Species in Decline?

Summary

More or Less – "Species in Decline?"

Podcast: More or Less (BBC Radio 4)
Host: Wesley Stevenson
Date: October 11, 2014
Theme: Critically examining statistics behind the claim that global wildlife populations have halved in 40 years.

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Overview

This episode of More or Less scrutinizes the often-cited statistic from the WWF and the Zoological Society of London (ZSL) that claims the world's wildlife populations have, on average, declined by 52% over 40 years. Wesley Stevenson investigates what the figure really means, how it was calculated, which animals are included, and what limitations and caveats are attached. Experts from the ZSL and biostatistics offer insight into the method and messaging behind the numbers.

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Key Discussion Points & Insights

1. The Headline Statistic and Its Origins

• Stat Cited: "Wildlife fallen by 50% in 40 years."
• Based on data from the Living Planet Index by the WWF and ZSL.
• Prompted skepticism and questions from listeners (00:19–01:16).

2. What Does a 52% Decline Mean?

• Louise McCrae (ZSL):
  • "The exact figure is 52%. And this is a decline in the size of vertebrate populations over 40 years." (01:32)
  • Over 10,000 population trends from over 3,000 species were analyzed.
  • Populations have "roughly halved on average"—but this doesn't mean half of all animals are gone.
• Clarified by Wesley Stevenson with an illustrative example (02:34):
  • Two populations (frogs and birds) can each decline by 50% on average, but the total number of animals may not halve—a nuance often lost in headlines.

3. What's Included (and What Isn't)?

• The index only covers vertebrates—not invertebrates (such as insects) or plants.
• Data is modelled, not based on direct global animal counts, and is therefore subject to sampling issues and uncertainty (03:31).

4. Data Limitations and Weighting Method

• Louise McCrae:
  • Data isn’t always representative—species like birds and mammals (easier to count in well-studied regions like Europe) are overrepresented.
  • ZSL tried to account for this by weighting data more heavily from tropical regions, where data is more sparse but biodiversity higher (03:31–04:21).
  • When insufficient data existed to individually weight groups, related species were bundled (e.g., amphibians and reptiles) (04:21).

5. Criticisms and Concerns About the Method

• Professor Steve Buckland (St Andrews University):
  • Surveys in temperate areas are high-quality, but biodiversity trends are actually more encouraging there than in tropical areas (05:25).
  • The tropical data is often of lower quality, with less reliable time series, even though tropical regions are home to most biodiversity.
  • The index relies on monitored populations, which are not necessarily representative; threatened populations or those in decline are more likely to be surveyed (06:01–07:07).
  • Quote: "I think it probably does [overplay the decline], simply on the basis that the populations being monitored are not representative." (07:07)

6. What Does the 52% Figure Really Tell Us?

• Professor Buckland:
  • "It tells us that the populations being monitored are in real trouble and that is a very large number of populations. What it doesn't tell us is that 52% of all vertebrate animals on the planet have declined by 52% in the last 40 years." (07:50)
• Headlines oversimplify a complex figure and don’t always communicate the scientific nuances (08:09).

7. The Problem With Messaging

• Balancing urgency with accuracy:
  • If you caveat the statistic too much, the warning loses impact.
  • But exaggeration or a lack of nuance can backfire, fostering public skepticism about future claims (08:09).

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Memorable Quotes & Moments

• Louise McCrae (ZSL, 01:32):
  "The exact figure is 52%. And this is a decline in the size of vertebrate populations over 40 years."
• Wesley Stevenson (02:34):
  "The average decline of these two populations is, is 50%, but the total number of animals hasn't halved."
• Professor Steve Buckland (07:07):
  "I think it probably does [overstate the decline], simply on the basis that the populations being monitored are not representative."
• Professor Steve Buckland (07:50):
  "[The figure] tells us that the populations being monitored are in real trouble and that is a very large number of populations. What it doesn't tell us is that 52% of all vertebrate animals on the planet have declined by 52% in the last 40 years."
• Wesley Stevenson (08:09):
  "Trying to exaggerate your message, even if you believe in the cause, can backfire and can make people more skeptical and about future claims."

