A terrestrial biome is a large region of land classified by its climate and the type of vegetation that dominates it. Temperature and rainfall are the two primary factors that determine which biome exists in a given area. The planet’s land surface is divided into roughly a dozen major biome types, from tropical rainforests near the equator to frozen tundra near the poles, each with distinct plant communities, animal life, and ecological dynamics shaped by local climate conditions.
How Climate Shapes Biome Boundaries
The biome concept is a way of organizing large-scale ecological variation across the globe. Regions that share similar temperature ranges and precipitation patterns tend to support similar types of plant life, even if they’re on different continents. A grassland in Argentina and a grassland in Kansas look and function in remarkably similar ways because the climate conditions driving plant growth are comparable.
As a general rule, plant height, density, and species diversity all decrease as you move from warm, wet climates toward cool, dry ones. Tropical forests near the equator support the tallest, most densely packed, and most species-rich plant communities on Earth. Move toward the poles or into arid interiors, and vegetation becomes shorter, sparser, and less diverse. This gradient is the backbone of how biomes are organized.
Tropical Forests
Tropical forests occupy the warmest, wettest parts of the planet, mostly within 23 degrees of the equator. Annual rainfall in these forests typically exceeds 2,000 mm, though some sites receive far more. Old-growth rainforests in Costa Rica, for example, receive between 4,000 and 9,000 mm of rain per year depending on elevation.
What makes tropical forests visually striking is their vertical layering. In lowland areas, canopy trees commonly exceed 15 meters, with scattered emergent trees rising far above. The difference between the tallest and middle-height trees can be 26 to 34 meters in lowland forests. At higher elevations, the canopy compresses dramatically, sometimes with only a 9-meter gap between median and maximum tree heights. This layered structure creates distinct habitats at every level, from the dark forest floor to the sunlit canopy, supporting more species per square kilometer than any other biome.
Deserts
Deserts sit at the opposite extreme, receiving the least rainfall of any biome. Most get fewer than 300 mm per year, with many averaging around 250 mm (about 10 inches). That scarcity of water defines everything about desert life.
Plants here have evolved specific strategies to survive prolonged drought. Cacti store water in thick stems and use it slowly over weeks or months. Shrubs minimize water loss by growing very few leaves or developing massive root systems that spread wide and deep to capture whatever moisture exists. Many perennial desert plants survive by going dormant during dry periods, then bursting into growth when rain finally arrives. These adaptations allow deserts to support more life than their barren reputation suggests, though plant cover remains sparse compared to wetter biomes.
Grasslands and Savannas
Grasslands cover roughly 22.8% of the Earth’s terrestrial land surface, making them one of the most widespread biome types. They occupy a middle ground between forests and deserts: enough rainfall to support dense plant cover, but not enough (or too seasonal) to support closed-canopy forest. When you combine grasslands with shrublands into a broader “rangeland” category, the coverage rises to about 37% of Earth’s ice-free surface.
What sets grasslands apart is their soil. Grassland soils, known as mollisols, develop deep layers rich in organic matter because grasses continuously grow and die back, depositing carbon underground. This makes grasslands nearly as important as forests for storing carbon, despite having far less visible plant material above ground.
Fire is essential to how grasslands function. Natural wildfires have shaped these ecosystems for over 300 million years, preventing tree seedlings from taking hold and keeping the landscape open. Humans have also used fire for millennia to manage grasslands, improving habitat for grazing animals and livestock. Without periodic burning, many grasslands would gradually convert to shrubland or forest. Today, prescribed burns remain a core tool in grassland management, while the frequency of unplanned wildfires continues to rise in semi-arid regions experiencing persistent drought.
Chaparral and Mediterranean Shrublands
Mediterranean-climate biomes exist in only five regions worldwide: the Mediterranean Basin, coastal California, central Chile, the Cape region of South Africa, and parts of southwestern Australia. Their defining feature is a seasonal rainfall pattern that reverses what most people expect. Winters are mild, humid, and rainy, while summers are warm and bone-dry. Annual precipitation falls between 25 and 100 cm.
This unusual pattern shapes the rhythm of life. Plants produce most of their biomass during the cool, rainy winter, then go dormant or die back in summer. Perennial grasses dry and wither by early summer, while annual plants set seed and die by mid-spring. Shrubs in these regions have evolved thick, waxy, evergreen leaves that resist water loss, a trait called sclerophylly. In drier areas, shrubs may drop their leaves entirely during summer, and annual wildflowers dominate the landscape only after favorable winter rains, spending unfavorable years stored as seeds in the soil.
Fire is a recurring force here, too. Chaparral shrubs have adapted to frequent burning, resprouting from root crowns or germinating from fire-triggered seed banks.
Tundra and Boreal Forest
At the cold end of the spectrum, tundra and boreal forest (also called taiga) are defined by extreme temperatures and short growing seasons. Tundra, found above the Arctic tree line and on high mountain plateaus, is characterized by permanently frozen ground called permafrost. This frozen layer lies beneath a thin “active layer” of soil that thaws during summer, and the depth and chemistry of these layers vary significantly. The surface organic layer, the mineral subsoil, and the permafrost transition zone each support distinct communities of soil organisms, reflecting how dramatically conditions change over just a few feet of depth.
Boreal forests form the largest terrestrial biome by area, stretching across northern Russia, Scandinavia, Canada, and Alaska. They’re dominated by cold-tolerant conifers like spruce, fir, and pine. Temperatures can plunge well below freezing for six months or more, and the growing season is compressed into a brief summer window.
How Humans Have Reshaped Biome Boundaries
The traditional biome map you see in textbooks shows an idealized version of the planet. In reality, between 75% and 95% of Earth’s terrestrial surface has been reshaped to some degree by human activity. This isn’t a recent phenomenon. Even 12,000 years ago, nearly three-quarters of Earth’s land was inhabited and influenced by human societies, including more than 95% of temperate woodlands and 90% of tropical woodlands.
Researchers now use a parallel classification called “anthromes” (anthropogenic biomes) to describe how human land use has transformed these landscapes. Anthromes classify land into three broad types. Wildlands have no human populations or intensive land use at all. Cultured anthromes have some human presence but less than 20% of the landscape converted to crops, pasture, or cities. Intensive anthromes have more than 20% of the land in intensive use. The spread of industrial agriculture, mechanized land clearing, chemical fertilizers, and global supply chains has accelerated the trend toward increasingly homogeneous landscapes, simplifying habitats and shuffling species around the world.
A recent assessment of 655 terrestrial ecosystem types across North and Central America found that 33% qualified as threatened, with 7% critically endangered, 14% endangered, and 13% vulnerable. These threatened ecosystems historically covered about 45% of the study area but now account for roughly 30%. Grasslands and Mediterranean shrublands tend to be among the most converted biomes globally, largely because their climates and soils are well suited for agriculture.