In forests and on mountaintops and savannahs around the world, endangered and threatened animals cling to a precarious existence. Whether we can save their habitats from pollution, development, and other threats will determine whether they survive.

A new study aims to identify the terrestrial areas that are truly irreplaceable—vital for the preservation of mammals, birds, and amphibians.

Previous research and conservation efforts in the area have focused on expanding the global network of protected areas, said study co-author Ana Rodrigues, a conservation biologist with the Center for Functional and Evolutionary Ecology in Montpellier, France. "We don't have enough protected areas and we need to expand the network."

"[But] it has become clear to us that you can't just do that," said Rodrigues. "You also need to ensure the existing areas work and do what we need them to do."

Protected areas encompass 13 percent of land on Earth and about two percent of Earth's oceans, according to a 2012 United Nations Environment Programme (UNEP) report.

But the management plans and enforcement procedures for those places vary widely. Some parks, refuges, and other protected areas focus on protecting charismatic species that aren't in as much trouble as smaller, less conspicuous animals, Rodrigues said.

Rodrigues and colleagues hoped to identify areas that are officially protected but that are nonetheless in most need of attention. She stressed that all protected areas are important: "We're not saying we should drop any."

The conservation biologist hopes the new analysis will help managers and conservationists pinpoint protected areas that bear the brunt of the responsibility for sheltering mammals, birds, and amphibians—at least half of which are classified as endangered or threatened.

The data for other animal and plant groups wasn't as complete, so they weren't considered in this study. Information on marine protected areas is still in the nascent stages, so they were also excluded.

Here are six irreplaceable places highlighted in the study, published Thursday in the journal Science.

Sierra Nevada de Santa Marta, Colombia

View Images Sierra Nevada de Santa Marta in Colombia is the highest coastal mountain range in the world. Photograph by Robin Moore.

This natural park sits on Colombia's Caribbean coast. "It's a mountain chain that goes from the Caribbean Sea up to 4,000 meters [13,123 feet],” says Rodrigues. “It peaks very quickly and it has a huge diversity of habitat."

That includes tropical areas at lower elevations, temperate areas, and peaks that are continually covered in snow and ice.

It's an area rich in endemic species—those that occur nowhere else on Earth. But the expansion of agriculture threatens the region, said Rodrigues. (Learn more about Sierra Nevada de Santa Marta.)

Canaima National Park, Venezuela

View Images The Roraima black frog is one amphibian species endemic to the tepuis in Canaima National Park. Photograph by Charles Besancon.

"The geology is quite extraordinary," Rodrigues noted. "Basically you have these tables coming out from the savannah and forests underneath. Each one is its own world."

The tabletop mountains sprout straight up about 3,200 to 9,800 feet (1 to 3 kilometers). Some are so isolated that the only way for humans to reach their peaks is by helicopter.

Each "island in the sky" has its own community of plants and animals, many of which are found nowhere else on Earth. Some, like a group of tree frogs known as Tepuihyla, are actually thought to have climbed up the sides of tepuis to the cloud forests above.

Rain Forests of the Atsinanana, Madagascar

Designated as a World Heritage Site in 2007, these rainforests are protected by a collection of six national parks along eastern Madagascar.

About 80 to 90 percent of all the plant and animal groups in this area are endemic, according to the United Nations World Heritage Convention.

An island nation, Madagascar broke off from Africa about 60 to 80 million years ago. The plants and animals that hitched a ride there have been isolated for a long time, resulting in many unique species, including primates like lemurs.

Western Ghats, India

View Images The hills and mountains of the Western Ghats in India. Photograph by Kalyan Varma.

The mountains in this World Heritage Site are older than the Himalayas.

Running along the west coast of India, the Western Ghats influence India's monsoon weather patterns and are considered one of the world's eight "hottest hotspots" of biodiversity.

The northern Western Ghats house about a third of the plants, roughly half the reptiles, and around three quarters of the amphibians found in India, according to the World Wildlife Fund.

View Images A Black-chinned Laughingthrush in the Western Ghats. Photograph by Kalyan Varma.

Logging and agricultural activity threaten these mountains as people clear land to grow tea, coffee, and rubber and oil palm.

Central Highlands, Sri Lanka

These mountain forests are home to such endangered animals as the western purple-faced langur and the Horton plains slender loris. Less than a hundred Horton plains slender loris are thought to exist in the world.

The Highlands, comprising the Peak Wilderness Protected Area, Horton Plains National Park, and Knuckles Conservation Forest, was designated as a World Heritage site in 2010. It contains the largest tract of undisturbed mountainous and sub-mountainous rain forest in the country.

Udzungwa Mountains National Park, Tanzania

Although this mountainous area was submitted to the World Heritage commission for consideration as a potential World Heritage Site in the late 1990s, the Tanzanian government withdrew its application in 2011.

The national park cuts across the Eastern Arc mountain range and supports endemic species like the Iringa red colobus monkey and the Sanje crested mangabey monkey.