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Important Timestamps

• 00:19 – Introduction to the claim and listener skepticism
• 01:32 – Method of calculating the 52% figure (Louise McCrae)
• 02:34 – Example illustrating how the average decline is calculated
• 03:31 – Data limitations and sample representativeness (Louise McCrae)
• 04:21 – How data was grouped when insufficient for individual weighting
• 05:25 – Concerns from Professor Buckland about sampling in the tropics
• 06:01 – Non-representativeness of monitored populations
• 07:07 – Potential overstatement of decline (Steve Buckland)
• 07:50 – What the figure actually tells us (Steve Buckland)
• 08:09 – The risks in oversimplifying or exaggerating statistical messaging

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Conclusion

The episode underscores how headline statistics—though powerful—often mask significant complexity. The 52% decline figure is based on partial, non-representative data focused on vertebrate populations and susceptible to biases linked to monitoring practices. While there's undoubtedly cause for concern about biodiversity loss, such figures must be used with care and sufficient explanation to avoid misleading the public or undermining trust in conservation messaging.

Transcript

[Wesley Stevenson]

This is the short edition of More or Less, first broadcast on the BBC World Service.

[BBC Announcer]

Thank you for downloading from the BBC. For details of our complete range of podcasts and our terms of use, go to bbcworldservice.com podcasts

[Wesley Stevenson]

hello, this is More or Less on the BBC World Service. I'm Wesley Stevenson. Let's take a trip into the jungle. And if the WWF and the Zoological Society of London are right, there are now fewer animals here than there were 40 years ago and they've fallen by an average rate of 50%. In fact, it's not just in the jungles, but all over the world. This news prompted startling headlines and it also got the More Or Less inbox buzzing. We've had a lot of emails like this one from Steve Welch.

[BBC Announcer]

The headline figure of wildlife falling by half seemed immediately suspicious. And I see from the text that only some types of animals are included. For example, there are no insects. But what does falling by an average of 52% mean? Please enlighten us.

[Wesley Stevenson]

More or Less listeners are a skeptical lot. So let's have a look at what the figure means and how it was arrived at. It came from the recently released Living Planet Index and the project manager is Louise McCrae from the Zoological Society of London.

[Louise McCrae]

The exact figure is 52%. And this is a decline in the size of vertebrate populations over 40 years. The way we came to this figure was that we gathered over 10,000 different population trends from over 3,000 species. And what we've done is we've calculated an average of these trends to get this figure. Now what it means is that it's the rate of decline. So it means that populations have roughly halved over this period.

[Wesley Stevenson]

So does it mean that if you counted all the vertebrates in 1970 and counted them all in 2010, the number is 50% less?

[Louise McCrae]

It's not quite the same. So it's not that we've lost half of all individual animals because some of the populations are very large and hundreds of thousands of individuals and some are very small. So it's all relative, but it just means that the rate at which populations are declining is that they've halved on average over that time.

[Wesley Stevenson]

Lets imagine a scenario. You have two populations, one of frogs and one of birds. And let's say in 1970 there were 10 frogs and 100 birds, giving a total of 110 animals. By 2010, the frogs had declined by 80% and the birds by 20%, leaving two frogs and 80 birds, a total of 82 animals. The average decline of these two populations is, is 50%, but the total number of animals hasn't halved, which is probably not what most people would have understood from the headlines. Another thing about the index, as loyal listener Steve points out, is this is just an index of vertebrates and doesn't include insects or plants. And it's not as if either the WWF or the Zoological Society of London have actually been out to count the animals themselves. This is a figure modelled using available data, some of which is patchy.

