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Home›Ecology and environment

Biomes

A single hectare of Amazon rainforest can hold 100 to 300 species of tree, while the Arctic tundra has a growing season of only 10 to 12 weeks. See how temperature and rainfall sort the land into eight major biomes, find them on the globe, and then go under water, from the sunlit top 200 m of the ocean to the hadal trenches.

Progress

About this course

A single hectare of Amazon rainforest, about two and a half acres, holds on average 100 to 300 species of tree, while the Arctic tundra has a growing season of only 10 to 12 weeks. The course asks what sorts the land into biomes. Temperature decides how long plants can grow each year and whether water is liquid or locked up as ice, and precipitation decides whether there is enough water for trees, only for grasses, or barely for anything. A tropical wet forest gets from 125 cm to 660 cm of rain a year.

The six units are what a biome is, tropical rainforests and savannas, deserts and chaparral, temperate grasslands and forests, boreal forest and tundra, and ocean and freshwater biomes. Much of the rainforest's variety comes from its layers, from a sparse forest floor and an understory up to a closed upper canopy topped by a few emergent trees. In a subtropical desert the soil surface can pass 60 °C in the day and fall close to 0 °C at night, so many plants are annuals whose seeds lie dormant until rain comes. Fire, mostly started by lightning, is a natural part of grasslands, and restoring a prairie often means setting controlled burns on purpose.

The last unit goes under water. Sunlight rarely reaches farther than 200 m into the ocean, yet the average ocean floor lies about 4,267 m down. Life in the dark depends on food from above as dead plankton sinks, and around hydrothermal vents chemosynthetic bacteria form the base of a food web of their own.

Before you start

Biomes are set mainly by temperature and rainfall, so Seasons and climate zones is the best companion course. Food webs also helps, since each biome is described through the plants and animals that live there.

When you finish

Afterwards you can name the two factors that decide a land biome and explain why the same biome appears on several continents. You can compare rainforest, savanna, desert, chaparral, grassland, temperate forest, boreal forest and tundra, say why grassland soils are so fertile and why conifers keep their needles, and describe how light fades below 200 m in the ocean.

  1. 1What 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.
  2. 2Tropical rainforests and savannasA single hectare of Amazon rainforest, about two and a half acres, holds on average 100 to 300 species of tree.
  3. 3Deserts and chaparralIn a subtropical desert the soil surface can pass 60 °C in the day and fall close to 0 °C at night.
  4. 4Temperate grasslands and forestsTemperate forests can swing from −30 °C in winter to 30 °C in summer, and their trees drop every leaf before the cold arrives.
  5. 5Boreal forest and tundraIn the Arctic tundra the average winter temperature is −34 °C, and plants get only about 10 to 12 weeks to grow each year.
  6. 6Ocean and freshwater biomesSunlight rarely reaches farther than 200 m into the ocean, yet the average ocean floor lies about 4,267 m down.

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Sources for this course (19)
  • OpenStax Biology 2e (, per the page notice on 2026-10-17) 44.3 Terrestrial Biomes: eight major biomes set by temperature and precipitation
  • tropical wet forest 20 to 34 °C, 125 to 660 cm, 11 to 12 h of sun, 100 to 300 tree species per hectare (2.5 acres)
  • savannas in Africa, South America and northern Australia, 24 to 29 °C
  • subtropical deserts 15° to 30°, under 30 cm, soil surface above 60 °C, nights near 0 °C, as low as 2 cm
  • cold deserts 25 to below −30 °C
  • chaparral 65 to 75 cm, winter rain
  • temperate grasslands 25 to 75 cm
  • temperate forests −30 to 30 °C, 75 to 150 cm
  • boreal forest 40 to 100 cm
  • arctic tundra winter average −34 °C, summer 3 to 12 °C, growing season 10 to 12 weeks, permafrost (openstax.org/books/biology-2e/pages/44-3-terrestrial-biomes)
  • 44.4 Aquatic Biomes: photic zone about 200 m, Challenger Deep about 11,000 m, average ocean depth 4,267 m, abyssal zone 4,000 m or more, coral reefs within 30° of the equator and more than 4,000 fish species, estuaries, lakes, rivers, wetlands, bogs (.../pages/44-4-aquatic-biomes)
  • 46.1: ocean over 70% of the surface, freshwater 1.8%, phytoplankton 40% of photosynthesis (.../pages/46-1-ecology-of-ecosystems)
  • National Geographic Education, Biomes collection: six, eight or as many as 11 biomes (education.nationalgeographic.org/resource/resource-library-biomes)
  • Mongabay, Tropical forest loss falls in 2025, 29 April 2026, citing Global Forest Watch: Brazil, DRC and Indonesia top three for remaining tropical forest (news.mongabay.com/2026/04/tropical-forest-loss-falls-in-2025-but-world-still-off-track-on-deforestation-goals/)
  • OpenStax 45.6: species richness highest near the equator, lowest near the poles (.../pages/45-6-community-ecology)
  • NOAA National Ocean Service, How far does light travel in the ocean?: sunlight zone to 200 m, twilight 200 to 1,000 m, midnight 1,000 to 4,000 m, abyss 4,000 to 6,000 m, hadal 6,000 m and deeper (oceanservice.noaa.gov/facts/light_travel.html)
  • NOAA news releases on the Gulf of America hypoxic zone: 2017 record 8,776 square miles (noaa.gov/media-release/gulf-of-mexico-dead-zone-is-largest-ever-measured)
  • 2025 forecast 5,574 and measured 4,402 (noaa.gov/news-release/gulf-of-america-dead-zone-below-average-scientists-find)
  • 2026 measured 1,332, second smallest in 40 years, goal 1,900 by 2035 (noaa.gov/news-release/noaa-and-partners-find-smaller-than-anticipated-hypoxic-zone-in-gulf-of-america).

Checked: 2026-10-17