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Oxford zoologists: Insect decline studies could erode confidence in science if not held to rigorous standards

Global rates of insect decline will be very hard to measure. Conclusions regarding drivers and rates of declines can be unreliable due to biases – including sampling location and omission of relevant publications through narrow search terms. Extrapolation of global species declines from a few regions or a sample disproportionately including population loss at range-margins is indefensible, and the projected global rate of loss can be seriously overestimated.

Meeting policy targets on preventing biodiversity loss requires measures of rates of loss, and insects will likely be a major fraction of terrestrial extinctions. Recent reports of insect decline have received considerable and often uncritical media reporting which, if overstating the rates, will erode public confidence in science and be detrimental to conservation. Here we demonstrate that it is not possible to measure directly the overall global rates of population decline, biomass decline or extinction in insects, since data are too sparse and measuring insect abundance is challenging, even in the short term.

[Editor’s note: Clive Hambler, co-author of this article, is Lecturer in biological and human sciences, Hertford College, University of Oxford]

To obtain a general global extinction rate would require a representative sample of population changes, such as a random sample from the planet. If the sample used is spatially unrepresentative (as in Sánchez-Bayo and Wyckhus, 2019) then extrapolation cannot be defended.

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The use of regional or national ‘red lists’ in a review (such as Sánchez-Bayo and Wyckhus, 2019) will inflate the calculated extinction rate. The percentage of ‘endangered’ species will be overstated because regional red lists include species that are globally not threatened – for example butterfly species in Britain. Regional ‘red listing’ using IUCN trend criteria is unacceptably influenced by short term-trends and start dates.


We suggest some minimal methodological requirements when reviewing declines and extinctions. Given these strictures, we reaffirm that declines in most non-marine invertebrate groups will have to be estimated using previously calibrated indicators such as birds and freshwater fish.

In the light of the biases and other problems discussed above, we suggest these minimal standards for reviews of population trends and extinctions:

  • Representativeness of the locations of regional samples if claims are to be made about global loss.
  • Representativeness of taxa if claims are to be made about much higher taxonomic levels and about biomass.
  • Representativeness of search terms for population time series, including those that can detect gains, losses, declines, increases, regional and global extinctions, colonization and no trends. Terms should ideally be in a range of languages. Publication and reporting bias against studies showing no trend should be acknowledged….

Read full, original article: Challenges in measuring global insect decline

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