So the 2020 version of the Living Planet Report has been released to massive headlines blaring catastrophe. The central claim is that vertebrate (i.e. fish, amphibian, reptile, bird, mammal) local populations declined, on average, by 68% from 1970 to 2016 (the report is released 4 years after the end of the data). The authors of the report have done a much better job of getting out the notion that this is an average decline. IE they’re not claiming that there are 68% fewer vertebrate individuals on the planet, but that the average decline is 68% (but see footnote)*.
To invert their claim, the average vertebrate population in 2016 is 32% (100%-68%) of the size that it was in 1970. If we look at the 2018 report it says that the average vertebrate population in 2014 is 40% of what it was in 1970. And the average vertebrate population in 2010 is 48% of what it was in 1970. So if a population in 1970 was of size N then, 2010=0.48N, 2014=0.40N, and 2016=0.32N. Wow! That is a 52% decline in the 40 years from 1970 to 2010, a 16.6% decline in four years from 2010 to 2014 and a remarkable 20% decline from 2014 to 2016. The math is a little complex because it is exponential, not linear, decline but that gives a 1.82% decline per year from 1970 to 2010, a 4.46% annual decline from 2010-2014, and a 10.6% per-year decline from 2014-2016. So not only are there huge declines, but the declines appear to be accelerating (admittedly with small samples for recent years). If we are conservative in the face of this accelerating trend and hold declines constant for the next 10 years (from 2016 so to 2026) at 10.6%/year and start in 2016 at 32% of 1970 numbers then we are down to 10% of the 1970 numbers by 2026. Do you believe that! In 6 years from now the average population will be just 10% of what it was in 1970. (To be clear, the LPI authors did not make this claim – I did, but it is just a 10 year extrapolation from their numbers). You would think such a decline would be more obvious to the casual observer. I’m old enough to remember 1970 and have spent a lot of time in the woods in my life. If there was a 20% decline (or increase) I’m not sure my fallible memory would reliable detect the change (in fact I’m pretty sure it wouldn’t). But if there were 90% less birds on average than my childhood, I would have thought I would have noticed. You would also think the world would be absolutely exploding with things vertebrates eat (e.g. insects and plants).
If this isn’t happening, then what is going on? Well for starters, it is pretty dicey to take short term rates and extrapolate them when things grow or decline exponentially. If you do that you are liable to find out everything is extinct or at infinity pretty quickly. So lets go back to the core claim straight from the report – there has been a 68% decline in the average vertebrate population since 1970. Not quite as extreme, but you would still think I (and a lot of other people) would have noticed declines in vertebrates of this extent not to mention the boom of insects and plants as they’re freed from predation.
If you don’t trust my fond recollections of my childhood nor my extrapolation to what should have happened to insects and plants (as you definitely shouldn’t!), then how about this. The LPI core result is completely different than other studies (not cited in the Living Planet Report for what it is worth). Several, like the LPI, track thousands of populations over decades. All (like the LPI) suffer from some observer bias – scientists have more data in temperate regions and near cities and for bigger animals, but there has been no evidence to date that this fact is biasing the results of any of the three studies. First, here is a plot very similar to the LPI plot but for invertebrates in the UK by Outhwaite and colleagues in Nature Ecology and Evolution:
Now this is invertebrates, not mammals, but what we see is 3 broad groups have abundances higher than they did in 1970 (freshwater species showing a spectacular recovery possibly due to clean water laws), and one broad group that is down just a smidge. The overall balance across all 4 groups is a 10% INCREASE.
Here is a paper by Dornelas and colleagues in Ecology Letters (disclosure I am a co-author):
They (we) used a slightly different method – we calculated the slope of the timeseries and then plotted histograms of the slopes. Note that there is a lot of variability with some real declines and real increases, but the overall trend across populations is strongly centered on (i.e. averages on) about zero (neither up nor down). In fact the title of that paper is “A balance of winners and losers in the Anthropocene” and finds that 85% of the populations didn’t show a trend significantly differently from zero, 8% significantly increased, and 7% significantly decreased. A lot of churn of which species are up or down, but NOT an across the board catastrophic decline. Maybe this is because Outhwaite and Dornelas didn’t study vertebrates? Unlikely. Dornelas et al did pull out different taxa and found that reptiles, amphibians and mammals skewed to more increases than decreases and no real difference from zero in birds and fish (their Figure 4). Or check out Leung et al who analyzed a subset of the LPI data (hence all vertebrates) focusing on the well sampled North American and European regions using a different methodology who got more groups increasing than declining. Or check out Daskalova et al who also found winners and losers were balanced (and most species were neutral). Even the most extreme result of the studies that exclusively use longer term data to look at this question that I am aware of (van Klink et al) shows a 35% decline over 45 years for terrestrial insects and 60% increase over the same period in aquatic insects. I think it is an interesting and challenging question why these studies received little press (despite also being published in high profile journals), but the LPI gets enormous coverage every time it comes out.
