Biomes

If you look at the two pictures in Figure 1 below, you will see very few similarities. The picture on the left shows a desert in Africa. The picture on the right shows a rainforest in Australia. What is the most obvious difference between the two places? It could be that the desert does not have any visible plants, whereas the rainforest is densely packed with trees. What causes these two places to be so different? The main reason is climate.

Biomes and Climate

The two pictures above represent two different types of biomes: deserts and rainforests. A biome is a group of similar ecosystems that cover a broad area. Biomes are major subdivisions of the biosphere. They can be classified into two major types:

• Terrestrial biomes: biomes on land

• Aquatic biomes: biomes in water

Figure 1: Sahara Desert in northern Africa (left). Rainforest in northeastern Australia (right).

It is important to understand how climate influences biomes. Climate is the most important abiotic (non-living) factor affecting the distribution of terrestrial biomes of different types. Climate determines the growing conditions in an area, so it also determines what plants can grow there. Animals depend directly or indirectly on plants, so the type of animals that live in an area also depends on climate.

What Is Climate?

Climate is the average weather in an area over a long period of time, whereas weather is a day to day explanation. Weather and climate are described in terms of factors such as temperature and precipitation. The climate of a particular location depends, in turn, on its latitude (distance from the equator) and altitude (distance above sea level). Other factors that affect an area’s climate include its location relative to the ocean or mountain ranges. Temperature and moisture are the two climatic factors that most affect terrestrial biomes.

Temperature

In general, temperature on Earth’s surface falls from the equator to the poles. Based on temperature, climates can be classified as tropical, temperate, or arctic, as shown in Figure 2. Temperature also falls from lower to higher altitudes, for example, from the base of a mountain to its peak. This explains why the tops of high mountains in tropical climates may be snow-capped year-round.

The ocean may also play an important role in the temperature of an area. Coastal areas may have milder climates than areas farther inland at the same latitude. This is because the temperature of the ocean changes relatively little from season to season, and this affects the temperature on nearby coasts. As a result, many coastal areas have both warmer winters and cooler summers than inland areas.

Figure 2: Major climate zones based on temperature include tropical, temperate, and arctic zones. The tropical zone extends from the Tropic of Capricorn to the Tropic of Cancer. The two temperate zones extend from the tropical zone to the arctic or antarctic circle. The two arctic zones extend from the arctic or antarctic circle to the north or south pole.

Moisture

Based on the amount of water available to plants, climates can be classified as arid (dry), semi-arid, semi-humid, or humid (wet). The moisture of a biome is determined by both precipitation and evaporation. Evaporation, in turn, depends on heat from the sun. Worldwide precipitation patterns result from global movements of air masses and winds, which are shown in Figure 3. For example, warm, humid air masses rise over the equator and are moved north and south by global air currents. The air masses cool and cannot hold as much water. As a result, they drop their moisture as precipitation. This explains why many tropical areas receive more precipitation than other areas of the world.

Figure 3: This model of Earth shows the direction in which air masses typically move and winds usually blow at different latitudes. These movements explain why some latitudes receive more precipitation than others.

When the same air masses descend at about 30° north or south latitude (see Figure 3), they are much drier. This explains why dry climates are found at these latitudes. These latitudes are also warm and sunny, which increases evaporation and dryness. Dry climates are found near the poles, as well. Extremely cold air can hold very little moisture, so precipitation is low in arctic zones. However, these climates also have little evaporation because of the extreme cold. As a result, cold climates with low precipitation may not be as dry as warm climates with the same amount of precipitation.

Distance from the ocean and mountain ranges also influences precipitation. For example, one side of a mountain range near the ocean may receive a lot of precipitation because warm, moist air masses regularly move in from the water. As air masses begin to rise up over the mountain range, they cool and drop their moisture as precipitation. This is illustrated in Figure 4.

Figure 4: The windward side of this mountain range has a humid climate, whereas the leeward side has an arid climate. On the windward side, warm moist air comes in from the ocean, rises and cools, and drops its moisture as rain or snow. On the leeward side, the cool dry air falls, warms, and picks up moisture from the land. How has this affected plant growth on the two sides of the mountain range?

