A million threatened species? Thirteen questions and answers

Prof. Andy Purvis

Coordinating Lead Author, IPBES Global Assessment and Life Sciences Research Leader at The Natural History Museum, London


The IPBES Global Assessment estimated that 1 million animal and plant species are threatened with extinction. It also documents how human actions have changed many aspects of nature and its contributions to people; but species threatened with extinction resonate with the media and the public in ways that degradation of habitats and alteration of rates of ecosystem processes perhaps don’t, so the figure was widely reported.

IPBES is the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, an independent intergovernmental body that was established in 2012 to strenthen links between science and policy to support conservation and sustainable use of biodiversity, long-term human well-being, and sustainable development

Because only the Summary for Policymakers has so far been made available, it wasn’t clear where the figure of 1 million threatened species came from. Some journalists and researchers asked me, so I explained it to them, and will explain it again here. Some other writers, often with a long history of commenting critically on reports highlighting environmental concerns, instead railed against the Global Assessment in general and the figure of 1 million threatened species in particular. Given that these writers often advance empty or bogus arguments, I thought it would be also be useful to explain why these arguments are wrong.

I have therefore written this blog post in the form of thirteen questions and answers.

Before that, however, there are two points I should make. First, I will declare my interest: I am one of the hundreds of authors of the Global Assessment, and I worked on the estimate of how many species are threatened. But, unlike most of the critics I’ve read or heard, I’ve received no payment for my work or for my opinions on it. Anyone who thinks that career scientists are part of some global conspiracy, or that we trot out the views of wealthy and influential paymasters, clearly doesn’t know many actual scientists or how science works. Maybe they are instead judging scientists by their own standards.

The second point to make clear is that I am of course not arguing that all criticisms of the Global Assessment are invalid or that all critics have ulterior motives. All research has its limitations and flaws; this Assessment is no exception. But it was produced painstakingly over three years by a team of experts from over 60 countries, and was peer-reviewed astonishingly thoroughly. (There were over 15,000 reviewer comments altogether!) So op-ed writers who, after an hour of superficial Googling, believe that they have spotted some fatal flaw that somehow everyone has missed – or that scientists have conspired to not mention – are more likely to be deluded than right.

Thirteen Questions and Answers

  1. How was the figure of 1 million threatened species calculated?

Across a wide and growing range of taxonomic groups, an average about 25% of species are threatened with extinction when assessed using the well-established and transparent IUCN Red List criteria. Some groups have a higher proportion, some a lower proportion, but the average has now settled down. The percentage of insects that are threatened may well be lower (which matters, because about 75% of species are insects), but evidence from the best-studied insects (dragonflies globally, and bees, butterflies and some beetles in Europe) suggests it is unlikely to be much below 10%. There is not yet any agreement about exactly how many species there are: around 1.7 million animal and plant species have been described, but most estimates of the true total are well over double this number. In the Assessment, we have used a recent mid-low estimate of 8.1 million animal and plant species, of which an estimated 5.5 million are insects (i.e., 75%) and 2.6 million are not. So 10% of the 5.5 million insects is 550,000, and 25% of 2.6 million is 625,000. Because of the imprecision in the estimates, there is no point in giving the total any more precisely than 1 million threatened animal and plant species.

Watch the video for Answer 1: https://youtu.be/N3SbKQOx8Kw

  1. How can IPBES estimate a million species at risk when IUCN estimates that fewer than 28,000 species are threatened with extinction? 

It is simply not true that the IUCN estimates fewer than 28,000 species to be threatened. They have documented over 27,000 threatened species, but they fully realise and openly acknowledge that this is nothing like a comprehensive list of all threatened animals and plants. Because IUCN have done such a great job of expanding their coverage to more plant and invertebrate groups (they had started with birds and mammals, because the data were easier to get together), it’s now possible use their detailed evidence to estimate the overall number of animal and plant species that are threatened – though doing so also requires estimates of just how many animal and plant species there are (we used a recent estimate of 8.1 million) and what fraction are insects (we assumed three quarters). Some people have suggested that the IUCN assesses the species most likely to be threatened first, so that its numbers for incompletely-assessed groups can’t be used. But this is wrong: IUCN want their assessments to give an accurate “barometer of life”, so they assess either all of a group’s species or sets of species that are chosen so as to give a representative snapshot of the whole.

Watch the video for Answer 2: https://youtu.be/VCbrbXd9s98

  1. But these species are just electrons on a hard drive – they exist only on computers. Can you even list any actual species that have recently gone extinct or might do soon? 