[Louise McCrae]

Our data set is a sample of all the vertebrates that are out there in the world. Now, unfortunately, this isn't a representative sample of what species that we would expect to see. This is because people historically have monitored species such as birds and mammals, which are very easy to count. And in places like Europe and North America, these monitoring schemes have been going on for a very long time. So we have a lot of data from these regions. Now, most of biodiversity is actually found in tropical areas. So we were trying to balance the data set so that we actually get an accurate picture of what is actually going on. The way we did this was to weight the data. So we're placing more weight on where

[Wesley Stevenson]

we see more species, but also you're placing more weight on those areas where the data isn't as rich as it is. For example, in Europe and other parts.

[Louise McCrae]

Yeah, that was a concern when we started thinking about this method, but what we did, when we saw that we didn't feel we had enough data to make this robust change, we combined the data together. So, for example, for amphibians and reptiles, we didn't have enough data to weight them individually, so we grouped them into a group on their own. So that was a concern, but we addressed it by combining data into subgroups.

[Wesley Stevenson]

Now, there's nothing wrong with modeling the figures, and Louise and her colleagues feel confident that the way they've extrapolated the data has given them a pretty solid figure with a margin of error of 10 percentage points either way. But there are some who aren't so confident that the figure really tells us what they think it's telling us. The weighting in favour of tropical areas where there's less data has caused concern for people like Professor Steve Buckland, professor of Biostatistics at St Andrews University in Scotland, and whose models are used in calculating the figure.

[Professor Steve Buckland]

Many of the surveys carried out in temperate areas, that's non tropical areas like Europe, are done to a very high standard, very good quality, but in temperate areas, the biodiversity trends are reasonably encouraging. The problem is the tropical areas account for most biodiversity in the world, and both the quality of time series and the trends tend to be less good in tropical areas.

[Wesley Stevenson]

And so what's your concern about those?

[Professor Steve Buckland]

Firstly, the index relies on those populations that are monitored which are not necessarily representative of populations in total. There are all sorts of reasons for monitoring populations. One of them is that a species or population of a species is thought to be under threat, and so a monitoring program is instigated. Species or populations that are more impacted by man's activities tend to be easier to monitor also, for example, rainforest populations that are under threat tend to be rainforest populations where there's activity by man. Areas of rainforest that are very inaccessible and remote, we don't have the time series of abundance estimates for those locations. So when you increase the weight of those monitored populations in tropical areas, the rate of decline increases quite substantially from the old index.

[Wesley Stevenson]

And does it overplay that, do you think?

[Professor Steve Buckland]

I think it probably does, simply on the basis that the populations being monitored are not representative. If they were representative of tropical and temperate populations as a whole, then that would be a correct thing to do. But the difficulty is that populations that are not in trouble are less likely to be monitored. They've done some assessment of whether there's bias towards surveying threatened species and they concluded that there wasn't evidence of bias from that source. But nevertheless, there probably is bias towards species that show decline rather than species that are largely unaffected.

[Wesley Stevenson]

What do you think the 52% figure tells us then?

[Professor Steve Buckland]

It tells us that the populations being monitored are in real trouble and that is a very large number of populations. What it doesn't tell us is that 52% of all vertebrate animals on the planet have declined by 52% in the last 40 years.

[Wesley Stevenson]

So the figure probably isn't as clear cut as the headlines or the press releases from the WWF and the Zoological Society of London. Suggestions. But Steve Buckland pointed out to me that to caveat the figure in lots of detail would have obscured the message that the WWF and the Zoological Society were trying to get across, namely that man is having a serious impact on wildlife. But as climate change scientists have found in the past, trying to exaggerate your message, even if you believe in the cause, can backfire and can make people more skeptical and about future claims. That's all for this week. Thank you very much for listening. If you have any numbers that you want us to investigate, please email more or lessbc.co.uk you can also listen to other editions of More or less exploring subjects from the chances of someone running a sub two hour marathon to whether the UK is really poorer than most of the states in America. And you could do this by going to our website bbcworldservice.com more or less.

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