Biomes

• Tropical rainforests are found at latitudes near the equator. In these areas, average temperatures and rainfall are high throughout the year. This warm, wet climate allows tropical rainforests to have high biodiversity.

• Temperate forests are generally found at latitudes between 30° and 60° north and south of the equator. These forests experience well-defined seasons, with a distinct period of winter.

• Temperate deciduous (seasonal) forests have cold, dry winters, and hot, humid summers. The deciduous trees of these forests shed their leaves each winter to save energy.

• Temperate rainforests have mild, frost-free winters and rainfall that is evenly distributed throughout the year.

• Coniferous forests are found at latitudes between 50° and 60° north of the equator. These forests experience short, warm, moist summers and long, cold, dry winters. The coniferous trees of these forests have thin, needle-like leaves that do not shed in winter. They also produce seeds in cones, which forms the root term of the name coniferous.

• Deserts exist at many latitudes, but most form at latitudes around 30° north and south of the equator. Rainfall in deserts is low and highly variable. Deserts closer to the equator have warmer temperatures than deserts further from the equator. Different types of plants live in warm or cold deserts, but they all have special ways to conserve water in these arid environments.

• Grasslands can occur at any latitude. Grasslands experience seasonal drought, occasional fire, and grazing by herbivores. These features stop trees and shrubs from overgrowing grasses. Warmer grasslands with scattered trees are called savannas. Milder grasslands with very few trees are called temperate grasslands, which include prairies and steppes.

• Scrublands (also known as shrublands, chaparrals and woodlands) are located in western coastal regions between 30° and 40° north and south of the equator. Scrublands usually get more rain than deserts and grasslands, but less than forested areas. As a result, scrublands are dominated by shrubs and short trees.

• The tundra biome is located primarily in the arctic, and is the most inhospitable of the biomes. It is cold and dry, with a short growing season and a layer of permanently frozen soil called permafrost. The tundra is relatively low in biodiversity, but is home to various species of plants and animals that are adapted to the tundra's cold, dry conditions.

Many of Earth’s biomes are aquatic, or water-based. The characteristics of aquatic biomes are heavily influenced by the concentration of dissolved oxygen and nutrients in the water.

Some aquatic biomes are freshwater biomes, where the water contains little or no salt. Freshwater biomes include ponds, lakes, streams, and rivers, and are a vital source of drinking water.

Ponds and lakes are natural bodies of standing water. Ponds are smaller and may be seasonal, whereas lakes are larger and more permanent. Large lakes have four main zones.

The littoral zone is the top, near-shore layer of a lake. This zone is shallow with plentiful sunlight and nutrient inputs from the surrounding land. As a result, this zone supports a wide variety of plants and animals.

The limnetic zone (or open-water zone) is a lake’s sunlit surface layer where most photosynthesis takes place. It starts away from shore, just past the littoral zone, and extends to the depth penetrated by sunlight. This zone supports phytoplankton and zooplankton, along with freshwater fish.

The profundal zone (or deep-water zone) is a lake’s deep, open-water layer, which is too dark for photosynthesis. The water in this zone is cooler and contains less dissolved oxygen than the shallower zones. This zone supports fish adapted to these cooler waters.

The benthic zone includes the soil and soil organisms that live at the bottom of a lake. This zone is inhabited mostly by decomposers and other organisms that feed on dead and decaying material, and can withstand a low-oxygen environment.

A cross section view of a lake shows the surface of the land descending under water gradually to the deep lake bottom. A tree and some land plants are growing on the upland portion of the lake. Submerged plants are growing where the water barely covers the land, and a bracket labels this region as the littoral zone. A dotted line delineates the water away from shore in both the shallow water areas and the deep water areas. Few to no plants are shown growing in these areas. The shallow waters away from shore are labeled open-water zone (photic zone) and the deep waters away from shore are labeled deep-water zone (aphotic zone).

Deep lakes normally consist of distinct zones that are defined by their depth and distance from shore. Image credit: “Primary zones of a lake" by Geoff Ruth, CC BY-SA 3.0.*

Streams and rivers are flowing bodies of water that drain the landscape, and are important agents of erosion. The flow of a typical stream or river can be characterized by three zones.

Streams and rivers start at the source zone, or headwaters, where water at high elevations collects from precipitation and snowmelt. This water is cold, fast moving, has lots of dissolved oxygen, and a low nutrient content. There are usually only a few plant species that grow near a river's source.

Headwaters merge together to form the transition zone of a river. Here, the river is wider, slower moving, has less dissolved oxygen, and carries more sediment than the source zone. The water is warmer and more nutrient-rich, so this zone tends to support a larger variety of plants and animals.

From the transition zone, a river meanders toward its mouth through a flat, wide floodplain zone. This zone forms because, occasionally, the bulk of water in the river becomes greater than the river channel itself, and water spills out onto the land forming wetlands and temporary lakes. The water in this zone usually contains more sediment and less dissolved oxygen than the transition zone. This zone's warm, nutrient-rich waters support the greatest variety of plants and animals.