By Paul Rincon
BBC News Online science staff
More than 300 critically endangered species have no conservation protection in any part of their ranges, experts say.
The Snake-necked turtle is threatened by the pet trade (Oliver Roempp)
Despite increases in the amount of protected land worldwide, many ecosystems fall outside this network of safe havens, scientists say in Nature magazine.
This is because current protected areas do not represent enough of existing global biodiversity, the team claims.
They propose a shift in conservation planning to avoid species extinctions in the coming decades.
Currently, 11.5% of the Earth's surface is protected; 1.5% higher than the target set at the Caracas Congress in 1992.
The researchers say these kinds of targets are now deeply ingrained in national and international conservation planning.
But they argue that this needs to be replaced with an approach that takes into account patterns of diversity.
Dr Ana Rodrigues, of the Center for Applied Biodiversity Science (Cabs), in Washington, US, and her international colleagues used a technique called gap analysis to assess the current network of protection and identify holes in its coverage.
This involved comparing a map of over 100,000 protected areas to maps of 11,633 species from four species groups worldwide.
They found the relationship between protected areas and patterns of biodiversity was uneven.
"Different countries need different levels of protection. Countries with many economic resources can afford that protection," Dr Rodrigues told BBC News Online.
"Most places where we've found these gaps are amongst the poorest countries in the world - poorest from an economic perspective, but richest in biodiversity."
Countries with high densities of gap species include China, India, Sri Lanka and Madagascar.
26.6% of threatened amphibians are gap species like this Harlequin mantella (Franco Andreone)
"The biodiversity that these countries hold is not just national, it is a global patrimony. So it's also the responsibility of other countries to provide support for biodiversity in these ones."
Overall, 20% of threatened species were identified as so-called "gap species" with no protection.
Of the threatened species, 14% of mammals, 19.8% of birds, 10.1% of turtles and 26.6% of amphibians were gap species.
Co-author Lincoln Fishpool, of BirdLife International, commented: "[The study] found that the world's larger protected areas currently exclude 37% of all threatened bird species.
"Although this is better than if these areas had been chosen randomly across the planet, it still shows that we need to designate more sites."
The authors claim the number of species covered by the current network in their Nature paper may be an overestimate because they had to assume that protected areas are adequate for protecting all species and that species can be protected equally effectively in any part of their range.
Gustavo Fonseca, executive vice president for programmes and science at Conservation International, commented: "We should focus specifically on those places with the greatest concentrations of threatened and endemic species."