Peter Raven is a botanist. He knows about photosynthesis, primary productivity and sustainable growth. He knows that all flesh is grass; that the richest humans and the hungriest alike depend ultimately on plants for food, fuel, clothing, medicines and shelter, and that all of these come from the kiss of the sun on warm moist soils, to quicken growth and ripen grain.
So botanists such as Raven begin with the big picture of sustainable growth and can calculate to the nearest planet how much land and sea it would take to sustain the population of the world if everybody lived as comfortably as the Americans, British or French. The answer is three planets.
The global population is about to soar from 6 billion to 9 billion in less than a lifetime. Around 800 million humans are starving, and maybe 2 billion are malnourished, while 3 billion survive on two dollars a day.
Valuable agricultural land is being poisoned or parched or covered in concrete, soils eroded, rivers emptied and aquifers drained to feed the swelling numbers. Something has got to give, and the first things to go are many of the plants and animals.
By many, Raven means perhaps half to two thirds of all the other species on the planet in the next 100 years. There could be 10m different kinds of fern, fungus, flowering plant, arthropod, amphibian, reptile, bird, fish and mammal on Earth. Nobody knows. People such as Raven, director of the Missouri Botanic Gardens in St Louis, are doing their best to count and preserve them.
But the human population is growing at the rate of about 10,000 an hour, and each human depends on a hectare or two of land and water for what economists now call "ecosystem services" - the organisms that ultimately recycle waste and deliver new wealth to provide oxygen, fresh food, clean water, fuel, new clothes, safe shelter and disposable income.
Some of these organisms are now being chased to oblivion by human population growth at levels that ecosystems cannot sustain.
Ecosystems, he says, can be whatever you like. Hedgerows in Hampshire are an ecosystem; so are weeds on a railway line at Hammersmith. Savannahs, grasslands, prairies, rainforests, dry forests, pine forests, uplands, heathlands, downlands, wetlands, mangrove swamps, estuaries, oxbow lakes and coral reefs are all ecosystems, and they survive on diversity. The greater the variety of microbes, plants and animals in an ecosystem, the more resilient it is and the better it works for all, including humans. So it would not be a good idea to evict at least half of these creatures, especially if nothing is known about them. But, Raven says, that is what is happening.
There are ways of confirming species loss, even if it cannot be established how many species there were in the first place. Look at the vertebrates and molluscs in fossil records, Raven says, just for the past 65m years or so. "You find that the average life of a species is two to three million years and you get about one species per million becoming extinct per year in the fossil record. Those particular groups are a small sample, but they are a real sample," he says.
"Then you can start with the literature in about 1600, when people began to care enough about organisms to be able to document them well, and for the groups that they were documenting - birds, mammals, amphibians, reptiles, butterflies and plants - then you can say, 'What was the rate over the past 400 years? It's tens of times or hundreds of times the level it was before.'"
That works out at hundreds of creatures per year over the past 400 years, and even more when humans, rats and other invaders started colonising islands: 2,000 species have vanished from the Pacific basin alone since the Polynesians got there 1,200 years ago.
There is another way of checking, Raven says, pioneered by, among others, sociobiologist and evolutionary psychologist Edward O Wilson. There is a logarithmic relationship between the area of habitat and the species that inhabit it. Measure a patch of forest and count a sample of the species in it. Then compare it with another patch of forest 10 times smaller. The smaller one will have only half the sample species count. This has been shown in thousands of individual observations, he says. So destroying forests, piecemeal, is a way of extinguishing creatures.
There are various wild creatures that get along with humans and follow them everywhere: cockroaches, fleas, ticks, rats, cats, pigs, cattle, scavenger birds, lusty weeds. These invade little islands of ancient biodiversity, take over, and see the natives off the premises. And not just islands: one-third of all endangered plants in the continental US are threatened because of alien invaders, Raven says. In Hawaii, it is 100%.
Global warming is not going to help, either. What happens to the unique assembly of plants in the Cape region of Africa as the thermometer rises? They cannot migrate south. There is no land south of the Cape. So many will perish.
Ecosystems are not static. They change, naturally. They burn, are grazed or browsed, they regenerate, flood and silt up. But left to themselves, they go on providing services that humans and other creatures value. A mangrove swamp provides a habitat for shrimps. It cannot be improved by draining it for a tourist beach, or building a large city on it. Its natural value would be dissipated.
