Preservation of the world's remaining wilderness could be the ultimate bargain. Scientists and economists calculate that forests, wetlands and other natural ecosystems are worth far more to human economies than the farm or building land that could replace them.
They report today in the US journal Science that the wilderness converted to human use each year actually costs economies $250bn a year, every year.
Put another way, it would cost the world $45bn to extend and effectively protect threatened areas of temperate and tropical forest, mangrove swamps, coral reefs and so on. But in return, these global reserves would supply humans with at least $4,400bn in "goods and services".
This is a benefit-cost ratio of around 100-1. And that, they say, is a low estimate of the likely benefits of better and more sustained conservation.
"The economics are absolutely stark. We thought the numbers would favour conservation, but not by that much," said Andrew Balmford of the University of Cambridge.
David Constanza of the University of Vermont said: "We've been cooking the books for a long time by leaving out the worth of nature. Economics has traditionally focussed on the market. But we have been finding that a lot of what is valuable to humans takes place outside of the market."
Humans depend on insects to pollinate crops, on forests to recycle carbon dioxide, slow erosion and prevent floods, on estuarine swamps as fish hatcheries and to buffer towns from storms and tidal surges. Ultimately, natural ecosystems provide humans with food, water, air, shelter, fuel, clothing and medicines. In 1997 economists tried to put a price on the things nature supplies, and arrived at a total of £33 trillion a year. This year, with backing from the Royal Society for the Protection of Birds and the Department for the Environment, Food and Rural Affairs, British and US scientists did their sums again.
They surveyed 300 case studies of what happened when the natural environment was converted to human use, and chose five for closer analysis. These included the intensive logging of a Malaysian forest, a Cameroon forest converted to small scale agriculture and commercial plantation, a mangrove swamp in Thailand turned to shrimp farming, a Canadian marsh drained for agriculture, and a Philippine coral reef dynamited for fishing.
In each case the value of the natural ecosystem - as storm and flood protection, for sustainable hunting and tourism, or to soak up carbon dioxide - outweighed the returns from human use. The Malaysian forest would have been 14% more valuable left standing. The Canadian marsh would have returned 60% if left alone for hunting, trapping and fishing.
The research is published as world leaders prepare for the Johannesburg summit on sustainable development. Two thirds of the world's fisheries are already harvested beyond sustainability. One fifth of the world's topsoil has been lost in the last 50 years, along with one fifth of agricultural land and one third of forests. Forest destruction has slowed, according to Science today - but an area twice the size of Belgium is still vanishing each year.
"Natural capital is going to be more valuable as it becomes more scarce. In many cases we have passed the point where development is worth more to us than conservation," said Professor Constanza.