These 5 other studies more closely match my childhood memories. There could be weaker trends (+ or – 10 or 20%). And for sure I could be seeing different species (winners replacing lowers). But these 5 studies completely contradict the LPI result (all 5 find a robust mix of increases and decreases and most find something like a balance between increases and decreases). So what is going on?
For one thing, I think the LPI bites off too much – it tries to reduce the state of vertebrates across continents and species to a single number (aka index). That has to sweep a lot of complexity under the rug! There is underlying variability in the LPI too – they just don’t emphasize it as that is not their point. And to a large extent these other papers are just unpacking that complexity by exposing the underlying high variability in trends.
But those other papers find a more neutral balance while the LPI most definitely does not. Something more has to be going on. It could be their data (but some of the aforementioned papers used the same or a subset of the data). Or it could be their methodology (but some of the aforementioned papers used similar methodologies). Personally, I think it is a complex interaction between the data they are putting in and the weaknesses of the methodology (in the sense that every methodology has weaknesses, not that their methodology is fundamentally flawed or wrong). There may be more to say about this in the future. But for now, I hope we can at least pause and think and do a sanity check.
I want to leave no doubt that I am convinced humans are hammering the planet and the vertebrates (and invertebrates and plants) that live on it. We’re removing >50% of the [terrestrial] primary production each year, have removed more than 50% of the tree biomass, modified >50% of the land, use more than 50% of the freshwater, have doubled the amount of nitrogen entering the biosphere each year and nearly doubled the amount of CO2 in the atmosphere since pre-industrial times. But I also don’t think it is possible for there to be a 68% decline in 46 years leading to a projection of a 90% decline over 56 years (10 years from now) nor does a 20% decline in the last two years seem possible. The consequences of 68-90% gone is just too large not to be observed anecdotally and through indirect effects. And the 68-90% decline story just doesn’t align with other major, comprehensive, 1000s of datasets analyses of this question.
What I believe the data show is we’re creating winners and losers – some really big winners and some really big losers and a lot in between, and that’s bad – humans ARE massively modifying the planet in ways that all but the most biodiversity-hating people care about, and the extinctions we are causing are irreversible,so please don’t cite this blog as evidence that “everything is OK”. Its not. Is there room for an “in between” (bad but not catastrophe) message?
But either way, please think twice before reporting that vertebrates are disappearing from the planet at these incredible rates. Because the logical conclusion is that nothing will be left in a very short time (decade or two) and that doesn’t pass the common sense test. This is not an “all scientists agree” scenario. I personally think the balance of evidence (such as cited above) points pretty strongly against the LPI conclusion. I worry how many more years scientists (and reporters) can report catastrophic trendlines that predict little to no life of any sort on the planet within our lifetimes and not have people notice that this isn’t actually happening.
Note: I am indebted to many colleagues who have talked about this topic with me over the years, some of them co-authors on the paper cited here, some of them co-authors on forthcoming papers, some of them not co-authors, but I want to stress that the opinions here are controversial and my own so I am not listing them here.
* The report averaged rates of decline in populations, not total decline in number of individuals (unlike this catastrophic headline). But shouldn’t they be the same thing? Well yes if there were the same number of individuals in each population and each species then a 68% decline of 100 here (to 32) and a 68% decline of 100 there (to 32) would still result in a 68% decline (from 200 to 64). But we know in fact number of individual varies wildly (100x-1000x) across populations and species. So It would be a 68% of 1000 to 320 and a 68% decline of 10 to 3.2 giving 1010 to 323.2 which is STILL 68%. But now the fact the 68% is an average comes in. What if the 1000 declined by 60% to 400 and the 10 declined by 76% to 2.4 or 1010 to 402.4. That’s not a 68% decline but a 60.2% decline even though average the rates 60% and 76% still give an average 68% decline. We don’t know for sure whether large populations are more likely to decline or small populations are more likely to decline, but we do know that at least in birds abundant species are declining while rare species are increasing, so if you assume that it would mean things are actually even worse than the 68% decline in terms of total number of vertebrate individuals increasing, but we don’t know for sure. But I don’t think this is the central reason why the LPI numbers don’t match my childhood memories, nor other studies. With such large data and no truly strong correlations between abundance and decline, most of this comes out in the wash. So theoretically this could be a mathematical reason the total number of individuals has decreased by less than 68% even when the average decline across all populations is 68%. But I don’t think it likely. In fact I think in a weird way, arguing this is a way of distancing the LPI from what it is really claiming/implying.