Getting Started
A biome is a large, distinct terrestrial region characterized by similar climate, soil, plants, and animals, regardless of where it occurs on Earth. The global distribution of these biomes is not random; it is a direct consequence of the planet's climate patterns, which are shaped by the uneven heating of the Earth, atmospheric and oceanic circulation, and geography. Understanding biomes is fundamental to understanding the distribution of life and natural resources across the globe.
What You Should Be able to Do
By the end of this section, you should be able to:
Describe how temperature and precipitation patterns determine the characteristics of a terrestrial biome.
Compare the climate, soil quality, and characteristic organisms of the world's major biomes.
Explain why the availability of natural resources, such as lumber and fresh water, varies significantly from one biome to another.
Analyze how global climate change could cause the geographic distribution of biomes to shift over time.
Key Concepts & Mechanisms
The defining characteristics of any terrestrial biome are rooted in its climate and the adaptations of the organisms living there. We can compare the major biomes by examining their key environmental features and the life they support. A biome contains characteristic communities of plants and animals that result from, and are adapted to, its climate.
| Feature | Tropical Rainforest | Temperate Seasonal Forest | Taiga (Boreal Forest) | Tundra | Desert | Savanna |
|---|---|---|---|---|---|---|
| Climate | Consistently warm and very high precipitation year-round. | Warm summers, cold winters; moderate precipitation spread throughout the year. | Cold winters, short, mild summers; low to moderate precipitation, mostly as snow. | Very cold, long winters; very short, cool summers; very low precipitation. | Can be hot or cold, but always very low precipitation. | Warm year-round with distinct wet and dry seasons. |
| Dominant Vegetation | Broadleaf evergreen trees forming a dense canopy; high species diversity. | Deciduous trees (e.g., maple, oak, hickory) that lose leaves in winter. | Coniferous trees (e.g., spruce, fir, pine) with needles and cones. | Low-growing plants like mosses, lichens, sedges, and dwarf shrubs. | Cacti and succulents with water-storing adaptations; sparse vegetation. | Grasses with scattered, drought-resistant trees like acacia. |
| Soil Quality | Nutrient-poor and acidic. Rapid decomposition and heavy rain mean nutrients are in the biomass, not the soil. | Fertile, nutrient-rich soil due to the slow decomposition of annual leaf litter. | Nutrient-poor, acidic soil due to slow decomposition of waxy conifer needles. | Thin soil with a layer of permafrost (permanently frozen subsoil). | Thin, rocky, or sandy soil with very low organic matter. | Porous soil with a thin humus layer; low fertility. |
| Human Impacts & Resources | Deforestation for agriculture and lumber; high biodiversity is a key resource. | Deforestation for agriculture and urban development; provides hardwood lumber. | Logging for timber and paper pulp; mining and oil extraction. | Vulnerable to climate change (melting permafrost); oil and gas extraction. | Off-road vehicle use damages fragile soil; some mineral extraction. | Overgrazing by livestock and conversion to agriculture can cause desertification. |
Key Models & Diagrams
The global distribution of biomes can be visualized based on the two most important climatic variables: average annual temperature and average annual precipitation. This relationship shows that specific combinations of these factors support the characteristic vegetation of each biome.
Biome Distribution by Climate Matrix
| Low Precipitation | Moderate Precipitation | High Precipitation | |
|---|---|---|---|
| Low Temperature | Tundra | Taiga (Boreal Forest) | (Not a major biome) |
| Moderate Temperature | Temperate Grassland / Shrubland | Temperate Seasonal Forest | Temperate Rainforest |
| High Temperature | Desert | Savanna / Tropical Seasonal Forest | Tropical Rainforest |
This matrix is a simplified model. Factors like soil type, geography, and seasonality also play crucial roles in determining a biome's exact location and characteristics.
Key Components & Evidence
Adaptation: A genetically determined trait that increases an organism's fitness in its environment. For example, the thick, waxy needles of coniferous trees reduce water loss during dry, frozen winters in the taiga.
Latitude: The distance from the equator. As latitude increases (moving toward the poles), average temperature decreases, which is a primary determinant of biome distribution.
