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Over recent years, studies from across the globe have indicated a decline in insect abundance, diversity, distribution and biomass. However, the trends for global insect declines are uncertain and may be over- or underestimated. For example, a well-reported 2019 global review has been recently criticised as its results were skewed by excluding studies that reported stable or increasing insect populations. Data on insects are limited because of the large number of species, with species continuing to be discovered and described. For example, there are an estimated average of 5.5 million insect species globally, with about 1 million of these already discovered and described.

Fully understanding the data on the drivers of insect decline is complex as there is limited evidence on how drivers influence each other, and which drivers are having the greatest impact. Also, some of the largest declines may have occurred prior to research being carried out, further complicating the data. Particular drivers, such as climate change, may also benefit some insects but be detrimental to others. Much of the data for the effects of drivers is based on research in controlled laboratory environments and focuses on individual organisms, or is undertaken over short time periods (1–2 years) that are not relevant for long -term population-level processes.10 Because of this, it is challenging to apply the findings of this research to assess the impacts of drivers on complex insect communities and ecosystems under natural environmental conditions that can’t be controlled for. This complexity has so far limited the evidence on the impact of declining populations on ecological processes.

Having standardised, systematic and long-term data sources is key to understanding insect decline and toward identifying the drivers behind the trends. Relevant and reliable data are needed to measure the effectiveness of interventions, such as those supported by agri-environment schemes.

This POSTbrief complements POSTnote 619 UK Insect Declines and Extinctions.

Acknowledgements 

POSTnotes are based on literature reviews and interviews with a range of stakeholders and are externally peer reviewed. POST would like to thank interviewees and peer reviewers for kindly giving up their time during the preparation of this briefing, including:

Dr Christopher Hassall, University of Leeds* 

Professor Bill Kunin, University of Leeds* 

Dr Richard Gill, Imperial Collage London* 

Dr Ben Woodcock, Centre for Ecology & Hydrology (CEH)* 

Dr James Bell, Rothamsted Insect Survey* 

Dr Chris Shortall, Rothamsted Insect Survey*  

Dan Blumgart, Rothamsted Insect Survey* 

Jon Curson, Natural England* 

Andy Brown, Natural England* 

James Philips, Natural England*  

Jon Webb, Natural England*  

*Denotes people who also acted as external reviewers of the briefing


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