Biomes Explained: Earth's Major Ecosystem Types

What Determines Biome Distribution

Two climatic factors, temperature and precipitation, are the primary determinants of biome distribution. Temperature decreases with increasing latitude and altitude, creating predictable gradients from equatorial regions to the poles and from lowlands to mountaintops. Precipitation patterns are more complex, influenced by global atmospheric circulation, ocean currents, proximity to large water bodies, and topography. Rain shadow effects, where mountain ranges block moisture-laden air masses, can create deserts immediately adjacent to well-watered regions.

The Holdridge life zone classification system maps biomes based on mean annual temperature, annual precipitation, and potential evapotranspiration. While useful as a framework, biome boundaries in nature are rarely sharp lines. They are broad transition zones called ecotones where characteristics of adjacent biomes intermingle. Climate change is shifting these boundaries, pushing biome transitions toward the poles and to higher elevations as temperatures warm.

Tropical Rainforests

Tropical rainforests occur near the equator where average temperatures remain between 25 and 28 degrees Celsius year-round and annual rainfall exceeds 2,000 millimeters, often considerably more. They are the most species-rich terrestrial biome, containing an estimated 50 to 75 percent of all terrestrial species despite covering only about 6 percent of the land surface. The Amazon Basin, Congo Basin, and Southeast Asian archipelago harbor the three largest remaining tropical rainforest blocks.

Rainforest structure is characterized by distinct vertical layers. The emergent layer consists of scattered giant trees that tower above the canopy, reaching heights of 60 meters or more. The canopy, a dense, continuous layer of treetops at 25 to 45 meters, intercepts most of the sunlight and is where the majority of photosynthesis occurs. Below the canopy, the understory receives only 2 to 5 percent of full sunlight, supporting shade-tolerant species. The forest floor is dark, humid, and covered with a thin layer of rapidly decomposing leaf litter. Tropical soils are often nutrient-poor because decomposition is so rapid that nutrients are quickly recycled into living biomass rather than accumulating in the soil.

Temperate Forests

Temperate forests occur at mid-latitudes where temperatures vary seasonally, with warm summers and cold winters. Temperate deciduous forests, found in eastern North America, Europe, and East Asia, are dominated by broad-leaved trees that shed their leaves in autumn to conserve water and energy during winter. These forests experience moderate precipitation of 750 to 1,500 millimeters annually and support rich understory vegetation during spring before the canopy fully closes.

Temperate rainforests, found along the Pacific coast of North America, southern Chile, New Zealand, and Tasmania, receive extremely high rainfall, often exceeding 2,000 millimeters, and are dominated by massive conifers or evergreen trees. The Pacific Northwest temperate rainforests contain some of the tallest and most massive trees on Earth, including coast redwoods, Douglas fir, and Sitka spruce. These forests store more biomass per hectare than any other terrestrial ecosystem, including tropical rainforests.

Boreal Forests and Taiga

The boreal forest, or taiga, is the largest terrestrial biome, stretching across northern North America, Scandinavia, and Russia in a nearly continuous band between roughly 50 and 65 degrees north latitude. It is dominated by cold-tolerant coniferous trees, primarily spruce, fir, pine, and larch. Winters are long and severe, with temperatures dropping below minus 40 degrees, while summers are short but can be warm. Annual precipitation is low, typically 300 to 900 millimeters, much of it falling as snow.

The boreal forest plays a critical role in the global carbon cycle. Its soils and peat deposits store approximately twice as much carbon as tropical forests, making it one of the largest terrestrial carbon reservoirs. Boreal forests are also particularly vulnerable to climate change, warming at roughly twice the global average rate. Increased wildfire frequency, insect outbreaks, and permafrost thaw are already altering boreal forest composition and carbon dynamics in ways that could accelerate global warming through positive feedback loops.

Grasslands and Savannas

Grasslands are biomes dominated by grasses and other herbaceous plants, with few or no trees. They occur where precipitation is sufficient to support dense vegetation but too low or seasonal to sustain closed-canopy forest, or where fire and grazing prevent tree establishment. Temperate grasslands include the North American Great Plains, the South American pampas, the Eurasian steppes, and the South African veld. They have deep, fertile soils rich in organic matter, making them among the most productive agricultural regions when converted to cropland.

