Has there been a huge decline in the number of insects?

Based on:

Several media stories in recent years have reported on catastrophic insect declines, often based on single reports or papers:

https://www.theguardian.com/environment/2020/feb/12/car-splatometer-tests-reveal-huge-decline-number-insects

https://www.theguardian.com/environment/2019/feb/10/plummeting-insect-numbers-threaten-collapse-of-nature

https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.5236

https://www.kentwildlifetrust.org.uk/sites/default/files/2020-02/Bugs%20Matter%20report%20website%20version_0.pdf

The fuller picture:

Background:

Recently, there have been several media reports of dramatic insect declines – these are often based on a particular study, or triggered by a specific report¹. Mainstream media began reporting on this subject in the mid-2010s with articles warning of an “insect Armageddon” (although some articles do present a more rounded reflection of the complexity in this field). However, scientists have been monitoring insects and aware of the problems facing them for many decades and there are even records going back to the late 1800s suggesting this is a long term phenomena ². The scientific literature is expanding and, with this renewed interest in the fate of insects, many important papers have been published on this topic in recent years. These have offered possible solutions by investigating what we see happening to insects and why this may be^3–6. A group of 25 authors published a paper earlier this year entitled “Scientists’ warning to humanity on insect extinctions” which detailed many drivers of insect population declines including habitat loss, climate change, overexploitation and pollution among other things⁷. The importance of insect populations and the threats, declines, or extinctions that they face is further shown by the recent publication of a POSTNOTE from the Parliamentary Office for Science and Technology – the UK parliament’s hub for scientific advice⁸.

How do we get reliable information?

A rounded understanding of what is happening to our insect populations is based on more than just a handful of papers that happened to catch popular attention at a particular time. There are several important features of a study that mean it can give us genuinely reliable, useful information about insect populations. These include:

• Long-term studies. Insect populations are susceptible to many different influences that vary year-to-year, for example, the weather, how the area they live in is being managed at that time, diseases that may affect them and so on. The start year and end year can have a large impact on trends – if abundance in these years is uncharacteristically high or low. So, to really understand an insect population it needs to be monitored thoroughly every year, for a long time, to reveal genuine trends. These should become clear despite year-to-year changes. The graph below shows the possible result of comparing data from just two time points and how this may not reflect the true changes in a population.

This example shows real data from beetles on the Sussex Study site. If just two time points were sampled, the straight lines show how the results could mistakenly find either a nine-fold increase (blue line), or a five-fold decrease (yellow line), with samples taken on subsequent years. The black dashed line shows the genuine trend monitored each year over a fifty-year period, which is fairly stable.

• Counting. Studies which look at the number of insects, rather than just which species are present, are relatively few. For example, some papers look only at what is called the occupancy – is the species found in a particular area or not? This gives useful information about the range of a group of insects, but not about how numerous they are, which is an important distinction. One example is a paper reporting a widespread loss of pollinating insects in Britain – of bees and hoverflies, especially rare species, and with losses especially focussed on the uplands. This paper provides important information about where but not how many insects were found⁹. Although the two are thought to be closely linked, with changes in occupancy often following changes in number, it is important to recognise which feature is being reported.
• Variety. Looking in several areas, over time, at many species gives much more valuable information than those that are limited in scope, for example only looking at one species or at one site.

Long-term monitoring studies

Several such long-term monitoring studies are ongoing in the UK in a variety of sites around the country, so we have some of the best data in the world on insect populations for the past 50 years or so. The Rothamsted Research in Hertfordshire, the GWCT Sussex Study on the South Downs between Arundel and Worthing, the Environmental Change Network and the UK butterfly monitoring scheme are among these.

The Sussex Study has been monitoring invertebrates in cereal crops across an area of farmland in the South Downs since 1970, recording farming and weather information. One paper from this study showed that two fifths of the insect groups studied had increased in abundance since the beginning of monitoring, and three fifths had decreased, including aphids10, and that insect populations are sensitive to extreme weather events, but usually stabilise within one or two years11. Climate change is likely to affect insect populations, but the most notable impact was from increased pesticide use. This is actually a fairly positive result because it implies that reduced pesticide use could be effective to combat insect declines – even in the face of climate change¹¹.

Various collaborative studies on moths between multiple groups including the Centre for Ecology and Hydrology, the charity Butterfly Conservation, York University, Rothamsted Research and others have helped us understand more about moth and butterfly trends^12,13. These reveal that approximately two thirds of species groups have declined in number, while around a third have increased. A paper published in 2014 found that both climate change and changes in land use appear to be driving these trends¹². A study from Rothamsted Research looking at aphids and moths since 1969¹⁴ found that populations had risen and fallen across the decades, with an overall loss of almost a third of moths while aphid numbers were stable, in contrast to the picture in the Sussex Study (see above). However, there were different results in different areas, with forests, urban and coastal habitats seeing more declines than parks, farmland and more open shrubby landscapes¹⁴.

Significant changes in ground beetle (carabid) populations have also been found¹⁵. Three quarters of the beetle species studied were found to be declining in number over a 15-year period, with half of these falling by around a third or more. However, the findings were very variable between sites and areas, with the majority of populations falling, but some on southern downland sites increasing by half. This highlights the differences that are found between species and sites when monitoring insect populations¹⁶.

