EcosystemDescription

Unit 4 : Ecosystems -5- www.learner.org

Figure 3. Biome type in relation to temperature and rainfall

Land biomes are typically named for their characteristic types of vegetation, which in turn influence what kinds of animals will live there. Soil characteristics also vary from one biome to another, depending on local climate and geology. compares some key characteristics of three of the forest biomes.

Table 1. Forest biomes.

Forest type Temperature Precipitation Soil Flora Tropical 20-25°C >200 cm/yr Acidic, low in

nutrients Diverse (up to 100 species/km2)

Temperate -30 to 30°C 75-150 cm/yr Fertile, high in nutrients

3-4 tree species/ km2

Boreal (taiga) Very low 40-100 cm/year, mostly snow

Thin, low in nutrients, acidic

Evergreens

Aquatic biomes (marine and freshwater) cover three-quarters of the Earth's surface and include rivers, lakes, coral reefs, estuaries, and open ocean (Fig. 4). Oceans account for almost all of this area. Large bodies of water (oceans and lakes) are stratified into layers: surface waters are warmest and contain most of the available light, but depend on mixing to bring up nutrients from deeper levels

Unit 4 : Ecosystems -5- www.learner.org

Figure 3. Biome type in relation to temperature and rainfall

Land biomes are typically named for their characteristic types of vegetation, which in turn influence what kinds of animals will live there. Soil characteristics also vary from one biome to another, depending on local climate and geology. compares some key characteristics of three of the forest biomes.

Table 1. Forest biomes.

Forest type Temperature Precipitation Soil Flora Tropical 20-25°C >200 cm/yr Acidic, low in

nutrients Diverse (up to 100 species/km2)

Temperate -30 to 30°C 75-150 cm/yr Fertile, high in nutrients

3-4 tree species/ km2

Boreal (taiga) Very low 40-100 cm/year, mostly snow

Thin, low in nutrients, acidic

Evergreens

Aquatic biomes (marine and freshwater) cover three-quarters of the Earth's surface and include rivers, lakes, coral reefs, estuaries, and open ocean (Fig. 4). Oceans account for almost all of this area. Large bodies of water (oceans and lakes) are stratified into layers: surface waters are warmest and contain most of the available light, but depend on mixing to bring up nutrients from deeper levels

Unit 4 : Ecosystems -6- www.learner.org

(for more details, see Unit 3, "Oceans"). The distribution of temperature, light, and nutrients set broad conditions for life in aquatic biomes in much the same way that climate and soils do for land biomes. Marine and freshwater biomes change daily or seasonally. For example, in the intertidal zone where the oceans and land meet, areas are submerged and exposed as the tide moves in and out. During the winter months lakes and ponds can freeze over, and wetlands that are covered with water in late winter and spring can dry out during the summer months. There are important differences between marine and freshwater biomes. The oceans occupy large continuous areas, while freshwater habitats vary in size from small ponds to lakes covering thousands of square kilometers. As a result, organisms that live in isolated and temporary freshwater environments must be adapted to a wide range of conditions and able to disperse between habitats when their conditions change or disappear.

Figure 4. Earth's marine and freshwater biomes

© United States Department of Agriculture.

Since biomes represent consistent sets of conditions for life, they will support similar kinds of organisms wherever they exist, although the species in the communities in different places may not be taxonomically related. For example, large areas of Africa, Australia, South America, and India are covered by savannas (grasslands with scattered trees). The various grasses, shrubs, and trees that grow on savannas all are generally adapted to hot climates with distinct rainy and dry seasons and periodic fires, although they may also have characteristics that make them well-suited to specific conditions in the areas where they appear.

Unit 4 : Ecosystems -9- www.learner.org

Figure 6. Energy and nutrient transfer through ecosystems

© Ohio Environmental Protection Agency. Nature Connections.

An ecosystem's gross primary productivity (GPP) is the total amount of organic matter that it produces through photosynthesis. Net primary productivity (NPP) describes the amount of energy that remains available for plant growth after subtracting the fraction that plants use for respiration. Productivity in land ecosystems generally rises with temperature up to about 30°C, after which it declines, and is positively correlated with moisture. On land primary productivity thus is highest in warm, wet zones in the tropics where tropical forest biomes are located. In contrast, desert scrub ecosystems have the lowest productivity because their climates are extremely hot and dry (Fig. 7).