THE two river boats with an international team of marine biologists ploughed up and down the world's busiest river. They searched the Yangtze in China for six weeks, a river motorway swollen with container ships, coal barges and speed boats.

But the freshwater dolphin they were searching for, so distinct that it qualified as a mammal family in its own right, was nowhere to be seen and the conservation project which had been developed had come too late. The experts were forced to concede that the Yangtze River dolphin was now extinct.

Its demise - the heavy river traffic deafened its sonar increasing its risk of being torn or hit by propellers - is the first large vertebrate to have become extinct in 50 years. Yet the extinction of the dolphin is just the tip of a melting iceberg.

According to Mark Wright, conservations science advisor with the World Wildlife Fund (WWF), there are 16,000 endangered animals on the IUCN's (World Conservation Union's) Red List of Threatened Species and there is one reason why - man.

"For every one million species that exists, you would expect one to become extinct every single year," he says. "At the moment, we're running at anything from 100 to 1,000 times faster than that.

"The biggest threat to wildlife, in general, is that we're reducing the amount of habitat, ie cutting down forests, and changing habitats. Then there's over exploitation, just look at what we're doing in the North Sea with our fish stocks. Unless you can address all these things then we will be faced with more cases of the Yangtze River dolphin."

According to the Red List, one quarter of all mammals, one eighth of all birds and a third of all amphibians are endangered now. But some species are more vulnerable than others simply because of where they live.

Says Mr Wright: "In Hawaii, 80 per cent of species exist in isolation, so if you do something that affects that group, there is a greater risk of extinction. Whereas, if the house sparrow became extinct in Britain you would still find them elsewhere."

Endangered species are also considered to be threatened by the effects of global warming. A study led by Leeds University in 2004 estimated that more than one million species would be lost by 2050 as a result of climate change. It was considered that species living in mountainous areas would have a greater chance of survival because they could simply move uphill to get cooler. Those in flatter areas, such as Brazil, Mexico and Australia, were more vulnerable because a small change in climate would cause migration over vast distances.

The Costa Rican golden toad is believed to have been one victim. It relied on having high humidity and that moisture has been lost.

Even size matters. The bigger an animal, the longer it will take to reach maturity, making it vulnerable for exploitation by hunters, because stocks cannot be replaced quickly enough.

Says Nicholas Polunin, professor of marine environmental sciences at Newcastle University: "Some species also have very low reproductive rates and that applies to birds where they may only lay a small number of eggs each year, so if their eggs go they're really vulnerable."

Professor Polunin and his team are currently studying coral reefs. The corals contain symbiotic algae which make it possible to lay down carbonate rock, forming the basis for them to live. But recently the corals have been expelling the algae, threatening its very existence.

"We have to say that global warming is certainly an issue but I think its impact in the future will be exceeded by the direct impacts of human population growth and human consumption," he says. "Fish stocks are not getting extinct because of global warming, but because people have hunted them."

How we address the issues affecting endangered species is a key question. There are conservation projects and laws in place around the world to protect rare species. But some of them, as in the case of the Yangtze River dolphin, come too late.

In certain countries, it is a simple belief that man must come first, whatever the consequences.

"In countries such as China with rapid growth and an extensive population it is very hard to argue that people have got to go without," says Professor Polunin. "That's why you need massive financial donations and land or river space to be set aside. We have to get away from finance and economic issues and consider the bigger picture."

But it is a picture which will no longer contain the Yangtze River dolphin.