Tropical savannas are grasslands with scattered trees, found across large areas of Africa, South America, Australia, and South Asia. The East African savanna is renowned for its large mammal populations, including wildebeest, zebra, elephants, and lions. Savanna ecosystems are maintained by the interaction of fire, grazing, and seasonal drought, all of which prevent trees from dominating. Removing any of these factors can shift savannas toward either forest or degraded grassland, depending on the direction of the change.

Deserts

Deserts are defined by aridity, receiving less than 250 millimeters of precipitation per year. They cover approximately one-third of the Earth land surface and occur on every continent. Hot deserts like the Sahara, Arabian, and Sonoran deserts experience extreme daytime temperatures but can be surprisingly cold at night due to the lack of cloud cover and humidity to retain heat. Cold deserts like the Gobi and Patagonian deserts have freezing winters and moderate summers.

Desert organisms have evolved remarkable adaptations to water scarcity. Many desert plants, including cacti and other succulents, store water in specialized tissues. Others have deep root systems that tap groundwater, waxy coatings that reduce evaporation, or the ability to complete their entire life cycle during brief rainy periods. Desert animals avoid heat through nocturnal activity, burrowing, estivation, and physiological adaptations that concentrate urine and reduce water loss. Despite their harsh reputation, deserts support diverse and highly specialized biological communities.

Tundra

Tundra occurs in the Arctic and at high elevations above the tree line, where cold temperatures, short growing seasons, and permafrost limit plant growth to low-lying vegetation including mosses, lichens, grasses, sedges, and dwarf shrubs. Arctic tundra circles the North Pole across Alaska, Canada, Russia, and Scandinavia. Alpine tundra occurs on mountaintops worldwide, sharing many characteristics with Arctic tundra despite often being at lower latitudes.

Permafrost, permanently frozen ground that can extend hundreds of meters deep, is a defining feature of Arctic tundra. Only the top layer, the active layer, thaws during summer, creating a waterlogged landscape dotted with ponds and bogs. Arctic tundra stores enormous quantities of organic carbon in frozen soils. As climate change thaws permafrost, this stored carbon is released as carbon dioxide and methane, creating a positive feedback loop that accelerates warming. The fate of permafrost carbon is one of the largest uncertainties in climate change projections.

Aquatic Biomes

Aquatic biomes are classified primarily by salinity (freshwater versus marine), water movement (still versus flowing), depth, and light availability. Freshwater biomes include rivers, streams, lakes, ponds, and wetlands. Marine biomes include the open ocean, coral reefs, estuaries, intertidal zones, and deep-sea environments. Aquatic ecosystems cover over 70 percent of the Earth surface and produce roughly half of global oxygen through phytoplankton photosynthesis.

Coral reefs, sometimes called the rainforests of the sea, are among the most biodiverse ecosystems on Earth, supporting an estimated 25 percent of marine species on less than 1 percent of the ocean floor. They are built by colonies of tiny animals called coral polyps that secrete calcium carbonate skeletons. Reefs depend on the mutualism between corals and photosynthetic zooxanthellae algae, a relationship that is disrupted by rising ocean temperatures, causing mass bleaching events that threaten reef ecosystems worldwide.

Human Modification of Biomes

Human activities have profoundly altered every biome on Earth. Agriculture has converted roughly 40 percent of the global land surface, replacing native vegetation with croplands and pastures. Temperate grasslands have been the most heavily transformed biome, with over 70 percent converted to agriculture in some regions due to their fertile soils. Tropical deforestation continues at alarming rates, driven by cattle ranching, soybean cultivation, palm oil production, and logging. Urban expansion replaces natural habitats with impervious surfaces, creating novel urban ecosystems with their own ecological dynamics.

Climate change is projected to shift biome boundaries significantly over the coming century. Boreal forests may advance northward into current tundra regions as temperatures warm, while southern boreal margins may transition to temperate forest or grassland. Alpine biomes are shrinking as tree lines move upslope. Some tropical forests may experience dieback if rainfall patterns change sufficiently, potentially converting to savanna or degraded grassland. These transitions have cascading effects on the species that depend on particular biome conditions and on the ecosystem services those biomes provide to human societies.