8.7 million species now estimated on Earth (and then some)

Estimates on the possible number of species vary wildly. Holger Hollerman/AFP

An estimated 8.7 million species exist on Earth, according to a paper published today in PLoS Biology. The figure is based on a new validated analytical technique that, it’s said, significantly narrows the range of previous estimates.

Of course the question of how many species there are has exercised our species from the year dot. Before there were gods or Darwin, we knew we were not alone.

Taxonomists (gatekeepers of life on Earth) have taken it upon themselves to document life on Earth. From the inception of scientific classification (starting in 1758 with the great Swedish botanist Carl Linnaeus), taxonomists have relentlessly ground away at formally describing planetary species. So far, almost 2 million species have been described.

That seems a lot, of course – it seems like plenty.

But in the mid-1980s, with the emergence of the biodiversity paradigm – a term used to describe the variety of life on Earth – the perennial question of “how many” was re-energised, and there was no ready answer.

Estimates on the possible number of species have varied over the last quarter century, from about 5 million to 100 million. Does that sound like beer-mat calculations?

Led by Dr. Camilo Mora of the University of Hawaii and Dalhousie University in Halifax, researchers in the PloS paper have rightfully questioned the subjectivity of previous assessments, and in the process provide a method using higher taxonomic categories. But are they right, or even close to being right, with their estimate of 8.7 million?

A bit of history helps our understanding of the complexity of the question and the challenges we face.

Terry Erwin

In the modern era, we credit a well known coleopterist (one who studies beetles), Dr Terry Erwin of the Smithsonian Institute, for stirring the pot. He was curious about how many species of insects could be found on a single species of rainforest tree.

In the early 1980s, he fogged the canopy of a few individuals of one species of rainforest tree (Luehea seemannii) in Panama with insecticide, and an endless supply of insects rained into his collecting trays on the rainforest floor.

Putting them all on pins, he found, with his other beetle mates around the world, there were more than 1,000 species of beetles, and undoubtedly came to the conclusion that:

1) that’s a lot of beetles
2) many of them had not been seen before.

He calculated there may be as many as 30 million tropical arthropods (insects, spiders, crustaceans and their allies) in the world.

Since that time, biologists have used a variety of methods to calculate the global number, based on extrapolations from surveys, species interactions, body size and higher taxonomic categories. Even Lord Robert May – former Chief Scientific Adviser to the UK Government – has had a crack at this holy grail of calculations.

So how many are there?

Of late, and by different methods – including what taxonomists “just reckon” – there’s a tacit agreement building there are about 10 million living species.

That informs and challenges us in a number of ways.

If we have described only 2 million (Mora and his colleagues estimate 1.2 million in their PLoS paper, but this is an underestimate), we clearly still have a way to go.

So have the taxonomists been slack? No, not at all.

Species are not immutable entities as Linnaeus thought, and determining their identity is no easy task. Natural variation is perplexing and species don’t self-identify for our benefit.

That said, within ten generations of human effort we have unraveled a good percentage of the outcomes of 3.5 million years of evolutionary history – the so-called “Tree of Life” – a job well done.

There has been a steady decline in the taxonomic capacity of universities. Angelika Warmuth/AFP

Australia

Australians may be curious to know what we know about our species and how we compare to the rest of the world. There are currently about 127,000 described Australian species.

A recent Commonwealth report provides an estimate of just over 500,000 species thought to exist in Australia.

If this estimate is reasonable, then we have described a quarter of the species that exist in Australia, which is a little more percentage-wise than we estimate globally.

Are we doing a good job? Again the answer is yes and no.

In the Northern Hemisphere, most of the species are described. In Britain it’s now rare for new species of any group of organisms to be described. But it’s all together another matter in the tropics and Southern Hemisphere.

The so-called taxonomic impediment (taxonomic gaps in our knowledge of species and workforce shortfalls) relates very closely to our lack knowledge of the tropics, where threats to biodiversity are so great.

But this is not the only gap, and here Australia comes into focus. It turns out that for some groups of organisms there is remarkable diversification in temperate and arid regions, which belies the orthodoxy that species richness increases towards the tropics.

For many groups of arthropods, diversity is greatest in places such as the southwest of Western Australia, and the scrublands and woodlands of eastern and western Australia.

The flora of southwest Western Australia is one of the richest areas globally, species-wise, and rivals any place in the world for its biodiversity values.

The great flowering plant diversity in this region is well known (more than 9,000 species) but what is not known is the enormous diversification of insects that has occurred upon these plants – the vast majority of which are unknown.

What’s being done?

In my laboratory at UNSW, we have discovered and described a large number of species from temperate and arid Australia, through a US National Science Foundation program – the Planetary Biodiversity Inventory program.

More recently, through the Commonwealth funded program Bushblitz, we have continued to discover and describe new species.

In the past four years, our lab has described about 100 new Australian species. Such efforts are repeated across our universities, herbaria and museums, but the taxonomic workforce has eroded significantly over the past quarter century, just when we need to ramp up the effort.

So to answer the question – how many species are there? Well, the answer is: a lot, and 10 million sounds likely.

The geneticist JBS Haldane said early in the last century: “God must have been inordinately found of beetles”.

And there’s the rub. Our knowledge of the less obvious organisms is where the taxonomic impediment lies.

Arthropods account for about two thirds of all species and yet it’s very easy to find new species, even within a city such as Sydney.

We know little about microfungi (fungi are not plants!) but we know they are very diverse, and microbial diversity is also clearly staggering, but we also have much to find out.

One could ask why we should care about the number of species. But thinking this way is to circle our wagons and shoot inwards. As human activity encroaches increasingly on Earth’s natural systems, comprehensive knowledge of species will count towards our quality of life, if not survival.

So with 2 million described, and another 8 million or so to go, we need to invest, and fast. The paper by Dr Mora and his colleagues supports the need for an enhanced taxonomic effort.

There has been a steady decline in taxonomic capacity in universities as identified at in the 2007 National Forum on Taxonomy.

The tertiary sector can provide leadership on this score by providing the framework for training the next generation of taxonomists, supporting taxonomic research, and forming strategic partnerships with museums and herbaria.

It follows that governments must see the need for increased investment in taxonomy.

Will this happen? It would be nice to think we could count on it.