Last Monday was World Biodiversity Day, and the focus was this time on marine biodiversity. This means a lot of press releases and other broohaha (and boohoo) about declining marine biodiversity. I can point to plenty of problems with marine biodiversity (for instance, that delicious fish species like eel and southern bluefin tuna are at the brink of extinction), but you can read all about that here. For now, I want to look at the following statement by the Dutch North Sea Foundation (my translation; links added by me):
The North Sea, The Netherlands’ largest nature area, is under pressure. In 2010 the Nature Balance announced that only 40% is left of the North Sea’s biodiversity. “So it is necessary that important areas and animal species are well-protected, so that the sea’s biodiversity can recover”, says Monique van de Water of the North Sea Foundation.
Rypke Zeilmaker of Climategate disagrees because, among others, the Nature Balance figures are based on reference levels for which no scientific foundation exists (translated rather liberally):
As reference level for porpoises the Nature Balance uses an unfounded historical figure of 50,000 porpoises along the Dutch North Sea Coast and 1000 in the Wadden Sea. They don’t say why.
As I said before, I follow the Climategate blog not because I’m a climate sceptic (I’m not), but between all the accusations and rhetoric you occasionally find interesting viewpoints. So I looked up the documents on which all this is based. Here are a few quotes.
Osinga et al, 2007; cited in Wortelboer 2010, which is cited by the Nature Balance:
With a density of 0.390 porpoises per square km and a Dutch North Sea with a surface area of 57,000 square km the total number of porpoises in the Dutch North Sea is estimated at about 22,230 individuals.
Based on the data of later analyses by SCANS II we estimated the number of porpoises at 11,000. (…) The Dutch Mammal Society (2007) estimates the current number of porpoises in the Dutch North Sea at roughly 17,000 (15,000 – 19,000) based on airplane surveys and SCANS II. (…) The Dutch Mammal Society estimates the porpoise population in the Dutch North Sea in 1950 at 32,500 (25,000 – 40,000) individuals. This is lower than the reference level according to Baptist and Jagtman (1997).
I don’t have the Dutch Mammal Society report but here is what Baptist and Jagtman (1997) say:
There are no measurements of porpoise populations before 1960. From the number of stranded animals we can say that the species was very common. (…) From biological monitoring studies we can deduce that there is a population of at least 30,000 individuals in the southern North Sea. This population was until recently present in the British part of the North Sea, but not in the Dutch part. Based on current knowledge the reference population can be estimated at more than 100,000 porpoises in the southern North Sea, of which 50,000 in the Dutch part of the North Sea.
Overestimated? Possibly. Shaky? Undoubtedly, but unavoidably in this kind of studies. Poorly referenced? I believe so: I would have liked to know what is meant by “current knowledge”. Unfounded? That’s too harsh. Scientific fraud, as Zeilmaker claims? Certainly not. There are simply no data, but the policy makers want figures to work with. And the affair after which the Climategate blog is named shows that if you don’t like the North Sea Foundation, PBL, or any organisation involved in environmental policy or research, it is always possible to find some mistake, misquote, omitted reference or whatever, and to blow that out of proportion.
But there is still something that bothers me about the 40% figure used by the North Sea Foundation. The indicator they use is a sort of average of a lot of other indicators, all of which express how well the environment is doing with respect to some reference level that is supposed to represent a pristine, untouched environment. So if the porpoise population would have been 50,000 in the absence of man, and it is now, say, 25,000, then the indicator for porpoises is 50%. There are loads of such indicators, and the method of aggregating them to a single figure can be debated at length also, of course. But that holds for any indicator that aims to express a multidimensional issue in a single figure (more on ecological footprinting in a later post).
No, what bothers me is that they also do this for commercial fish species like herring, plaice, and cod. Didn’t we agree under the Convention for Biodiversity that fish stocks should be maintained at Maximum Sustainable Yield? As a rule of thumb, a species’ MSY stock size is mostly about half what it would be in the absence of fishing, although for some species it can be as low as 25%. So if we meet our CBD targets and maximize the harvests we can maintain sustainably, environmentalists still have plenty to complain as these stocks have a “nature quality index” of 50% or less. So yes, please do take the 40% with a grain of salt.