Travel from the equator to the poles, or from the foot of a mountain to its summit, and you pass through a sequence of very different landscapes: rainforest, savanna, desert, grassland, forest, tundra and ice. These large-scale ecosystems, each defined by its climate and dominant vegetation, are called biomes. Understanding biomes explains why lions live on savannas and not in rainforests, and why Kenya and northern Australia look so alike despite having completely different animals.
What you will learn
- What a biome is and what determines where biomes occur
- How to read a Whittaker biome diagram
- The main features, plants and animals of the major land and aquatic biomes
- Why unrelated animals in similar biomes often look alike
What controls where biomes occur
Two climate variables do most of the work:
- Temperature, especially its yearly average and seasonality.
- Precipitation, including how much falls and when.
Global air circulation creates a predictable pattern. Warm, moist air rises at the equator, cools and drops heavy rain, producing rainforests. The now-dry air descends at about 30° north and south, producing the great deserts: the Sahara, Arabian, Kalahari, Australian and Sonoran. Temperature falls towards the poles and with altitude, so climbing a tall tropical mountain such as Kilimanjaro passes through biomes similar to a journey from the equator to the Arctic.
Soil, fire and grazing animals also shape biomes. Savannas, for example, are maintained by fire and by elephants; without them, many would become woodland.
The Whittaker biome diagram
In 1975 the ecologist Robert Whittaker plotted the world’s biomes on a simple graph of mean annual temperature against mean annual precipitation. Explore it below: hover over or tap each biome to see its climate and example wildlife.
The major land biomes
| Biome | Climate | Vegetation | Example animals | Where |
|---|---|---|---|---|
| Tropical rainforest | Hot and wet all year, usually over 2,000 mm of rain | Tall, layered evergreen forest | Jaguar, orangutan, gorilla, toucan, poison frog | Amazon, Congo Basin, Southeast Asia |
| Tropical seasonal forest | Hot, with a long dry season | Deciduous and semi-evergreen trees | Tiger, Asian elephant, sloth bear | India, Southeast Asia, Central America |
| Savanna | Hot, with distinct wet and dry seasons | Grassland with scattered trees | Lion, elephant, zebra, giraffe, kangaroo | Africa, northern Australia, Brazil’s Cerrado |
| Hot desert | Very dry, under about 250 mm of rain | Sparse shrubs, succulents | Camel, fennec fox, thorny devil, roadrunner | Sahara, Arabia, Australia, Sonoran |
| Mediterranean (chaparral) | Hot dry summers, mild wet winters | Evergreen shrubs, fire-adapted | Iberian lynx, California quail, many reptiles | Mediterranean, California, Chile, South Africa, southwest Australia |
| Temperate grassland | Cold winters, hot summers, moderate rain | Grasses, few trees | Bison, pronghorn, saiga, rhea | Great Plains, Eurasian steppe, Pampas |
| Temperate deciduous forest | Four seasons, moderate rain | Broadleaf trees that lose leaves in winter | Deer, black bear, badger, woodpeckers | Eastern North America, Europe, East Asia |
| Temperate rainforest | Mild and very wet | Giant conifers, mosses | Grizzly bear, spotted owl, salmon | Pacific Northwest, southern Chile, New Zealand, Tasmania |
| Boreal forest (taiga) | Long cold winters, short summers | Conifers such as spruce, fir and pine | Moose, lynx, wolf, wolverine | Canada, Alaska, Scandinavia, Russia |
| Tundra | Very cold, short growing season, permafrost | Mosses, lichens, low shrubs | Caribou, Arctic fox, musk ox, snowy owl | Arctic, and alpine zones on mountains |
| Polar ice | Extremely cold | None on land | Penguins (south), polar bears (north) | Antarctica, Greenland |
Aquatic biomes
Water covers about 71 percent of Earth’s surface, and aquatic biomes are defined mainly by salinity, depth, light and water movement.
- Freshwater: rivers and streams, lakes and ponds, and wetlands such as marshes and swamps. Freshwater covers less than 1 percent of Earth’s surface but holds about a tenth of all known species, and its wildlife is declining faster than that of any other biome.
- Estuaries and mangroves, where rivers meet the sea, are highly productive nurseries for fish and shellfish.
- Coral reefs cover less than 1 percent of the ocean floor but support around a quarter of all marine species.
- The open ocean is vast but mostly nutrient-poor, apart from upwelling zones where cold, nutrient-rich water rises, such as off Peru, Namibia and California.
- The deep sea, below about 200 m where sunlight fades, is the largest habitat on Earth, home to anglerfish, giant squid and hydrothermal vent communities.
- Polar seas are extremely productive in summer and support krill, whales, seals and penguins.
Convergent evolution: similar biomes, similar animals
Animals living in similar biomes on different continents often evolve similar shapes and habits, even though they are not closely related. This is called convergent evolution.
- The fennec fox of the Sahara and the kit fox of North American deserts both have huge ears for losing heat.
- The cacti of the Americas and the euphorbias of Africa look almost identical, with water-storing stems and spines, but belong to different plant families.
- Australia’s thylacine looked remarkably like a wolf despite being a marsupial.
- Hummingbirds in the Americas and sunbirds in Africa and Asia both hover to feed on nectar.
Case study: climbing Kilimanjaro
At 5,895 m, Kilimanjaro rises from savanna at its base through a belt of montane rainforest (with colobus monkeys and blue monkeys), a heath and moorland zone of giant groundsels and lobelias, an alpine desert where temperatures swing from freezing at night to hot by day, and finally to the glaciers of the summit. In a few days, climbers pass through a sequence of habitats resembling a journey from the equator to the poles, although the mountain’s glaciers are shrinking fast.
Try it yourself
Look up your town’s average annual temperature and precipitation. Where does it fall on the Whittaker diagram? Does the natural vegetation around you match the predicted biome? If not, what might explain the difference: farming, fire, soil or elevation?
Common misconceptions
- “Deserts are always hot.” Deserts are defined by low rainfall. The Gobi and Antarctica are cold deserts.
- “Rainforests have rich soils.” Most tropical rainforest soils are poor; nutrients are held in living plants and recycled quickly.
- “Biomes have sharp borders.” Most change gradually through transition zones called ecotones.
Key terms
- Biome: a large region defined by climate and dominant vegetation.
- Ecotone: a transition zone between two biomes or ecosystems.
- Permafrost: ground that stays frozen for at least two consecutive years.
- Upwelling: rising of cold, nutrient-rich deep water to the surface.
- Convergent evolution: unrelated species evolving similar traits in similar environments.
Quick quiz
Test what you have learned. Scoring 60 percent or more marks this module as complete.
Further reading
- Whittaker, R. H. (1975). Communities and Ecosystems.
- Olson, D. M. et al. (2001). “Terrestrial ecoregions of the world: a new map of life on Earth.” BioScience.