By the time the air masses reach the other side of the mountain range, they no longer contain moisture. As a result, land on this side of the mountain range receives little precipitation. This land is in the rain shadow of the mountain range. Many inland areas far away from the ocean or mountain ranges are also dry. Air masses that have passed over a wide expanse of land to reach the interior of a continent usually no longer carry much moisture.

Climate and Plant Growth

Plants are the major producers in terrestrial biomes. Almost all other terrestrial organisms depend on them either directly or indirectly for food. Plants need air, warmth, sunlight, water, and nutrients to grow. Climate is the major factor affecting the number and diversity of plants that can grow in a terrestrial biome. Climate determines the average temperature and precipitation, the length of the growing season, and the quality of the soil, including levels of soil nutrients.

Growing Season

The growing season is the period of time each year when it is warm enough for plants to grow. The timing and length of the growing season determine what types of plants can grow in an area. For example, near the poles the growing season is very short. The temperature may rise above freezing for only a couple of months each year. Because of the cold temperatures and short growing season, trees and other slow-growing plants are unable to survive. The growing season gets longer from the poles to the equator. Near the equator, plants can grow year-round if they have enough moisture. A huge diversity of plants can grow in hot, wet climates.

The timing of precipitation also affects the growing season. In some areas, most of the precipitation falls during a single wet season (such as in California), rather than throughout the year (such as in New England). In these areas, the growing season lasts only as long as there is enough moisture for plants to grow.

Soil

Plants need soil that contains adequate nutrients and organic matter. Nutrients and organic matter are added to soil when plant litter and dead organisms decompose. In cold climates, decomposition occurs very slowly. As a result, soil in cold climates is thin and poor in nutrients. Soil is also thin and poor in hot, wet climates because the heat and humidity cause such rapid decomposition that little organic matter accumulates in the soil. The frequent rains also leach nutrients from the soil. Thin, poor soil is shown in the left drawing of Figure 5. The right drawing shows thick, rich soil. This type of soil is generally found in temperate climates and is best for most plants.

Figure 5: The soil on the left has a thin layer of topsoil, the part of soil where most plant roots obtain moisture and nutrients. The topsoil is light in color, which means that it is poor in nutrients and organic matter. The soil on the right has a thicker layer of topsoil. Its dark color indicates that the topsoil is rich in nutrients and organic matter.

Biome Biodiversity and Adaptations

Because plants are the most important producers in terrestrial biomes, anything that affects their growth also influences the number and variety of other organisms that can be supported in a biome. Therefore, climate has a major impact on the biodiversity of biomes.

Biodiversity

Biodiversity refers to the number of different species of organisms in a biome (or ecosystem or other ecological unit). Biodiversity is usually greater in warmer biomes. Therefore, biodiversity generally decreases from the equator to the poles. Biodiversity is usually greater in wetter biomes, as well. Remember the desert and rainforest pictured in Figure 1? The biodiversity of these two biomes is vastly different. Both biomes have warm climates, but the desert is very dry, and the rainforest is very wet. The desert has very few organisms, so it has low biodiversity. Some parts of the desert may have no organisms, and therefore zero biodiversity. In contrast, the rainforest has the highest biodiversity of any biome on Earth.

Adaptations

Plants, animals, and other organisms evolve adaptations to suit them to the abiotic factors in their biome. Abiotic factors to which they adapt include temperature, moisture, growing season, and soil. This is why the same type of biome in different parts of the world has organisms with similar adaptations. For example, biomes with dry climates worldwide have plants with similar adaptations to aridity, such as special tissues for storing water (see Figure 6).

Figure 6:The large hollow leaves of an African aloe plant store water and help the plant survive in its arid biome. (right) Cacti like these are found in arid biomes of North America. They store water in their thick, barrel-like stems.

In biomes with a severe cold or dry season, plants may become dormant during that season of the year. In dormant plants, cellular activities temporarily slow down, so the plants need less sunlight and water. For example, many trees shed their leaves and become dormant during very cold or dry seasons. Animals in very cold or dry biomes also must adapt to these abiotic factors. For example, adaptations to cold include fur or fat, which insulates the body and helps retain body heat.