So, are insects declining?

Overall, many insect populations are declining. Different species, or groups of species (known as taxa to scientists), are affected to different extents, with some evidence that more specialised types may be more at risk than those that are more adaptable (known as generalists)^12,15. However, although many insect groups are declining, this effect is not universal. Some are stable or increasing^14,17, especially those which tend to benefit from our effect on the environment. This is an extremely complicated area, with data that is extremely complex, often incomplete, and sometimes seemingly contradictory. However, some general points are:

• Butterflies and moths are sometimes considered an indicator species for broader insect populations, and many of these species have been found to be in decline in the UK, although again some are stable or increasing, and one recent paper found that moth numbers rose and then fell over the course of their study^12,13,17,18.
• Many pollinators such as bees and hoverflies declined sharply in the early part of the 20th century, but those losses have slowed more recently².
• Losses are ongoing for some species groups, and many have become extinct this century although trends for some others are rising or unclear¹⁹.
• Some species are colonising or moving north in response to climate change effects, such as butterflies²⁰.
• We do not understand very well what these declines will mean for ecosystems or the environment.

It is important to remember that the term “insects” encompasses a huge number of species. Insects are the largest and most diverse group of animals on earth, with more than a million species so far described (more than half of all known living organisms), and between six and ten million thought to exist in total²¹. Enormous variety is found within the insects, with species adapted to survive in all sorts of environmental niches almost the world over, therefore the blanket phrase “insect declines” is misleading. Some insects are indeed declining, but others are stable, and some are increasing. Most are not monitored. Some of the losses that we know of are marked, some are long-term and some are worrying. Entomologists are studying trends in insect populations to understand what changes we can make to better support those species.

There are many reasons for changes to insect populations including loss of habitat, climate change and pesticides. There are also management techniques which can benefit insects including creating new wildflower areas²², uncropped field margins^23,24, allowing hedgerows to flower and increasing hedgebank plant diversity^25,26, among many others. Agri-Environment Scheme (AES) options are available for UK farmland, including support for conservation headlands, beetle banks^27,28 and pollinator mixes²⁹. If you would like to know more, a document looking in more detail at why insect populations are changing, and what we can do to help will be published soon.

This article was edited on 3rd November 2020 to correct the line that reads ‘Many pollinators such as bees and hoverflies declined sharply in the early part of the 20th century’. It had incorrectly read ’21st century’.

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References:

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18. Thomas, J.A., Telfer, M.G., Roy, D.B., Preston, C.D., Greenwood, J.J.D., Asher, J., Fox, R., Clarke, R.T. & Lawton, J.H. (2004). Comparative Losses of British Butterflies, Birds, and Plants and the Global Extinction Crisis. Science, 303:1879–1881.

19. Carvalheiro, L.G., Kunin, W.E., Keil, P., Aguirre-Gutiérrez, J., Ellis, W.N., Fox, R., Groom, Q., Hennekens, S., Van Landuyt, W., Maes, D., Van de Meutter, F., Michez, D., Rasmont, P., Ode, B., Potts, S.G., Reemer, M., Roberts, S.P.M., Schaminée, J., Wallisdevries, M.F. & Biesmeijer, J.C. (2013). Species richness declines and biotic homogenisation have slowed down for NW-European pollinators and plants. Ecology Letters, 16:870–878.

20. Pateman, R.M., Hill, J.K., Roy, D.B., Fox, R. & Thomas, C.D. (2012). Temperature-dependent alterations in host use drive rapid range expansion in a butterfly. Science, 336:1028–1030.

21. Collen, B., Böhm, M., Kemp, R. & Baillie, J.E.M. (2012). Spineless: status and trends of the world’s invertebrates. United Kingdom.

22. Haaland, C., Naisbit, R.E. & Bersier, L.F. (2011). Sown wildflower strips for insect conservation: a review. Insect Conservation and Diversity, 4:60–80.

23. Kells, A.R., Holland, J.M. & Goulson, D. (2001). The value of uncropped field margins for foraging bumblebees. Journal of Insect Conservation: 5:

24. Vickery, J., Carter, N. & Fuller, R.J. (2002). The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK. Agriculture, Ecosystems and Environment, 89:41–52.

25. M’Gonigle, L.K., Ponisio, L.C., Cutler, K. & Kremen, C. (2015). Habitat restoration promotes pollinator persistence and colonization in intensively managed agriculture. Ecological Applications, 25:1557–1565.

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28. MacLeod, A., Wratten, S.D., Sotherton, N.W. & Thomas, M.B. (2004). ‘Beetle banks’ as refuges for beneficial arthropods in farmland: long-term changes in predator communities and habitat. Agricultural and Forest Entomology, 6:147–154.

29. Smith, B.M., Aebischer, N.J., Ewald, J., Moreby, S., Potter, C. & Holland, J.M. (2020). The Potential of Arable Weeds to Reverse Invertebrate Declines and Associated Ecosystem Services in Cereal Crops. Frontiers in Sustainable Food Systems, 3:118.

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