“Electrons on a hard drive” is a dismissive phrase favoured by some commentators wishing to belittle computer models in the minds of their readers; they are using a rhetorical trick to suggest that the models are a scientific trick – anything, really, to avoid readers thinking about the models and what they suggest. And although the Global Assessment makes use of many complex computer-based models (that have been described in detail and peer-reviewed), the estimate of 1 million threatened species is actually really thoroughly grounded in IUCN’s painstaking, thorough, peer-reviewed and transparent assessments of the conservation status of nearly 100,000 animal and plant species, supplemented with other information as outlined in the answer to Question 1 above. So, yes, there are long lists of actual species that have gone extinct since 1500 and of those that are now at risk, and you can see all the species accounts.

Watch the video for Answer 3: https://youtu.be/pfJhKvIivBA

  1. The IPBES estimate uses IUCN Red List data. But isn’t it true that IUCN warned specifically against using their data to make these kinds of estimates? 

IUCN have welcomed the Global Assessment and endorsed our use of the data for this purpose. They wisely caution against using the Red List data without an understanding of its strengths and weaknesses, but we were suitably careful and considered in our use of the data.

Watch the video for Answer 4: https://youtu.be/MO4you1put4

  1. If only 2 million species are known, how can we say that a million are threatened? 

There is no comprehensive list of the world’s species. Embarrassingly, there isn’t even a comprehensive list of the animal and plant species that have been formally described and named by taxonomists, so we only have estimates even for that number; it’s around 1.7 million. Around 10,000-15,000 new species descriptions are published each year, suggesting that we are nowhere near running out of species to describe. We know many more species still remain to be discovered, but don’t know exactly how many. Several different approaches have been used to estimate the actual total. None of these methods is perfect. They give a very wide range of answers – from 3 million to over 100 million. Most of the more recent estimates based on thoughtful approaches are in range of 5-20 million. For this assessment, we used an estimate from a paper published in 2011, of 8.1 million animal and plant species. We chose this one because it was towards the lower end of the reasonable range (so our results would tend to be conservative), was based on a clear methodology (using how the numbers of described higher taxa – phyla, classes, orders, families and genera – have increased through the history of taxonomy, along with a consistency in the ratio of subtaxa per taxon up the hierarchy), and wasn’t just restricted to species on the land or those in the sea. Because the proportion of species that are threatened differs between insects and the rest, we also needed to know how many species are insects. We used an estimate that 75% of animal species are insects, from a global compendium in 2009.

Watch the video for Answer 5: https://youtu.be/6SAjOmpR2kQ

  1. Why make estimates of extinctions based on millions of unknown species? 

Because it would be silly not to! Patrick Moore, a consultant for hire who is part of the CO2 Coalition, has asserted that any estimate of the total number of threatened species cannot be viewed as scientific if it considers species that have not yet been described. However, he is exactly wrong: there is no doubt at all that very many – probably several million – animal and plant species have not yet been formally described and named; and there is also no doubt at all that at least some of these undiscovered species are threatened with extinction. Ignoring them would therefore be not only silly but decidedly unscientific. The question is, how best to consider them? Our approach has been conservative. We have assumed they are neither more nor less likely than named species to be threatened. In practice, they are likely to be more threatened on average (see Question 5 above). As-yet-undescribed species will tend to have narrower geographic distributions than their relatives that have already been named; and species with narrower distributions tend to be more threatened than their more widespread relatives. Add to that the fact that the as-yet-undescribed species are likely to be disproportionately found in ‘biodiversity hotspots’ where much habitat has already been lost, and it is hard to argue seriously that they are less likely to be threatened than the species we have already named.

Watch the video for Answer 6: https://youtu.be/Rp7j2SYG7rg

  1. But isn’t the rate of extinction slowing down, rather than speeding up? 

No, it isn’t. Time lags in reporting mean that recent extinctions are also under-recorded. There is almost always a time lag – often decades long – between when extinction takes place and when it is confirmed.  A paper last year highlighted this in birds, for which the data are best (see figure below). Confirmed extinctions are in black, while hollow bars show the species listed as “Critically Endangered (Potentially Extinct)”, most of which will turn out to have already gone extinct. While it is true that the very highest rate of confirmed extinctions was in the late 19th century, as flightless island birds fell prey to cats and rats introduced by Europeans, the rate was as high again by the end of the 20th century. The rightmost time-bin covers 2000-2017, so is shorter, and there are reporting lags even for birds, so it is unsurprising that fewer extinctions are recorded for it.

1-s2.0-S0006320718308012-gr4_lrg Butchart for Andy PurvisNumbers of bird species confirmed (Extinct/Extinct in the Wild (solid bars) or suspected (Critically Endangered (Possibly Extinct), open bars) to have gone extinct per quarter-century. Values for the latest time period are for 2000–2017. From Butchart et al. (2018). With permission: see acknowledgements

Note that the recent numbers would be even higher but for successful conservation actions that have prevented some extinctions in recent decades. Again, the data are best for birds: 16 extinctions were prevented by ongoing conservation actions from 1994-2004 (e.g., the US California Condor), a time period for which only 10 extinctions or probable extinctions are recorded from birds.

