ago a group of German entomologists reported that there had been a huge reduction in the biomass of insects caught using Malaise traps sited in 63 German nature reserves since 1989 (Sorg et al., 2013). This shocking observation went almost unnoticed until a reanalysis of the data appeared recently (Hallmann et al., 2017). The latter paper generated a flurry of media activity and the phrase 'Ecological Armageddon' swiftly circled the globe. Although not denying the decline reported, there are a number of caveats that should be considered when reading the two papers; the data are based on biomass, not species, the sites were not sampled continuously and are not globally representative (Saunders, 2017). 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."[3]

References

  1. ^ a b c d Hallmann, CA; Sorg, M; Jongejans, E; Siepel, H; Hofland, N; Schwan, H (18 October 2017), "More than 75 percent decline over 27 years in total flying insect biomass in protected areas", PLoS ONE, 12 (10): e0185809, doi:10.1371/journal.pone.0185809, PMC 5646769, PMID 29045418{{citation}}: CS1 maint: unflagged free DOI (link)
  2. ^ McCarthy, Michael (21 October 2017), "A giant insect ecosystem is collapsing due to humans. It's a catastrophe", The Guardian
  3. ^ a b c d e f g Leather, Simon (20 December 2017), ""Ecological Armageddon" – more evidence for the drastic decline in insect numbers", Annals of Applied Biology, 172: 1–3, doi:10.1111/aab.12410
  4. ^ Jarvis, Brooke (27 November 2018), "The Insect Apocalypse Is Here", The New York Times
  5. ^ a b Carrington, Damian (18 October 2017), "Warning of 'ecological Armageddon' after dramatic plunge in insect numbers", The Guardian
  6. ^ a b c d e f g h i j k l Sánchez-Bayo, Francisco; Wyckhuys, Kris A.G. (31 January 2019), "Worldwide decline of the entomofauna: A review of its drivers", Biological Conservation, 232: 8–27, doi:10.1016/j.biocon.2019.01.020
  7. ^ Labandeira, Conrad (1 January 2005), "The fossil record of insect extinction: new approaches and future directions", American Entomologist, 51: 14–29, doi:10.1093/ae/51.1.14
  8. ^ Schwägerl, Christian (2016-07-06). "What's Causing the Sharp Decline in Insects, and Why It Matters". Yale School of Forestry & Environmental Studies. Retrieved 28 March 2019.
  9. ^ a b Reckhaus, Hans-Dietrich (2017). Why Every Fly Counts: A Documentation about the Value and Endangerment of Insects. Springer International Publishing. pp. 1–5. ISBN 978-3-319-58765-3.
  10. ^ a b McGrath, Matt (11 February 2019), Global insect decline may see 'plague of pests', BBC
  11. ^ Carrington, Damian (10 February 2019). "Plummeting insect numbers 'threaten collapse of nature'". The Observer.
  12. ^ Vogel, Gretchen (10 May 2017), "Where have all the insects gone?", Science
  13. ^ a b c d Main, Douglas (14 February 2019). "Why insect populations are plummeting—and why it matters". National Geographic. Archived from the original on 15 February 2019. {{cite news}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
  14. ^ Dirzo, Rodolfo; Young, Hillary; Galetti, Mauro; Ceballos, Gerardo; Isaac, Nick; Collen, Ben (25 July 2014), "Defaunation in the Anthropocene" (PDF), Science, 345 (6195): 401–406, doi:10.1126/science.1251817, PMID 25061202
  15. ^ Sorg, M.; Schwan, H.; Stenmans, W.; Müller, A. (2013). "Ermittlung der Biomassen flugaktiver Insekten im Naturschutzgebiet Orbroicher Bruch mit Malaise Fallen in den Jahren 1989 und 2013" (PDF). Mitteilungen aus dem Entomologischen Verein Krefeld. 1: 1–5.
  16. ^ "The insect apocalypse is not here but there are reasons for concern". The Economist. 21 March 2019.
  17. ^ McGrane, Sally (4 December 2017), "The German Amateurs Who Discovered 'Insect Armageddon'", The New York Times
  18. ^ a b Bélanger, J.; Pilling, D., eds. (2019), The State of the World's Biodiversity for Food and Agriculture (PDF), Rome: FAO Commission on Genetic Resources for Food and Agriculture, p. 133
  19. ^ Lister, Bradford C.; Garcia, Andres (October 2018), "Climate-driven declines in arthropod abundance restructure a rainforest food web", Proceedings of the National Academy of Sciences, 115 (44): E10397–E10406, doi:10.1073/pnas.1722477115, PMC 6217376, PMID 30322922
  20. ^ Guarino, Ben (15 October 2018), "'Hyperalarming' study shows massive insect loss", The Washington Post
  21. ^ a b c d Wagner, David L. (4 March 2019). "Global insect decline: Comments on Sánchez-Bayo and Wyckhuys (2019)". Biological Conservation. doi:10.1016/j.biocon.2019.03.005.
  22. ^ a b c d Yong, Ed (19 February 2019), "Is the Insect Apocalypse Really Upon Us?", The Atlantic
  23. ^ Saunders, Manu (16 February 2019). "Insectageddon is a great story. But what are the facts?". Retrieved 16 March 2019.
  24. ^ a b "Nothing in the cry of cicadas", The Economist, 430 (9135): 71, 23 March 2019
  25. ^ Alexandra Sifferlin (14 February 2018), "Fewer Scientists Are Studying Insects. Here's Why That's So Dangerous", TIME
  26. ^ Leather, Simon (January 2007), "British Entomology in terminal decline?", Antenna, 31 (4): 192
  27. ^ Gangwani, Kiran; Landin, Jennifer (12 December 2018), "The Decline of Insect Representation in Biology Textbooks Over Time", American Entomologist, 64 (4): 252–257, doi:10.1093/ae/tmy064
  28. ^ Blakemore, Erin (12 December 2018), "Insects are disappearing from science textbooks—and that should bug you", Popular Science
  29. ^ Hart, Adam, "Inside the killing jar", The Biologist, 65 (2): 26–29
  30. ^ Fischer, Bob; Larson, Brendan (25 February 2019), "Collecting insects to conserve them: a call for ethical caution", Insect Conservation and Diversity, doi:10.1111/icad.12344
  31. ^ a b Global Insect Biodiversity:Frequently Asked Questions (PDF), Entomological Society of America, March 2019
  32. ^ Action program insect protection (in German), Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit, 10 October 2018
  33. ^ Dangles, Olivier; Casas, Jérôme (February 2019), "Ecosystem services provided by insects for achieving sustainable development goals", Ecosystem Services: Science, Policy and Practice, 35: 109–115, doi:10.1016/j.ecoser.2018.12.002
  34. ^ Thomas, Chris D.; Jones, T. Hefin; Hartley, Sue E. (18 March 2019). ""Insectageddon": A call for more robust data and rigorous analyses". Global Change Biology. doi:10.1111/gcb.14608.
  35. ^ Kirby, Alex (1 September 2004), Scarce insects duck UK splat test, BBC
  36. ^ Katherine Roth (15 January 2019), Apps let everyone help track health of insect populations, Associated Press

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Further reading

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