Thousands of species in danger of extinction in the wild may survive only in captivity. The annual 'Red List' of extinct and endangered species to be published on Wednesday by the World Conservation Union is expected to show another increase in the numbers under threat of being wiped out by habitat loss, hunting, alien predators and climate change.

Last year the union warned that the world faced 'the sixth great extinction of life on earth' as mammals, amphibians, birds, insects, fish and plants were being lost at 'unprecedented rates'. One in four mammals and one in eight bird species have been labelled 'threatened'.

News that the list will show another deterioration will prompt fresh warnings about the danger of ecosystems collapsing, leading to problems with food supply and other 'biological services' such as the provision of clean water.

Although some species are expected to be put into 'safer' categories after successful projects to protect them, more are believed to face a vulnerable future.

'It's a natural process: species are born, they evolve and they die out,' said Mark Wright, science adviser to WWF, formerly the World Wildlife Fund. 'What's different now [is] for every million species we'd expect one to die out per year; but we're ranging between 100 and 1,000 times faster than that.'

The union says that 784 species of plant and animal have become extinct in the past 500 years, with 65 recorded as extinct in the wild. It blames 'human actions' for this: including habitat loss due to urbanisation, agriculture and deforestation; hunting for meat, medicines and luxury products, and other modern problems - such as drugs for cattle which are blamed for poisoning 99 per cent of three species of vultures in India. Experts also warn that expected climate change will push many species out of their habitats and some close to extinction - most dramatically polar bears.

Vulnerable species range from the Iberian lynx and snow leopard to the less glamorous partula snail from French Polynesia. Some, like the Arabian oryx in Oman and Saudi Arabia, have been pushed to extinction in the wild, reintroduced, and are under threat again.

The Red List already monitors 41,000 species and is trying to increase that to 50,000-60,000 by next year.

The growing threats to species across the world should be a warning for humans, said Wright: 'These habitats provide us with food, water and so on: that's why we should be concerned.'

Experts also warn the world cannot rely on captive breeding programmes to 'save' species which become extinct in the wild. One reason is the interconnectedness of complex eco-systems: 'Every species on the planet has a role to play in nature: losing that is going to have an impact,' warned Nick Lindsay, acting head of field conservation for the Zoological Society of London. 'It's not just the food chain, take the linkage to prey: rhinos and elephants play a key role in keeping what would otherwise be closed [forested] environments open for other animals.'

Another fear is that when species die there is no incentive to protect their habitat; and in the case of 'flagship' species protected areas can lose their special status, leaving remaining ecosystems at the mercy of human damage, said Raul Matamoros, campaigns officer for the International Fund for Animal Welfare.

However, for many the problems of relying on animals in captivity are more personal. 'Animals have evolved to be in their environments and that's where we want to see them,' said Lindsay.

iucnredlist.org