What is a biome?

The chaparral of California and the scrub around the Mediterranean Sea are thousands of kilometres apart, yet both are shrublands where rain falls mostly in winter.

Same climate, same look

Fly from Los Angeles to southern Spain, or to the coast near Perth in Australia, and the hills look surprisingly alike: tough, low shrubs, dry summers and a fire season. The plants are different species, but they solve the same problems in similar ways. A biome is a large group of ecosystems that share this kind of climate and the kind of plants and animals it supports. On land, OpenStax Biology lists eight major terrestrial biomes, each set mainly by two things: temperature and the amount of precipitation.

The eight are tropical wet forest, savanna, subtropical desert, chaparral, temperate grassland, temperate forest, boreal forest and arctic tundra. Polar ice and high mountains are shown separately on most maps. National Geographic Education points out that scientists do not agree on one number: some count six biomes, some eight and some as many as 11. The categories are a tool for comparing places, not walls in nature, and every biome hides great variety. OpenStax compares the Sonoran Desert in Arizona, full of saguaro cacti, with the bare rocky desert of Boa Vista in Cape Verde.

Two numbers do most of the sorting

Temperature decides how long plants can grow each year and whether water is liquid or locked up as ice. Precipitation decides whether there is enough water for trees, only for grasses, or barely for anything. The table above, with values from OpenStax, shows how wide the rainfall ranges are. A tropical wet forest gets from 125 cm to 660 cm a year; a subtropical desert gets under 30 cm. Between them sit grasslands with 25 cm to 75 cm, and temperate forests with 75 cm to 150 cm. Where rain is too scarce for trees, grass takes over; where it is too scarce for grass, desert plants do.

Timing matters too. The chaparral gets 65 cm to 75 cm a year, about as much as a wet grassland, but most of it falls in winter, so summers are very dry and many plants rest through them. Two places with the same yearly total can therefore grow very different vegetation.

Where the climate comes from

If you have done the course on seasons and climate zones, you have seen why the tropics are warm all year and the poles have long dark winters: the height of the Sun and the length of the day change with latitude. Biomes follow the same pattern. Tropical wet forests lie along the equator. Subtropical deserts sit between about 15° and 30° north and south, centred on the Tropics of Cancer and Capricorn. Boreal forest stretches across Canada, Alaska, Russia and northern Europe, and tundra lies north of it. Oceans, mountains and winds bend the pattern, which is why the same biome turns up on different continents at similar latitudes.

You can try the sorting yourself with the table. A place with about 50 cm of rain a year, hot summers and cold winters fits a temperate grassland. Add more rain, around 100 cm spread through the year, and the same winters, and it fits a temperate forest. Keep the rain but make it warm all year and rain-soaked, and you move towards tropical forest. Real boundaries are blurry, and soil, fire and grazing animals shift them too.