At first blush, the idea of biodiversity seems simple enough. It is essentially the variety of all life on Earth. But making sense of biodiversity in a way that can help us halt or even reverse its decline is anything but straightforward.
鈥淧eople often use the word biodiversity just to mean any characteristic of life out there that we might care to protect,鈥� says , a biologist at the University of Sherbrooke in Quebec, Canada. 鈥淭hat鈥檚 not a definition I find useful in science because if it鈥檚 everything, it鈥檚 nothing.鈥�
For biodiversity to be a valuable concept, he says, it needs to be a measure of biological variety. That way, we can not only assess where we are and where we are headed, but also how best to conserve the biodiversity we have left.
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The problem is that variety itself comes in many forms, especially in biology. 鈥淵ou can鈥檛 just come up with a single number for biodiversity in the same way as you can for carbon,鈥� says at the University of Oxford. 鈥淚t鈥檚 way, way more complicated.鈥�
We already have ways to measure biodiversity. That鈥檚 how we know it is in steep decline. They boil down to what biologists think of as dimensions of biodiversity. One of the most basic is species richness, which is simply the number of species in a given place at a given time. This has been used extensively and can sometimes be a useful proxy for other dimensions of biodiversity, says Hector.
Measuring biodiversity
One of those is the relative abundance of the different species. Two ecosystems can be equally rich in species, but not in diversity. 鈥淭he way I like to explain it is if you walk through a forest or swim through a coral reef and you see two organisms in sequence, what are the odds that they鈥檙e going to be different things?鈥� says Vellend. 鈥淵ou could have a thousand types of things in there, but if 99 per cent of them are of one type, then the odds are, when you see two in a row, it鈥檚 going to be the same thing.鈥�
A third dimension is how different the species are from one another in some important aspect. 鈥淔unctional diversity鈥�, for example, looks at the range of different roles that species play in an ecosystem, such as in photosynthesis, nutrient recycling, predation or pollination.
But there is also a fourth dimension, which tracks how the other three change over time. Every measure of biodiversity worth its salt captures one or more of these aspects, weighted according to what data is available and the project鈥檚 goals. 鈥淚t all depends what you want,鈥� says Hector. 鈥淎re you trying to conserve biodiversity for biodiversity鈥檚 sake or is it more human-centric?鈥�
And here鈥檚 where things get knotty, because there are myriad ways of measuring each dimension. That means the whole thing risks becoming frighteningly fractal and indeed fractious. When discussions started on how to define the 2020 global biodiversity targets, there were nearly 100 suggestions on the table, according to at the University of Halle-Wittenberg in Germany.
In 2013, researchers led by Pereira began trying to standardise the way biodiversity is measured. They distilled biodiversity to six key dimensions: genetic composition, species distribution and abundance, species traits, community composition, ecosystem functioning and ecosystem structure. These capture the essence of biodiversity and how it is changing in a format that biologists can measure and share, says Pereira.
Not everyone is on board. But there is at least a growing realisation that the time for such quibbling has long passed. There isn鈥檛, and probably never will be, a comprehensive measure of biodiversity, says Hector. And ultimately, 鈥渨e don鈥檛 have the luxury of waiting until all life is documented鈥�, he says.
