Mountains are vital economic and ecological resources, high in biodiversity and minerals alike. Their height triggers heavy precipitation which, coupled with the water-storing capacity of glaciers, gives them a vital hydrological role. Mountain regions are the sources of most of the world's major rivers and half the world's population is reliant on mountain water. A billion Chinese, Indians and Bangladeshis drink from rivers flowing out of the Himalayas. In arid countries such as Egypt, mountain sources provide more than 90 percent of the available water².

Their elevation allows mountains to harbor a great diversity of species and habitats within a small area, often forming islands of biodiversity that take their own evolutionary path and create high levels of endemism, such as in the Peruvian Andes. More than half the world's endemic bird species occur in tropical mountain regions³. Mountains also provide natural refuges for species during times of climatic change and stress.

Mountain terrain has deterred dense human occupation, for cities have nowhere to spread and access is difficult. Mountain communities have traditionally been isolated, developing particular skills to survive – such as transhumance pastoralism and cutting terraces on hillsides to protect soils, conserve water and provide flat land for cultivation. But such communities have often remained poor and at the margins of society. Mountain nations such as Bhutan, Lesotho, Nepal, Rwanda, Burundi and Ethiopia are among the poorest 20 in the world⁴. Within nations, mountains are often home to tribal groups and other minorities, such as the Tibetans, the Quecha in the central Andes and the Kurds in Turkey and Iraq. While some such groups find themselves increasingly marginalized, there are also opposite trends towards social and economic integration with the lowlands.

In countries with land shortages and growing populations, lowlanders may invade hill regions, causing deforestation and cultivating erosion-prone soils. Areas containing tropical mountain forests have had the fastest rates of both annual population growth and deforestation in recent years⁵. Examples include the Guatemalan Highlands and the Bolivian Altiplano.

Elsewhere, mountain regions are being abandoned by farming communities who tire of the meagre earnings and hard work. This is as true in developing countries such as Peru as in the European Alps and Pyrenees. Abandonment does not necessarily reduce environmental degradation but may increase it. Left untended, terraces on steep hillsides swiftly start to crumble away.

But mountains have other attractions for lowlanders. Some contain valuable minerals. And many are scenically beautiful, enticing skiers, mountaineers, trekkers and environmentalists, and offering alternative livelihoods to local communities. The Alps accommodate 100 million visitor-days a year. In the Himalayas, more than 250 000 pilgrims and trekkers climb to the Gagotri glacier, sacred source of the River Ganges, each year.

Valleys within mountain ranges have their own vulnerabilities. They become transport arteries and sites for urban development, where the surrounding mountains can trap urban air pollution. Valleys are also attractive sites for building reservoirs to supply water, generate hydroelectricity or provide flood protection. But reservoirs not only flood valleys and disrupt fluvial ecosystems, they also force displaced inhabitants into the hills, where they may cause further environmental damage. The Three Gorges dam currently being constructed on the River Yangtze in China is expected to displace up to 3 million people into surrounding hills.

Mountain ranges often become zones of conflict, particularly as many contested national borders run through them, for instance in Kashmir. Mountains also play host to disputes between national governments and ethnic minorities such as the Chechens in Russia, the Kosovans in Serbia and the East Timorese. Their rugged terrain may serve to house refugees from such conflicts as well as providing sanctuary for guerrillas and outlaws. Two thirds of the 34 armed conflicts in the world in 1993 took place primarily in mountain areas⁶.

Such conflicts may protect the environment by discouraging organized development and inward migration. But they may equally encourage illegal and environmentally destructive activities, such as logging. In Liberia, Cambodia and the Thai-Myanmar border region, intensive logging helped fund warring groups in the 1990s. Virtually all the world’s heroin and cocaine comes from three small mountain regions: on the borders of Pakistan-Afghanistan, Myanmar-Thailand-Laos and Bolivia-Colombia, causing massive deforestation and soil erosion⁷.

Mountain ecosystems face a massive test of their robustness from projected climate change. Warming is already melting many glaciers, fundamentally altering hydrology both in the mountain regions and downstream. For instance, glaciers cover 17 percent of the Himalayas and provide two thirds of the flow of the River Ganges. But at their present rate of decline all the glaciers in the middle and eastern Himalayas will have disappeared by 2035. Many mountain valleys are threatened by floods as lakes formed by melting ice are breached⁸.