Humans have done too little to find sustainable ways of sharing the Earth's resources, a US scientist says.
Dr Peter Raven, says the rich world in particular is not confronting the extinction crisis.
He believes we know scarcely 15% of animal and plant species alive today.
And most of those we are driving to extinction will vanish without us ever having known they were here.
Dr Raven is director of the Missouri Botanical Garden and Engelmann Professor of Botany at Washington University in St Louis.
Deep ignorance
He was delivering the Darwin lecture in London on Wednesday, the eve of the UN-designated International Biodiversity Day. His lecture was entitled Our Choice: How Many Species Will Survive The 21st Century?
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There may be as few as 20,000 lions left across Africa - a terrifyingly small number, and a plausible one
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Professor David Macdonald, University of Oxford
Dr Raven said there were perhaps 10 million species alive today, of which only 1.5 million had been recognised and named scientifically.
Humans knew no more than one in six of the Earth's animal and plant species. We know so little about fungi, he said, we had no accurate picture of their geographical distribution.
In the tropical rainforests, only one species in 20 had so far been catalogued, scientists estimated.
Over the last half-century, Dr Raven said, drastic human changes to the Earth included:
"We are likely never to have seen or to be aware of the existence of most of the species we are driving to extinction."
It was not "morally or ethically right" to destroy things as we were. Yet despite the 1992 Earth Summit, relatively little progress towards sustainable development had been made.
Dr Raven said the industrialised nations had not generally risen to the challenge - and if everyone lived at their standard, it would take another two planets to support the Earth's population.
His prescription was simple and demanding: a stable population, a globally sustainable consumption level, and acceptance of social justice as the norm for development.
Dr Raven described the UK's Darwin Initiative, which has provided £30m ($49.25m) to biodiversity conservation projects in developing countries, as "a brilliant concept".
Mistaken perception
The meeting heard details of one Darwin-funded project, which seeks to protect lion populations in Zimbabwe.
It is led by the Wildlife Conservation Research Unit (WildCRU), University of Oxford, working with the Zimbabwe wildlife department.
Professor David Macdonald, director of WildCRU, told BBC News Online: "People think lions are common, but a survey to which we contributed came up with a very different picture.
"There may be as few as 20,000 lions left across Africa - a terrifyingly small number, and a plausible one.
"The lions we're looking at in Hwange are killed by farmers, and by trophy hunters, and it's mainly males who die.
Knock-on effects
"Lions live in extremely complex societies. If you kill one male, the lion who replaces him will usually kill his cubs.
"And we found males serving three, four or five prides of females, not just one. So the take is completely unsustainable because the consequences of one kill just cascade.
"We've managed to get the hunting quota halved, and local youths are getting the conservation message across in the villages."
Another Darwin project is trying to save the guanoco, an animal of the high Andes which is thought to be the ancestor of the llama.
Other species to benefit include orang-utans in Sabah, Malaysia, South African penguins, and fruit bats in Madagascar.
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