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Unit 4: Ecosystems // Section 2: Major Terrestrial and Aquatic Biomes

Geography has a profound impact on ecosystems because global circulation patterns and climate zones set basic physical conditions for the organisms that inhabit a given area. The most important factors are temperature ranges, moisture availability, light, and nutrient availability, which together determine what types of life are most likely to flourish in specific regions and what environmental challenges they will face.

As discussed in Unit 2, "Atmosphere," and Unit 3, "Oceans," Earth is divided into distinct climate zones that are created by global circulation patterns. The tropics are the warmest, wettest regions of the globe, while subtropical high-pressure zones create dry zones at about 30° latitude north and south. Temperatures and precipitation are lowest at the poles. These conditions create biomes—broad geographic zones whose plants and animals are adapted to different climate patterns. Since temperature and precipitation vary by latitude, Earth's major terrestrial biomes are broad zones that stretch around the globe (Fig. 2). Each biome contains many ecosystems (smaller communities) made up of organisms adapted for life in their specific settings.

Earth's major land biomes

Figure 2. Earth's major land biomes
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Source: United States Department of Agriculture.

Another way to visualize major land biomes is to compare them based on their average temperature ranges and rainfall levels, which shows how these variables combine to create a range of climates (Fig. 3).

Biome type in relation to temperature and rainfall

Figure 3. Biome type in relation to temperature and rainfall
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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. Table 1 compares some key characteristics of three of the forest biomes.

Table 1. Forest biomes. Source: UC-Berkeley Museum of Paleontology, "The World's 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 (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.

Earth's marine and freshwater biomes

Figure 4. Earth's marine and freshwater biomes
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Source: 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.

Species are not uniformly spread among Earth's biomes. Tropical areas generally have more plant and animal biodiversity than high latitudes, measured in species richness (the total number of species present) (footnote 1). This pattern, known as the latitudinal biodiversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. Figure 5 shows the gradient for plant species, but it also holds true for animals.

Plant species diversity

Figure 5. Plant species diversity
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Source: Barthlott, W., Biedinger, N., Braun, G., Feig, F., Kier, G., and Mutke, J. (1999): Terminology and methodological aspects of the mapping and analysis of global diversity. Acta Botanica Fennica 162, 103–110.

Why is biodiversity distributed in this way? Ecologists have proposed a number of explanations:

Of these hypotheses, evidence is strongest for the proposition that a stable, predictable environment over time tends to produce larger numbers of species. For example, both tropical ecosystems on land and deep sea marine ecosystems—which are subject to much less physical fluctuation than other marine ecosystems, such as estuaries—have high species diversity. Predators that seek out specific target species may also play a role in maintaining species richness in the tropics.

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