"An ecosystem itself undamaged is very, very resilient, and the more simplified it gets, the less resilient. Globally, what we are doing is simplifying them all, simultaneously, which is a very dangerous large-scale experiment," Raven says.
Plants are a lifelong obsession of Raven - any plants. "I was so excited and pleased by so many kinds of plants where I was first getting used to them as a teenager, and even now I can look at individual kinds of plants and be very, very excited.
"The florid nature of a really beautiful orchid or some kind of very rare plant that you see for the first time is really amazing. There are some Chinese monkshoods, for example, in a garden outside my office, and every year they come up and each time I see them I just get completely excited by the intricacy of their flowers, and how beautiful they are, and the fact that they are blooming.
"Then when I see photographs of really bizarre species of monkshoods from high elevations in southern China, I just say, 'Oh my gosh.' "
Raven was born in China in 1936 and educated in California. "I grew up in San Francisco and took plants and collected them - and then through the rest of the Pacific states - and it never occurred to me that things were becoming extinct rapidly. I thought of the world as a natural place divided between cultivated and urban areas and what have you, and in the 1950s, the global population was far less than half of what it is now and certainly standards of affluence were nothing like they are now. By the mid-1960s, we really began to think in terms of environmental problems."
Even then, the concerns were more about the domestic environment, how people lived, the gap between rich and poor, and the dramatic, all too visible swelling of the human investment. Population growth rates were moving towards the highest percentages the world had ever seen.
"I remember an article in the New Scientist in the mid-1960s, where a physicist had calculated that at the rates of growth then prevalent, in something like several hundred years the mass of human bodies would be expanding away from the surface of the Earth at the speed of light, which began to put a fine point on it," he says. "By the end of the 1960s, it was beginning to become evident that species were becoming extinct rapidly."
He got letters about extinction from Norman Myers (once a district officer in Africa, now a professor at Green College, Oxford) which provoked some serious thinking.
In 1972, Raven chaired a National Science Foundation committee on the future of systematic and evolutionary biology. By then, it was obvious that tropical forests were being lost, and very rapidly. "Since we knew far less about them than we did about organisms found anywhere else, it was obvious that if we were going to derive biological generalities and really understand the structure of life on Earth, we needed to understand the interactions between them, the ways that they evolved, whether the kinds of behaviours that took place in them were like those in the well-known temperate communities or not," he says.
Raven went from Stanford University to the Missouri Botanic Gardens in 1971, and began turning a small city recreation with one or two researchers into a world-class research institution - mentioned in the same breath as Kew and New York Botanic Gardens - with 50 scientists, 100 support workers and big research projects in places such as Madagascar.
He has, for the past three decades, been one of a highly vocal scholarly group that has banged the drum for the environment. Time magazine dubbed him a "hero of the planet". He can - and on public platforms does - paint an alarming picture of the great human takeover; the domination, by just one species, of a home fashioned by 3bn years of evolution to be shared by 10bn species. The world is clearly becoming more homogeneous, Raven says. "But the way I see it, we are not dying. We are simply losing opportunities, and at some point we have got to become sustainable. The choice is not whether we are going to reverse things. They are not going to be reversed. The real choices are where to stabilise it or how far to go."
As he keeps pointing out, the human species is living as if it had more than one planet to occupy. Forty years ago, at Stanford, he and colleagues tried to calculate the economic cost of exporting humans to a star system likely to be orbited by habitable planets. They worked out that it would cost the entire gross economic product of the planet to ship just 12 people a year to Proxima Centauri or beyond. His message for the planet is, "Think, look at the big picture, and think again".
"If both the population and standards increase, then obviously you come up with an impossible picture, which is a clear signal that we must [change]. It is not a matter of choice, it is not a matter of social justice alone, it is not a matter of morality, it is not a matter of creating a sustainable world so that industrialised countries can benefit from it.
"We must reach a sustainable population level, sustainable levels of affluence or consumption, and we must find technologies that replace the ones we are using now."
Life at a glance
Born: Shanghai, 1936.
Education: Graduated, highest honours, Berkeley, 1957; doctorate, Los Angeles, 1960.
Career: Foreign member of more than 20 learned institutions, including the Chinese and Indian Academies of Science, and the Royal Society. First book was Native Shrubs of Southern California, 1966; author of more than 480 books and papers. Director of Missouri Botanic Gardens since 1971.
Family: Married, with three daughters and a son, none of them botanists.