The polar bear and hippopotamus are for the first time listed as species threatened with extinction by the world's biodiversity agency.
They are included in the Red List of Threatened Species published by the World Conservation Union (IUCN) which names more than 16,000 at-risk species.
Many sharks, and freshwater fish in Europe and Africa, are newly included.
The IUCN says loss of biodiversity is increasing despite a global convention committing governments to stem it.
"The 2006 Red List shows a clear trend; biodiversity loss is increasing, not slowing down," said IUCN director-general Achim Steiner.
"The implications of this trend for the productivity and resilience of ecosystems and the lives and livelihoods of billions of people who depend on them are far-reaching."
Overall, 16,119 species are included in this year's Red List, the most detailed and authoritative regular survey of the health of the plant, fungi and animal kingdoms.
This represents more than a third of the total number of species surveyed; the list includes one in three amphibians, a quarter of coniferous trees, and one in four mammals.
"The more species we assess, the more threatened species we find," commented Jean-Christophe Vie, deputy co-ordinator of IUCN's species programme.
"And because it is such a massive effort to assess a species, to gather all the data, get it all peer-reviewed and so on, 16,000 is a massive underestimate of the true problem," he told the BBC News website.
Climate and hunting
Polar bears are particularly affected by loss of Arctic ice, which the IUCN attributes to climatic change.
They need ice floes in order to hunt seals and other prey; without it, their food supply will decline. There is also evidence that the snow caves where they raise their young are melting earlier in the year.
Polar bears are listed as Vulnerable to Extinction based on forecasts that their population will decline by 50% to 100% over the next 50 to 100 years.
In the tropics, the common hippopotamus has entered the Red List for the first time because the population in the Democratic Republic of Congo has declined spectacularly - by about 95% in a decade.
The country's turbulent political situation has allowed unregulated hunting for meat and for the ivory in their teeth.
"Regional conflicts and political instability in some African countries have created hardship for many of the region's inhabitants, and the impact on wildlife has been equally devastating," said IUCN chief scientist Jeffrey McNeely.
The common hippo's decline in DRC has led to a Vulnerable listing even though other African populations including the largest, in Zambia, have held up well.
The much less well known pygmy hippo has suffered from illegal logging and poor protection in several West African nations, leading to an upgrade in its status from Vulnerable to Endangered.
Included on the Red List for the first time is the black-tailed godwit, a bird familiar to British and European enthusiasts, which gains a Near Threatened classification.
The species returned to Britain in 1952, a century after persecution and habitat drainage made it locally extinct, and now appears to be impacted by habitat changes across Europe.
"It is a bird which we think has to have favourably managed areas in order to survive," said Grahame Madge, a spokesman for the Royal Society for the Protection of Birds (RSPB).
"It joins two other species found in Britain on the Red List, the corncrake and red kite, which also have a Near Threatened classification," he told the BBC News website.
"But conservation successes mean it is not inconceivable that red kites might be taken from the UK to bolster populations in other European countries."
For the first time, this year's Red List includes a comprehensive region-by-region assessment on some groups of marine animals.
It shows that sharks and rays - members of the elasmobranch group of fish - are disappearing at an unprecedented rate across the globe.
About 20% of the 547 species surveyed merit inclusion on the Red List.
Some of these are fish which were once common on dinner plates in the UK and surrounding countries. The angel shark has been declared Extinct in the North Sea and Critically Endangered globally, while the common skate's status has also been upgraded to Critically Endangered.
The IUCN says that with fisheries extending into ever deeper zones of the ocean which are largely unregulated, populations of many species are set to decline sharply.
"The desperate situation of many sharks and rays is just the tip of the iceberg," said Craig Hilton-Taylor of the IUCN Red List Unit.
"It is critical that urgent action to greatly improve management practices and implement conservation measures, such as agreed non-fishing areas, enforced mesh-size regulations and international catch limits is taken before it is too late."
In the Mediterranean, freshwater fish are faring even worse than their sea-going counterparts.
Fifty-six percent of the 252 species endemic to the Mediterranean are threatened with extinction, the IUCN says; while in East Africa, a quarter of freshwater fish are at risk, which could carry important consequences for a human population highly dependent on fish for protein.
It is not all doom and gloom. The first optimistic note is that the overall number of species in this Red List is not significantly higher than in the last edition published in November 2004, which numbered 15,589 species on the brink.
And the number of species believed to have gone extinct over the last 500 years has not changed, a reflection of the fact that conservation efforts tend to intensify as final oblivion approaches.
The IUCN notes some marked conservation successes among the much more frequent stories of a slide towards oblivion.
The number of white-tailed eagles has soared in many European nations, and the bird's status has been downgraded from Near Threatened to Least Concern.
A recent decision by the Indian government to phase out a veterinary drug which was poisoning the common vulture, causing numbers to fall by 97%, is also cited as a simple measure which can bring great success.
But the overall message is that biodiversity continues to decline, despite the UN Biodiversity Convention which commits governments to halt the trend by 2010.
Jean-Christophe Vie believes there is a vast gap between what governments have promised and the action they are taking.
"Everything we find shows that it doesn't happen," he said, "and there is very little chance to achieve this goal unless there is a drastic change and governments decide to tackle the roots of species extinction."