Conservationists in the US say the world is losing plant species at a rate which threatens its ability to grow enough food, and to exploit other plant-based products on which hundreds of millions of people depend.

A report by the Worldwatch Institute, based in Washington DC, says the genetic diversity of the plants humanity relies on is being eroded at a dangerous rate.

The report, "Nature's Cornucopia: Our Stake in Plant Diversity", says widespread loss of species and varieties is attacking the foundations of agricultural productivity.

The report's author, Dr John Tuxill, said: "The genetic diversity of cultivated plants is essential to breeding more productive and disease-resistant crop varieties".

No help from genetic manipulation

"But with changes in agriculture, that diversity is slipping away.

"Biotechnology is no solution. We are increasingly skilful at moving genes around, but only nature can create them.

"If a plant bearing a unique genetic trait disappears, there is no way to get it back."

Highly domesticated crops sometimes need wild genes

Yet plant breeders often turn to wild relatives of domestic crops in search of desirable traits like disease resistance.

Fifty years ago Chinese farmers were estimated to have been growing 10,000 varieties of wheat. By the 1970s, that figure had fallen to about 1,000.

The range of corn varieties grown by farmers in Mexico today is 20% of what they were raising in the 1930s.

Apart from food, there are other serious implications. The report says a quarter of all medicines prescribed in the USA are based on chemical compounds originally found in plants.

Globally, about 3.5 billion people rely on plant-based medicine for primary health care, and in one form or another plants supply most of their material needs.

According to the International Union for the Conservation of Nature, one plant species in eight is now at risk of extinction.

The main threats they face are habitat loss, pressure from competing non-native species, and over-harvesting.

Vanishing knowledge

The African cherry tree, for example, is in great demand, because its bark is widely used in developed countries for treating prostate disorders.

So the number of cherries in the highlands of Cameroon and other African countries is now severely depleted.

The report says that fewer than 1% of all plant species have been screened for bio-active compounds, and traditional knowledge of medicinal plants is declining even faster than the plants themselves.

Kew Gardens: not only prize exhibits, but a seed bank as well

Dr Tuxill says the world needs to do more to safeguard plant and seed diversity, and sometimes this will mean policy changes.

Organisations like the Convention on Biological Diversity require governments to develop policies for managing plant resources wisely.

But the report singles out the World Trade Organisation as an example of the sort of body that demands the dismantling of protective measures in the name of liberating trade.

Dr Tuxill says: "The bottom line is that we have to share both the economic benefits of plant diversity and the obligation for protecting it".