In case you’ve been missing it, all the big news people have been reporting this:
A new report from the World Wildlife Fund indicates a nearly 50% decline in marine life populations between 1970 and 2012
The statistic comes from the 2014 Living Blue Planet Report.
Here are some graphs from the report:
So… we need re-write the headline as:
“Nearly Half of Marine Life Disappeared between 1970 and 1990 – not much has changed in the last 20 years though…”
Another headline re-write:
“Commercial fish populations decline by almost 50% between 1970 and the mid-2000s – appear to be stabilising.”
Yet another revised headline:
We could go on.
Which is not too say that we shouldn’t be concerned. I mean, Tuna Sashimi is an important part of my diet – let’s get the sustainable farm fishing going – come now.
But the journalists are being a little alarmist.
Also: the index!
I also want to say something quickly about the way that these figures were arrived at (because it’s important).
The measurement being used here is the Living Planet Index (and some subsets of that index).
How this is calculated:
- Take thousands of marine species.
- Calculate their total population change as a percentage of their 1970 population.
- Average all the population changes.
- Put that average change into an index form.
Let me use a somewhat extreme example to illustrate the problem here:
- Let’s say that I start with two populations in 1970:
- 10 billion sardines; and
- 100 rare sea turtles.
- Sadly for the sea turtles, due to an oil spill, their breeding grounds were lost. By 2012, there are only 2 left.
- But there are still 10 billion sardines.
- Under the Living Planet Index Calculation:
- Sardines have had a 0% population change.
- Turtles have had a -98% population change.
- And therefore, the average population change is -46%*.
*(0% + -98%) ÷ 2 populations
- Meaning that the Living Planet Index in 2012 will be 54 (using 1970 as the base year of 100), showing a near 50% decline.
- Even though, in terms of biomass, there used to be about 10 billion creatures in the total population, and there are still about 10 billion creatures in the total population.
That is: because the index is unweighted for population size, it is a measure of biodiversity, not biomass.
Which is not to say that the biomass has not also been hit as badly (or, possibly, even worse!). But you can’t really say that by looking at the index alone. All you can say is “Oh look – there seems to be less biodiversity in the sea. That is suggestive of a decline in overall biomass, but we can’t really say how much.”
Again – that’s not to say that 50% of the marine fish population hasn’t disappeared – but you won’t know one way or the other until the index is weighted for population size.
Point is: numbers mean things, and they mean things in context. And you can’t just presume a context.
It’s as absurd as saying:
- Oh look, that Dad just said “I love you” to his daughter.
- But “I love you” is what romantic partners say to each other.
Uh – no. That’s doing English wrong. And life, quite possibly.
Anyway, let’s still save the fish.
And go and check out the “More or Less” podcast from BBC Radio 4. It’s filled with this kind of thing (and it’s where I first heard anyone talk about the calculation of the Living Planet Index!).