Watch the video for Answer 7: https://youtu.be/syjUBdax8Hc

  1. Fewer than 900 species have actually gone extinct since 1500 – how can IPBES possibly say that a million are threatened? 

Contrary to some recent suggestions, a lot of evidence is needed before a species gets recorded as having gone extinct, because it’s essentially trying to prove a negative. Surveys have to look hard for it throughout its entire geographic range, and fail to find it.

This level of effort is only expended for a very few species, usually those in charismatic groups like birds and mammals, and/or for species with very restricted geographic distributions (like endemics on small islands). By contrast, it is virtually certain that most species of animal and plant are not yet even known to science (they may have been seen, but the people who saw them didn’t realise they were looking at an undescribed species).

If we focus on the mammals and birds, where recording of extinctions is better, then about 1.5% of species have gone extinct since 1500. For amphibians it’s probably rather higher, maybe 3% or so. These fractions, though small, are big enough to represent a huge acceleration in the rate of species extinction already: tens to hundreds of times the ‘background’ (normal) rate of extinction, or even higher.

It’s important to recognise the difference between threatened and extinct. The million threatened species don’t have to die out, but they are likely to if we don’t reduce the pressures that are driving their decline – land/sea use change, direct exploitation, climate change, pollution and invasive alien species.

Watch the video for Answer 8: https://youtu.be/hDJPFZ09jf4

  1. Didn’t most of the human destruction of nature happen centuries ago? 

It’s true that much of Western Europe and the Mediterranean had been heavily modified many hundreds – even thousands – of years ago. And it’s true that the wave of discovery and colonisation brought with it a wave of extinctions – but that was a small ripple compared to the tsunami that will come unless the drivers of biodiversity loss are mitigated.

Earlier human-caused extinctions were nearly all of intrinsically susceptible species – e.g., flightless birds on islands, large mammals with very slow reproductive rates. But now a much wider range of species are threatened with extinction, on mainlands as well as on islands.

The Red List Index tracks species’ average survival probability, and is estimated from successive Red List assessments of the species in a taxonomic group. One third of the total fall to date in average survival probability (or, equivalently, one third of the total increase in species extinction risk) has taken place since 1994. So the human destruction of nature has been accelerating, not slowing down.

Watch the video for Answer 9: https://youtu.be/_F5z1buryqQ

  1. Previous reports said we would have lost hundreds of thousands of species by now. They were wrong. Why should we believe this report? 

Those previous reports, with the arguable exception of the Millennium Ecosystem Assessment (MEA) in 2005, were simply not on the same scale as the IPBES Global Assessment. They didn’t have comparable levels of expertise available and the reports weren’t reviewed and scrutinised in anything like the same way that this has been. Even the MEA suffers by comparison because it was 14 years ago and the science has moved on a long way.

Additionally, we’re actually proving some of those earlier reports right – not saying they were wrong. Some of the reports were talking about numbers of species that would become committed to extinction (i.e., they don’t have enough habitat to survive in the long run, but have not died out yet; this is a related idea to species being threatened with extinction, but the analysis and data are totally independent), not the number that would have gone extinct by now; though it is true that press headlines and commentators often didn’t notice the distinction. Our assessment estimates that half a million terrestrial species of animal and plant have indeed already become committed to extinction. Their extinction can in principle be prevented by rapid habitat restoration, but they will go extinct unless their habitats are improved.

Watch the video for Answer 10: https://youtu.be/2BT4nQxShCM

  1. But most of these ‘threatened’ species aren’t really at a high risk of extinction. Isn’t IPBES being alarmist about their extinction rate? 

Species are considered ‘threatened’ if their Red List status is Vulnerable, Endangered or Critically Endangered.

Vulnerable species have the lowest extinction risk of any of these three levels, but they still have at least a 10% chance of going extinct in 100 years, the way things are now. So at least 10 out of every hundred would go extinct within a century; and we’d expect to lose half of them in about 600 years. To put that into context, without human actions, it would take at least a million years for half of the species to go extinct – so even Vulnerable species have an extinction rate that’s at least about 1700 times the natural ‘background’ rate.

To put these higher rates into a more everyday currency, consider a 50-year old person and how long they might expect to live. In the developed world, our 50-year old can expect to live for around 30 more years. Let’s take that as being the background. A “Vulnerable” 50-year old (with a mortality rate multiplied by 1700) would expect to live for about a week, rather than 30 years. An “Endangered” 50-year old could expect to live for 18 hours, and a “Critically Endangered” 50-year old would probably be dead inside 3 hours. The extinction risk of threatened species is seriously high!

