Biosphere

DEFINITION: The zone within which all life on earth exists

The conceptualization of all life on earth as part of one large, integrated system, the biosphere, helps to encourage understanding of the interrelatedness of all ecosystems and biotic communities.

The concept of the biosphere was introduced in the nineteenth century by Austrian geologist Eduard Suess. The biosphere is the zone, approximately 20 kilometers (12 miles) thick, that extends from the floors of the earth’s oceans to the tops of the mountains, within which all life on the planet exists. It is thought to be more than 3.5 billion years old, and it supports nearly one dozen biomes, regions of similar climatic conditions within which distinct biotic communities reside.

Compounds of hydrogen, oxygen, carbon, nitrogen, potassium, and sulfur are cycled among the four major spheres—biosphere, lithosphere, hydrosphere, and atmosphere—to make the materials that are essential to the existence of life. The most critical of these compounds is water, and its movement between the spheres is called thewater cycle. Dissolved water in the atmosphere condenses to form clouds, rain, and snow. The annual precipitation for a region is one of the major controlling factors in determining the terrestrial biome that can exist. The water cycle follows the precipitation through various paths leading to the formation of lakes and rivers. These flowing waters interact with the lithosphere to dissolve chemicals as they flow to the oceans, where about one-half of the biomes on earth occur. Evaporation of water from the oceans resupplies the vast majority of the moisture existing in the atmosphere. This cycle supplies water continuously for the needs of both the terrestrial and the oceanic biomes.

The biosphere is also dependent on the energy that is transferred from the various spheres. The incoming solar energy is the basis for all life. Light enters the life cycle as an essential ingredient in the photosynthesis reaction. Plants take in carbon dioxide, water, and light energy, which is converted into chemical energy in the form of sugar, with oxygen generated as a by-product. Most animal life reverses this process during the respiration reaction, where chemical energy is released to do work by the oxidation of sugar to produce carbon dioxide and water.

The incoming solar energy also has a dramatic interaction with the water cycle and the worldwide distribution of biomes. Because of the earth’s curvature, the equatorial regions receive a greater amount of solar heat than do the polar regions. Convective movements in the atmosphere (such as winds, high- and low-pressure systems, and weather fronts) and the hydrosphere (such as water currents) are generated during the redistribution of this heat. The weather patterns and general climates of earth are responses to these energy shifts. The seven types of climates are defined by mean annual temperature and mean annual precipitation, and there is a strong correspondence between the climate at a given location and the biome that will flourish.

Desert Biome

The major deserts of the world are located between 20 to 30 degrees latitude north and south of the equator. The annual precipitation in a desert biome is less than 25 centimeters (10 inches) per year. Deserts are located in northern and southwestern Africa, parts of the Middle East and Asia, Australia, the southwestern United States, and northern Mexico.

Deserts are characterized by life that is unique in its ability to capture and conserve water. Deserts show the greatest extreme in temperature fluctuations of all biomes: Daytime temperatures can exceed 49 degrees Celsius (120 degrees Fahrenheit), and night temperatures can drop to 0 degrees Celsius (32 degrees Fahrenheit). Most of the animals that live in desert biomes are active at night and retreat to underground burrows or crevices during the day to escape the heat. The water cycle in deserts rarely provides surface water, so plant life usually finds water through a wide distribution of shallow roots to capture the near-surface infiltration or a deep taproot system that finds groundwater located below the surface of dry streambeds. The plant life is characterized by scattered thorny bushes, shrubs, and occasional cacti. Animal life consists of an abundance of reptiles (mostly lizards and snakes), rodents, birds (many predatory types such as owls and hawks), and a wide variety of insects.

Deserts and semideserts cover approximately one-third of the land surface on earth. They continue to grow in size because of human influences such as and overgrazing.

Grassland Biome

Grasslands are found in a wide belt of latitudes higher than those in which desert biomes exist. Large grassland regions occur in central North America, central Russia and Siberia, subequatorial Africa and South America, northern India, and Australia. This biome flourishes in moderately dry conditions, having an annual rainfall between 25 and 150 centimeters (10 and 60 inches). Precipitation and solar heating are unevenly divided throughout the year, providing a wet, warm growing season and a cool, dry dormant season.

The animal life in grassland regions is characterized by large grazing mammals, such as wild horses, bison, antelopes, giraffes, zebras, and rhinoceroses, as well as smaller herbivores, such as rabbits, prairie dogs, mongooses, kangaroos, and warthogs. This abundance of herbivores allows for a large development of secondary and tertiary consumers in the food chain, such as lions, leopards, cheetahs, wolves, and coyotes. Grasslands have rich soils that provide the fertile growing conditions for a wide variety of tall and short grasses. Within a single square meter of this healthy soil, several hundred thousand living organisms can be found, from microbes to insects, beetles, and worms. The profusion of these smaller life-forms fosters an abundance of small birds.

Grasslands have been environmentally stressed as humans have converted them to farmland because of their rich soils and to rangeland because of the grass supply. It is estimated that only 10 percent of the world’s original grasslands remain undisturbed by human development. Worldwide, overgrazing and mismanagement of rangeland have caused large tracts of fertile grassland to become desert or semidesert.

