This magic number was stated in that flawed entomofauna paper, without any explanation of how this number was calculated – see why that paper is flawed here.
Since then, it has been stated regularly in popular media, scientific papers and technical reports, often without citation, just a number pulled out of the air and presented as fact.
Globally, there are about 5 million estimated insect species in total. Only 1 million species have scientific names. So, conservatively, the 40% claim suggests that at least 400,000 species are threatened with extinction.
So is it an accurate prediction? No. Here’s why:
Based on current knowledge, it’s actually impossible to predict extinction rates for the 1 million species of insect known to science. This is because the vast majority of them have been left unstudied, some are still in museum drawers waiting to be named. It’s extremely difficult to predict an extinction rate of a species without data on its population distribution, dynamics and ecological interactions.
The mind-blowing diversity within the insect world means you also can’t extrapolate from one species or group to other groups, let alone to the entire class of insects. Just as you can’t extrapolate what’s happening in one location, to other locations. The population dynamics and ecology of one Australian megachilid bee may be a good start to guess how other Australian megachilid bees will respond to environmental change, but it is not a good baseline to predict what colletid bee populations are doing, or what dragonfly, or scarab beetle populations are doing.
The IUCN Red List is currently the main summary of available knowledge on global insect population trends (I have a co-authored book chapter coming out on this soon). At last count, only 8131 species have had their global populations formally assessed through this process…that’s 0.8% of all known insects (or 0.16% of the estimated global insect fauna).
Of these 8131 species, 19% are in a threatened category. Overall, only ~10% of the 8131 species have been assessed as having declining populations (that is less than 0.1% of all insects).
Most of the Red List assessed species (~75%) are data deficient, meaning there aren’t enough data available to even determine the state of their population, and the remaining ~15% have stable populations.
Sure, IUCN data are limited and the real percentages are probably different – these are rough estimates, not rigorous scientific analysis, so please don’t quote this as evidence. For most species, we know nothing. But if you want a quantitative estimate based on best available data, that’s all we have to work on.
So where did the 40% come from?
© Manu Saunders 2020
Ecology is not a dirty word 
You’re being too generous. It’s impossible, plain and simple.
Great post, by the way; concise and to the point.
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I’m on board with the assertion that we cannot extrapolate from one species to another, but there’s actually a 40% hidden in IUCN Red list numbers you give.
If 75% of the IUCN species lack data to estimate population trends, then 25% of them can actually be estimated. And 10% of the IUCN species are declining, which is 0.1/0.25, or 40%.
This is definitely NOT supporting the media narrative that 40% of insect species are going extinct. But it does seem fair to say that for the insect species we can estimate population dynamics for, 40% are in decline.
Of course there’s a big difference between “declining” and “facing extinction”. And as you say, we don’t know what’s happening with all the unmeasured species (although it seems unwise to assume that the only species in decline are those we have measured).
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One problem for Ecologists is that their subject speciality has been plundered by other non-scientist media figures who make little distinction between theory and evidence or data. Whilst true Scientists are careful to proffer an opinion accompanied by caveats and reservations, Media figures play down or dumb down the cautious perspective views to make unequivocal pronouncements
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