Simply counting the number of species may not be the best measure of biodiversity. Credit: Westend61/Getty

The high-street coffee shop has long been used as a measure of urban gentrification. But are all coffee shops the same? Not so, claimed the London edition of Time Out in 2014. In fact, it said, there are eight types in London just in the independent sector, away from the global mega-chains. These separate species of capital brew house could be distinguished by the presence of table service, for instance, and whether the barista could remember your name and favourite order.

Time Out, then, would see a high street with one of each of these individual outlets as diverse. But most of us, especially tea drinkers, would probably prefer to swap a few of them for, say, a butcher, a baker and, if not a candlestick maker, then perhaps a newsagent. Despite their differences, all coffee shops provide essentially the same service. In those terms, a street of different types of coffee shop is anything but diverse. It doesn’t offer as good a service, and so it’s not such a great place to live.

What does the supply of caffeine have to do with this week’s special issue of Nature that discusses biodiversity, the extinction of species and how to conserve them? Everything. For, as some biologists argue, too much current thinking on conservation agrees with Time Out. The standard definition of biodiversity focuses too heavily on counting the number of different species, when perhaps it should concentrate on what each of those species contributes to the ecosystem.

Functional diversity, properly applied, could be a pragmatic and necessary step.

Carry your coffee to drink at the rocky seashore, for example. Within a square metre or so you might find four species — a mussel and three different species of barnacle. A bit farther along, in another square metre, you find another four species, but this time the mussel is joined by a starfish, an anemone and a seagrass (see Under current conservation measures, each community has equal biodiversity and deserves equal attention. That’s because a thatched barnacle is considered to be as different from an acorn barnacle as it is from the seagrass. Just as a barista who remembers your name is as different from a forgetful one as he or she is from a librarian.

To see and designate the second seashore community as different from the first, some biologists argue that we should consider what these species do, individually and collectively. The idea is called functional diversity, and it’s catching on. Many biologists have felt for decades that the starfish, anemone and seagrass make up a more diverse community than the barnacle trio. But as a News Feature explores this week, the concept is gaining ground in policy circles. And it’s being used to set priorities and to determine how conservation resources are allocated.

Intuition is not evidence, and there are already concerns that proponents of functional diversity are trying to run before they have worked out if they want to walk. Which functional traits should be considered and how can they be compared? How can biologists ensure that all functions of a species are accounted for, and not just those that are the most obvious? Do we have sufficient data to link diversity of traits to the health of an ecosystem? What if table service at a coffee shop is the only reason that a rich couple visit, and spend money in other shops while there?

To consider the utility of creatures in a habitat and not just their number can certainly throw up counter-intuitive findings. Some measures of functional diversity, for example, judge degraded post-logging secondary forests in the tropics to be as healthy as the primary forests they replace (see C. A. Sayer et al. Biol. Conserv. 211 (A), 1–9; 2017). (That is not an argument to stop protecting primary forest, but it might be a reason to give the degraded areas equal status.)

What is clear — and laid out in much detail in a series of other articles this week — is that existing attitudes and measures are failing to halt the global loss of habitats, species and ecosystems. To address the decline and stem the damage to the natural world, new approaches and new thinking are needed. Functional diversity, properly applied, could be a pragmatic and necessary step. All species are equal. But perhaps some are more equal than others.