Mountain Biome
The mountain biome is unusual because it is really several biomes stacked vertically. As you climb a mountain, the air gets thinner and colder, and you move through climate zones almost like travelling north from the equator. The bottom slopes might be forest; the middle slopes grassland or rocky scrub; the top alpine tundra with only the toughest plants surviving. The tallest mountains end above the snow line in a zone of permanent ice. Almost a quarter of Earth's land surface is classed as mountain country, and over a billion people live in or just below it.
- Definition600 m+ above landLower than that and it is just a hill
- TallestMount Everest8,849 m, in the Himalayas
- Temperature dropapprox. 6.5 °C/kmPer kilometre of altitude
- Tree lineapprox. 3,500 to 4,000 mAbove this, no trees grow
- % of Earth's Landapprox. 25%Most are in Asia and the Americas
- Famous mountain animalSnow leopardHigh Himalayas and central Asia
Mountain zones from bottom to top
Temperature drops approx. 6.5 °C for every km of altitude gained.
High mountains let you experience polar and tropical climates within a day's walk.
Climate zones with altitude
Every 1 km you climb, the temperature drops by roughly 6.5 °C. Climb Mount Kilimanjaro from base to summit and you cross five climate zones in a few days. The base is hot tropical farmland and forest. Above approx. 1,800 m the rainforest takes over, dripping with moss and ferns. Higher still, the heather zone gives way to a strange alpine moorland with giant lobelias and groundsels found nowhere else on Earth. Above approx. 4,000 m almost nothing grows, and the final 1,000 m is bare rock, ice and snow. It is one of the few places on Earth where you can walk from the equator to the Arctic in less than a week.
The same pattern shapes every big mountain. Foothills feed into montane forest; above that lies sub-alpine woodland of stunted trees; then comes the alpine zone above the treeline, where only grasses, dwarf shrubs and cushion plants survive; and the very top is the nival zone, dominated by snow, ice and bare rock.
Where mountains are found
Most of the world's great mountain ranges sit along the edges of tectonic plates. The major ranges are:
- Himalayas (Asia): the tallest range, containing all 14 of the world's 8,000 m peaks including Everest and K2.
- Andes (South America): the longest mountain range on land, stretching 7,000 km along the western edge of South America.
- Rocky Mountains (North America): 4,800 km long, from British Columbia to New Mexico.
- Alps (Europe): home of the Matterhorn, Mont Blanc and the source of the Rhine, Rhone, Po and Danube.
- Atlas Mountains (North Africa): separate the Mediterranean from the Sahara.
- Great Dividing Range (Australia): runs the entire length of the east coast.
- Transantarctic Mountains: split Antarctica almost in two.
Life on a mountain
Mountain plants are typically short and tough, hugging the ground to avoid the wind. Cushion plants, alpine wildflowers and small bushes survive where trees cannot. The edelweiss of the Alps is covered in tiny hairs to protect it from UV light and cold. The giant senecios of the African mountains close their leaves around their growing tips at night to protect them from frost. Above the snow line, almost nothing grows except lichens stuck to bare rock.
Mountain animals include the snow leopard of the Himalayas, the Andean condor (a vulture with a 3 m wingspan), mountain goats, ibex, marmots, pikas, the yak of Tibet, and the chinchilla and vicuña of the Andes. Many high-altitude animals have specially adapted blood that can carry more oxygen in thin air. Tibetan people have a genetic adaptation, inherited from an ancient cousin of modern humans called the Denisovans, that helps them breathe at altitude.
People in mountain biomes
People have lived in mountains for thousands of years. The Sherpas of the Himalayas, the Quechua of the Andes, and the herders of the Atlas Mountains have all developed bodies and lifestyles adapted to thin mountain air. Sherpa lungs work more efficiently at altitude; Andean people have unusually high red blood cell counts. Mountain peoples often live by herding hardy animals (yak, llama, alpaca, sheep, goats) and growing crops on terraced fields cut into the steep slopes.
Mountain glaciers also provide drinking water to over a billion people downstream. The Himalayas alone feed seven of Asia's great rivers: the Indus, Ganges, Brahmaputra, Yangtze, Mekong, Salween and Huang He (Yellow River). Two billion people depend on those rivers for water, food and farming.
Threats and conservation
Mountain biomes are warming faster than the global average. The treeline is creeping upwards, squeezing alpine species into smaller and smaller habitats at the very top. Glaciers are retreating worldwide: the Alps have lost more than half their glacier volume since 1850, and Mount Kilimanjaro's famous ice cap may vanish entirely within a few decades. As glaciers shrink, the rivers they feed swing between dangerous summer floods and dry winters.
Other threats include mining (in the Andes and Rockies), tourist pressure (the litter and human waste left by climbers on Everest), and the breaking-up of mountain habitat by roads, ski resorts and dams. Many mountain animals, including the snow leopard, the Asiatic black bear and the Spanish ibex, are still classed as threatened despite years of conservation effort.
Deeper dive: how do mountains form?
Almost every great mountain range on Earth is the slow-motion result of plate tectonics. The Earth's crust is broken into around a dozen huge slabs of rock called tectonic plates, drifting on the partly molten layer beneath. When two plates collide, the rock has nowhere to go but up, and over millions of years it crumples into mountains. The Himalayas are a classic example: they formed when the Indian Plate slammed into the Eurasian Plate around 50 million years ago, and they are still rising approx. 5 mm a year as India keeps pushing north.
Other mountains form by volcanism, where molten rock rises through the crust and piles up at the surface. Mount Fuji in Japan, the Cascade Range in the United States and most of the Andes are volcanic. A few mountain ranges form by faulting, where a block of crust is pushed up along a fault line: the Sierra Nevada in California works this way, with one side slowly tilting up while the other drops.
Once mountains are up, weather starts pulling them back down. Wind, rain, ice and rivers wear them away in a process called erosion. The Scottish Highlands and the Appalachians of the United States are very ancient mountains, once as tall as the Himalayas, but hundreds of millions of years of erosion have worn them down to rolling hills. Almost every mountain range is in this slow tug-of-war between tectonic uplift pushing it up and erosion grinding it down.
For famous individual mountains and mountain ranges, including Everest, K2, Kilimanjaro and the Himalayas, see the Mountains section.