One in three of all types of amphibians may yet to be found by scientists and remote tropical forests should get extra protection as the likely homes of such "unknown" creatures, a study said on Wednesday.
Despite centuries of research by biologists, the report estimated that 3,050 types of amphibians -- a group that includes frogs, toads, salamanders and newts -- were still to be described, compared to 6,296 species known to science.
Likewise, it estimated that at least 160 types of land mammals were yet to be found, about 3 percent of a known total of 5,398 ranging from elephants to tiny shrews.
"Most of these species are likely to be found in tropical forests," Xingli Giam, of Princeton University in the United States and lead author of the report, told Reuters. The Amazon, the Congo basin and Papua island were among likely sites.
The study estimated the number of unknown species from factors including past rates of discovery of new animals and the extent of unexplored habitats. As a rule, creatures found in recent years tended to be ever rarer, limited to small ranges.
"Many of the undescribed species ... are probably in danger of extinction and could well disappear before they are discovered," according to the study in the journal Proceedings of the Royal Society B by experts in the United States, Mexico and Singapore.
They urged conservation policies to target the least-disturbed tropical forests -- few of which were now set aside as formal protected areas.
Past studies have shown that human destruction of habitats -- such as forest clearance to make way for farms and towns, climate change, pollution and introduction of new species -- is a mounting threat to the diversity of life.
"Today's 'hidden' biodiversity need not vanish without a trace. It is up to us to try to prevent such a tragedy," they wrote.
Amphibians, living both in water and on land and breathing through their skin, are often important in food chains ranging from fish to birds. "They link the terrestrial and aquatic habitats," Giam said.
There were likely to be more undiscovered amphibians than mammals because they were often harder to spot -- living in swamps, or sitting immobile in trees. Mammals were often more active. The study did not consider other types of creatures.
Among recent discoveries, scientists found three new species of amphibian in Colombia last year including a toad with ruby-colored eyes. Among mammals, experts identified a snub-nosed monkey in remote forests in Myanmar in 2010.
And some vanishing species may have valuable genes.
The Australian gastric brooding frog, which incubated its young in its stomach, went extinct in the 1980s before scientists could study how it did not simply digest its young.
Its trick might have given clues to help people suffering from stomach ulcers.
Amphibians may also have clues for developing anti-microbial drugs or controlling malaria-spreading mosquitoes, Giam said.
Giam acknowledged that it may be hard to focus public attention on unknown species. "Here we try to make the unknowns more known," Giam said.