Altitude: The height above sea level. Increasing altitude leads to lower temperatures, causing a succession of biomes up a mountain that can mirror the changes seen with increasing latitude.
Permafrost: A layer of permanently frozen soil found in the tundra. It prevents deep root growth, limits decomposition, and stores a massive amount of carbon.
Deciduous Trees: Trees that shed their leaves annually, typically during a cold or dry season. This adaptation helps conserve water and energy. They are characteristic of temperate seasonal forests.
Coniferous Trees: Cone-bearing, needle-leaved evergreen trees. Their shape helps shed snow, and their needles are adapted to cold, dry conditions, making them dominant in the taiga.
Resource Variability: The uneven distribution of biomes directly causes the uneven distribution of nonmineral resources. For example, lumber is abundant in the taiga and temperate forests but virtually nonexistent in the tundra or desert.
Biome Shift: The concept that the geographic ranges of biomes are dynamic. As the global climate warms, biomes are expected to shift towards the poles and to higher elevations, forcing species to migrate or adapt, or face extinction.
Skill Snapshots
Causation:
Low annual precipitation leads to the sparse vegetation and specialized animal life found in desert biomes.
The combination of cold temperatures and a short growing season in the tundra results in slow decomposition and the formation of a permafrost layer.
High, consistent rainfall in tropical rainforests causes leaching, which leads to nutrient-poor soils despite the high biodiversity.
Comparison:
The taiga is dominated by coniferous trees adapted to cold winters, while the temperate seasonal forest is characterized by deciduous trees that lose their leaves in winter.
Tropical rainforests have high biodiversity and rapid nutrient cycling, whereas the tundra has low biodiversity and extremely slow nutrient cycling.
A savanna's climate is defined by distinct wet and dry seasons, while a temperate grassland's climate is defined by distinct hot and cold seasons (summer and winter).
Change and Continuity Over Time:
Baseline: For millennia, the global distribution of biomes has been relatively stable, determined by long-term climate patterns.
Change 1: Due to anthropogenic climate change, the taiga is migrating northward, encroaching on areas that were previously tundra.
Change 2: Increased frequency of drought and fire, linked to climate change, is threatening to convert parts of temperate forests into shrubland or grassland ecosystems.
Continuity: The fundamental principle that climate (temperature and precipitation) is the primary driver of a biome's location and characteristics remains constant, even as climate patterns themselves change.
Common Misconceptions & Clarifications
Misconception: Deserts are always hot.
- Clarification: Deserts are defined by their lack of precipitation (typically less than 25 cm per year), not their temperature. Cold deserts, such as the Gobi Desert in Asia, exist where winters are extremely cold.
Misconception: The soil in lush tropical rainforests must be incredibly fertile.
- Clarification: Tropical rainforest soil is actually nutrient-poor. The vast majority of nutrients are locked up in the living biomass. Heavy rainfall leaches soluble nutrients from the soil, and rapid decomposition means nutrients are immediately reabsorbed by plants rather than accumulating in the soil.
Misconception: Biomes have sharp, clear boundaries.
- Clarification: The transition from one biome to another is typically gradual, occurring over a transition zone called an ecotone, which contains a mixture of species from both adjacent biomes. Biome maps are generalizations of these complex gradients.
Misconception: All grasslands are the same.
- Clarification: Grasslands vary significantly. Tropical savannas are warm year-round with wet and dry seasons, supporting grazing herds. Temperate grasslands (prairies) have cold winters and hot summers and have historically supported large grazers like bison.
One-Paragraph Summary
Terrestrial biomes are large-scale ecosystems defined by their dominant plant life and shaped by regional climate, primarily temperature and precipitation. The global distribution of these biomes—from the icy tundra and coniferous taiga at high latitudes to the temperate forests and grasslands in the mid-latitudes, and the deserts and rainforests in the tropics—reflects the Earth's climatic gradients. This distribution directly influences the availability of natural resources like timber and water. Because the location of each biome is so tightly linked to climate, the worldwide distribution of biomes is dynamic; it has shifted in response to past climate fluctuations and is predicted to shift again significantly as a result of modern global climate change.