Introduction to Land Ecosystems (Biomes)

Large ecosystems are the largest divisions of the biosphere. They have a typical climate and special adaptations of life. There are between 7 and 15 of these large ecosystems, also called biomes, depending on how you count them.

In this unit we will explore the various land ecosystems. A select group of them are listed below.

Large ecosystems differ from one another in two primary ways -temperature and moisture. Precipitation (rain, fog, snow) alone actually determines whether a region is a forest biome, a desert ecosystem, or a grassland ecosystem; temperature determines the specific kind of forest, grassland, or desert.

Temperature

Temperature is a key defining feature of an ecosystem, affecting not only the energy available to that area but also its climate. The intensity of the Sun in a given area determines the warmth of the ground layer and the rate at which decomposition occurs. Since the Sun’s rays fall most strongly around the equator, the organic material in the soil at the equator quickly decomposes. More fungi and animals can live in warm soil, and they can work around the clock breaking down organic matter. Metabolic processes go faster in warmer settings.

Rainfall and Evaporate

Rainfall determines whether an area is a forest, desert, or grassland. But there are significant variations within these categories. For a forest to prosper, for example, rainfall has to be at least 10 to 15 inches in the cold temperate zone; 35 inches or more in the warm temperate zone; and 60 inches or more in the tropical zone. Can you guess why these areas have different rain requirements? Water evaporates faster in warmer areas, so more rainfall is required.

Intensity of Light

Large ecosystems closer to the equator receive much more light, and hence energy, than biomes further away from the equator. It is therefore possible for plants near the equator to photosynthesize more and longer, providing a richer food source for animals.

The length of the growing season also determines what can live there. For example, annual plants, which have to be pollinated, come to flower, and seed, could not reproduce during the short growing season of the far north. Seeds in cones can take several years to mature. The tree line is set by the length of the growing season, as well as the warmth of summer in the North. There is not a marked seasonal difference in areas near the equator.

Soil Quality

Soil depth and quality are other important characteristics of biomes. Soils vary in how much plant material is available in the soil and how rapidly it decomposes. The absence of some minerals in soil can be enough to prevent a grassland from becoming a forest.

Plant and Animal Adaptations:

Each animal fills its own special place in an ecosystem, living usually in a particular place, associated with particular food sources. Consider how rabbits tend to wander in the grasslands and low shrubs eating leaves, whereas amphibians prefer still water to hunt for the insects, and the woodpecker knocks on dead branches to scour out insects.

Plants and animals have evolved over time to adapt to the conditions in various environmental habitats, or biomes. Even very close species have different adaptations.

An Example of Adaptation: Ground Squirrels

Ground squirrels in South Africa have adapted to desert areas by staying out of the sunlight at the hottest times of the day, by excreting dry urine and feces, and by lying flat on rocks to absorb their lower temperatures. To stay cool, they also moisten their heads and raise their tails over their heads as umbrellas.

Ground squirrels of the Alaskan tundra are forced to burrow deep into the ground to find warm dens. They then must refurbish those dens, mate, and make a new family all in the space of a few short months. During these months they are up at least 17 hours a day and are quite busy!

Human Impact

Humans have considerable impact on their local ecosystem. They remove plants, animals and soil and add different materials -- asphalt, concrete, steel. They reroute and dam rivers, and dump their extra industrial waste into water nearby. That’s just a start.

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(See next page for table of large ecosystems)

IMPORTANT LAND/VEGETATION ECOSYSTEMS:

Grasslands Grasslands are located in the center of continents. Their soils are rich, with a layer of decomposed grasses called humus. Some names for grasslands around the world include pampas, campo, and llanos in South America; steppes between Hungary and Mongolia; veldt in southern Africa; and prairie in North America.
Boreal Forest or Taiga The “spruce-moose” forests extend around the polar areas from Scandanavia and Sibera to Alaska and Canada. Average monthly temperatures range between –30 deg. C in the winter to 20 deg.C in the summer. Many evergreens, including spruce, fir, cedar, and pine.
Temperate Deciduous Forest Deciduous forests grow in areas where there are four distinct seasons, a lot of moisture and a long growing season for plants. Most of the eastern half of the United States is in this biome, as is much of central and northern Europe, eastern China, and Japan. Deciduous forests drop their leaves in colder seasons. Their flowers develop to seeds. The soil is cold, wet, and acidic, with a permafrost in the northern sections.
Tropical rainforest Temperate rain forests are found in coastal areas. Tropical, broad-leafed rain forests are concentrated around the equator, and are found in South and Central America, Africa, and Asia. Rain forests represent about one half the forests that remain on Earth. They receive typically 200 cm of rain per year and little light variation. They have many broadleaf plants, high biodiversity, and a long growth cycle.
Desert A desert has a lot of heat and not much water. Deserts receive less than 25 cm of rainfall each year and rainfall is very irregular. Parts of the Sahara can go years without any rain at all. (Polar regions have moisture but not enough heat.) The soil is dry, without organic content. Days are hot and nights are cold.