Image copyright AP Image caption News species of animal are constantly being discovered by scientists

How many more fish in the sea? Plenty, according to research commissioned by the Census for Marine Life, which puts the number of species in the world's oceans at 2.2m, and the total for the whole planet at 8.74m (plus or minus 1.3m).

You can read more about the study, published in the journal PloS Biology, and the novel way that it was calculated here, but the figure is a vast improvement on previous estimates, which range form 3 to 100 million - close to useless when it comes to understanding the complex web of ecological interactions underpinning life on earth.

And the number of species we share the planet with does matter. As one of the co-author's of the study, Boris Worm from Dalhousie University points out,

"If we didn't know - even by an order of magnitude (1m? 10m? 100m?) - the number of people in a nation, how would we plan for the future?"

At its most basic, if we don't know what we've got, we can't protect it, and we can't even be sure what we're losing.

And we are losing species at an alarming rate. Again estimates vary wildly, but the distinguished biologist EO Wilson put the figure at some 30,000 a year, or three every hour. It's a rate that compares with previous extinction events - like the one that wiped out the dinosaurs 65m years ago - and has been dubbed the 6th mass extinction event in the history of life on earth.

Listening to the government's former chief scientific adviser, Lord May, on the programme this morning I was reminded of a scene in the science fiction blockbuster The Matrix.

Having subdued our hero the villain of the piece, agent Smith, pauses - as Hollywood bad guys so often do - to deliver a soliloquy: Humans, he declares, are like a virus, multiplying until every natural resource on the planet is consumed.

"Human beings are a disease. A cancer of this planet. You are a plague, and we are the cure."

Image copyright AP Image caption Can humanity avoid a species-depleted sci fi future?

Lord May didn't go quite that far, but he's not above employing the occasional sci fi metaphor to make his point.

Speaking some years ago as president of the Royal Society about the rate at which human activity was driving other species to extinction, he suggested that people were probably ingenious enough to survive in the damaged and depleted world of the movie Blade Runner, but the question was did we really want to live in that world?

"We are astonishingly ignorant," Lord May told the programme, "about how many species are alive on earth today, and even more ignorant about how many we can afford to lose while still maintaining the vital ecosystem services that humanity ultimately depends on."

That may be why so many biologists believe the biodiversity crisis is actually a much more profound threat to our future than global warming.