5.16pm GMT

David Roy, head of the Biological Records Centre at the Centre for Ecology & Hydrology, tells me that any attempt to model the number of species will be imprecise.

"Every model's got underlying assumptions and generalisations, and it's how realistic those generalisations are and whether you violate the assumptions which is a matter for debate. "You're trying to estimate an unknown based on imperfect information."

But he says the endeavour, flawed and probably impossible as it is, is vital "because of the value of species and biodiversity if you like, which can be considered in different ways".

He says biodiversity has an important spiritual and aesthetic value. "Then there are more human-centric views which are to do with discovering new chemicals, new medicines for example."

He also mentions services through which other life supports human existence - pollination of crops, nutrient recycling and carbon storage.

When I ask Roy about the concept of a species he lets out an audible sigh. This is evidently a matter of deep conjecture. He says DNA evidence had uncovered a raft of new species and had also made many disappear - the processes known in the taxonomy world as lumping and splitting.

Lumping is when some species that were thought to be different, or were described by different scientists at different times, are found to be genetically the same. Splitting is the reverse.

"Both of those things can happen and they do happen as techniques improve and knowledge improves particularly through the use of DNA techniques," says Roy.

Sina Adl from the University of Saskatchewan was one of the authors of the 2011 Mora et al. paper, which estimated 8.7 million species inhabit the planet. He wrote to me this evening.

In our paper “how many species are there?” we showed a new way of estimating the number of species. What our approach indicated was that for many groups of organisms, we seem to have found most of them, although not by any means all of them. For example, even if there are families to discover, or under-described families or genera of organisms among the Plants and Animals, most are now known. What most people forget is that most of the eukaryotic diversity is microscopic, and in the Protists. It is clear that we do not have enough information at this time to estimate how many species of protist there are, as only a fraction are described. Therefore the estimate in the manuscript is a gross under-estimate. As for bacteria, the eukaryotic “species” concepts do not seem to work well, and it is harder to come up with something to count.

Geoff Boxshall is a Researcher at the National History Museum. He says the study of marine biodiversity often gets swept away by our fascination with our terrestrial habitat. While the biodiversity of the ocean at a species level is lower than on land - the ocean covers 70% of the planet but only seems to account for around a quarter of the species - on the the upper taxonomical levels the ocean contains many more phyla that survive on land. So on this measure of diversity, the ocean leads.

Boxshall says we need to understand how many species there are in order to protect them.

"In order to live sustainably on the planet, which we are not managing to do very well, we are going to have to be able to model how biodiversity is generated, how biodiversity is maintained and how biodiversity is lost."

He says having rigorous science is vital to convincing policy makers to take action - much like with climate change.

He agrees that the numbers found by Mora et al. are fairly consistent with current scientific thinking.

"Over the past couple of decades we've had wildly varying estimates, 30 million, 100 million. But I think there is a consensus now that the number is between five and ten million globally."

Although he says studies of parasites have showed that it might be necessary to rethink these numbers.

"There's a rough estimate for every multi-celled animal species in the ocean, there's at least one species of parasite that is specialised upon that host. So if you are thinking about the diversity of parasites, it might actually be necessary to double the diversity estimates."

He says terrestrial parasite studies have shown their abundance could be the same on land.

As to where we might discover new species, Boxshall says it will be mostly in places we haven't looked. The major factors in this are either difficulty of access, or a low level of past exploration. He says a museum colleague found 300 new species in shallow water off Kuwait - simply because the area was histoprically poorly studied.

Joe Walston, executive director at the Wildlife Conservation Society Asia program tells me: