Increasing numbers of birds, mammals and amphibians have moved closer to extinction in the last several decades -- but not as far as they would have if no conservation measures at all had been enacted, according to a report published online Tuesday by the journal Science.

To assess the status of the world's vertebrates, a large, international research team lead by Michael Hoffmann of the International Union for the Conservation of Nature (IUCN) and Conservation International analyzed data for over 25,000 vertebrate species categorized on the IUCN Red List.

They report that one-fifth of species is classified as Threatened, and this figure is increasing. On average, 52 species of mammals, birds and amphibians move one category closer to extinction each year. The tropics, especially Southeast Asia, are home to the highest concentrations of Threatened animals, and the situation for amphibians is particularly serious.

Most declines are reversible, but in 16 percent of cases they have led to extinction. The researchers also asked whether conservation efforts such as establishing protected areas and adopting national legislation have made any measurable contribution to preserving biodiversity. By looking at species whose conservation status has improved in response to some type of conservation measure, Hoffmann and colleagues estimate that overall declines would have been approximately 18 percent worse without any conservation actions. Efforts that dealt with invasive species have been more effective than those attempting to address habitat loss or hunting, the researchers report.

A set of projections in an accompanying Review article also forecasts biodiversity declines during the 21st century, but with a wide range of possible outcomes.

"These papers move us toward a comprehensive picture of the current and future trajectories of the world's biodiversity. Such a picture is important for targeting the areas of greatest concern and also the areas of greatest uncertainty, thus providing a focus for policy and research," said Andrew Sugden, Science's Deputy and International Managing Editor.