In practice, conservation actions have prevented some of the extinctions that would have otherwise taken place, so we haven’t seen quite as many extinctions of Critically Endangered species – but ongoing conservation ‘life support’ is all that’s keeping them going, unless we tackle the root causes.

Watch the video for Answer 11: https://youtu.be/k8pAgwZq_gQ

  1. Isn’t it true that extinction doesn’t matter – there’s more biodiversity than ever before? 

No on both counts!

First, extinction does matter because the loss of species can destabilise ecosystems and so disrupt the flow of benefits from nature to people. We have geared our use of ecosystems in ways that assume they will continue to ‘work’ – so extinction (local and global) does matter.

Second, the apparent rise in biodiversity over the last 200 million years (trotted out again in some recent blogs) suffers from a well-known artefact called “The Pull of the Recent”. It has three main components:

–   Fossils are found in exposed rock strata, and there is generally more exposed rock from the recent past than the distant past (because older rock gets covered or subducted back inside the earth).

–   The diversity curve usually shown, like the one above, is a count of ‘range-through’ taxa, i.e., a taxon is considered to have been alive for all the time between its earliest fossil and its last appearance. All today’s taxa are still present, so appear in the graph even if only a single fossil has been found. (Earlier taxa have to have been found at least twice.)

–   We understand extant species better than extinct forms, so can interpret their morphology better. That means we are better at spotting differences among fossils with close living relatives than among fossils that don’t have them. So our classification of recent fossils is likely to be more fine-grained (inflating the numbers of higher taxa such as genera).

Analyses that control for any of these biases carefully – like this one – don’t show such a strong increase towards the present day. The only group with a detailed enough fossil record to overcome all three shows a diversity curve whose peak was over 40 million years ago (though that group might not be typical).

Watch the video for Answer 12: https://youtu.be/h7052P9GdCE

  1. Why do some people say there isn’t a biodiversity crisis? 

I can think of three reasons.

First, the reality is scary, so people might find comfort in choosing not to believe in it. I think that’s a mistake, because there’s lots we can do to help to make things better, but only if we recognise the problem.

Second, I guess some people might be open-minded about it but they’re not yet persuaded. I hope the Global Assessment, with all the evidence it has marshalled as even-handedly as possible, will persuade these people; and I’m always willing to answer their questions and debate the science with them as best I can.

But there is also a third group: people who say there’s no biodiversity crisis because it is in their own self-interest to say it whatever they actually believe. The Global Assessment highlights that nature and its contributions to people can be safeguarded only by transformative change to a sustainable global economy. Not everyone wants that to happen! As the assessment says, such change “can expect opposition from those with interests vested in the status quo”, but the good news is that “such opposition can be overcome for the broader public good.” Many people and businesses have become wealthy through using natural resources unsustainably while not paying the costs of the damage; inevitably, some of these will attempt to discredit the Global Assessment.

So please beware of professional contrarians who are trying to persuade you that the Global Assessment can’t be trusted, that it considered only half of the evidence whereas they have considered the other half. In truth, we tried to consider all of the evidence (over 15,000 papers) whereas they have no interest in doing so.

Some of these contrarians have actually likened themselves to Galileo, persecuted by dogmatic authority for his heretical views. But the resemblance is not strong – Galileo had new evidence, rather than old pointless points; he was being persecuted by a rich organisation for his views, rather than being funded by one; and the Church was denying the issue, rather than agreeing it is important.

Please also ask yourself whether there’s any sign that critical commentators really know what they are talking about. They have been taught (often at great expense) to write confidently and persuasively, whether or not they understand a topic. Some of them are now so confident in their own abilities that mere ignorance of a topic will never hold them back. It’s only natural for readers to mistake such confidence for actual competence – but please try to avoid this trap. My main goal in writing this post has been to explain the science and evidence behind our work, and to make it easier for you to make your own decisions about the strength of the Global Assessment in comparison to the smoke and mirrors being peddled by those who have every reason to prefer your continued confusion and inaction.

Watch the video for Answer 13: https://youtu.be/DbeQPwFYTo8



The original post of this blog is on the IPBES website https://www.ipbes.net/news/million-threatened-species-thirteen-questions-answers

Figure in Question 7 – Reprinted from Biological Conservation, Volume 227, November 2018, Pages 9-18, Butchart, H.M., Lowe, S., Martin, R.W., Symes, A. Westrip, J.R.S. & Wheatley, H., Which bird species have gone extinct? A novel quantitative classification approach. https://doi.org/10.1016/j.biocon.2018.08.014.   Copyright (2018), with permission from Elsevier.


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