Tropical Rain-Forest Biome

Rain forests receive heavy rainfall almost daily, with an annual average of more than 240 centimeters (95 inches). Temperatures in these areas are fairly constant from day to day and season to season, with an annual mean value of about 28 degrees Celsius (82 degrees Fahrenheit). The combination of plentiful rain and high temperatures causes high humidity, allowing some plants to utilize the atmosphere for their water supply via “air roots.” This biome is also unique because the chemical nutrients needed to sustain life within it are almost entirely contained in the lush vegetation of the biosphere itself and not in the upper layers of soil of the lithosphere. The soils are thin and poor in nutrients.

The tropical rain forests contain a wider diversity of plant and animal species per unit area than any other biome. It is estimated that nearly two-thirds of all the plants and insects found on earth are contained in tropical rain forests. This enormous biodiversity is accommodated in part because each form of plant or animal occupies a specialized niche based on its ability to thrive with a particular level of sunlight that corresponds to a given height above the forest floor within the forest canopy. Numerous exotic insects, amphibians, reptiles, birds, and small mammals can coexist within a single canopy level. The plants growing in tropical rain forests provide ingredients found in 25 percent of the world’s prescription and nonprescription drugs. It is estimated that at least three thousand tropical plants contain cancer-fighting chemicals.

Tropical rain forests are being destroyed at a fast rate for farmland, timber operations, mining, and grazing. It has been estimated that most of the animal and plant diversity of the rain forests will be lost by the year 2050. Further, if the high rate of destruction continues, the release of carbon dioxide into the atmosphere from the burning of biomass and fossil fuels may no longer be offset by plant consumption, and the balance of the carbon cycle will shift toward a higher concentration of carbon dioxide in the atmosphere.

Temperate Forest Biome

Temperate forests exist in areas where temperatures change dramatically during the four distinct seasons. Temperatures fall below freezing during the winter, with warmer, more humid conditions during the summer. Rainfall averages between 75 and 200 centimeters (30 and 80 inches) per year. This biome is often divided into forests with broad-leaved deciduous (leaf-shedding) trees and those with coniferous (cone-bearing) trees. Deciduous forests develop in regions with higher precipitation values, whereas the needle-like evergreen leaves of conifers have scales and thick, waxy coatings that allow them to flourish at the lower end of the precipitation range.

Deciduous forests develop more solid canopies with widely branching trees such as elm, oak, maple, ash, beech, and other hardwood varieties. The forest floor often contains an abundance of ferns, shrubs, and mosses. Coniferous forests are usually dominated by pine, spruce, fir, cedar, and hemlock trees. Some deciduous trees, such as aspen and birch, often occur with the conifers. The coniferous forest floor is so acidic from decomposing evergreen needles that often only lichens and mosses can grow.

Temperate forests are home to diverse animal life. Common mammals of deciduous forests include squirrels, chipmunks, porcupines, raccoons, opossums, deer, foxes, black bears, and mice. Snakes, toads, frogs, and salamanders exist alongside a large bird of thrushes, warblers, woodpeckers, owls, and hawks. The larger animals of the coniferous forests include moose, elk, wolves, lynx, grouse, jays, and migratory birds.

Forests have traditionally been viewed as a limitless timber resource by the lumber industry. The vast forests of Europe were cleared one thousand years ago. This liquidation of temperate forest continues in Siberia, where 25 percent of the world’s timber reserves exist.

Tundra Biome

Tundras occur in areas near the Arctic ice cap and extend southward across the far northern parts of North America, Europe, and Asia. During the majority of the year, these largely treeless plains are covered with ice and snow and are battered by bitterly cold winds. Tundras are covered with thick mats of mosses, lichens, and sedges (grasslike plants). Because the winters are long and dark, tundra vegetation grows during the three months of summer, when there is almost constant sunlight.

Bogs, marshes, and ponds are common on the summer landscape because permafrost, a thick layer of ice that remains beneath the soil all year long, prevents drainage of melted waters. These wet areas provide perfect breeding grounds for mosquitoes, deerflies, and blackflies during the brief summer. These insects in turn serve as a source of food for migrating birds. Larger mammals, such as caribou, reindeer, musk ox, and mountain sheep, migrate in and out of the tundra. Some animals, such as lemmings, arctic hares, grizzly bears, and snowy owls, can be found in the tundra during all times of the year.

The tundra is the earth’s most fragile terrestrial biome. Vegetation disturbed by human activity can take decades to replenish itself. Roads and pipelines must be constructed on bedrock or layers of added gravel; otherwise, they will melt the upper layers of the permafrost.

Oceanic Biomes

Oceans cover 70 percent of the earth’s surface and contain 97.6 percent of the water of the hydrosphere. They play a primary role in regulating the earth’s distribution of heat, and they are central to the water cycle. Oceans are instrumental in the survival of all life on earth. In addition, oceans house more than 250,000 species of marine plants and animals that occur as six common biomes.

Coral reefs are coastal biomes that develop on continental shelves in regions of clear, tropical waters. Collectively, coastal biomes make up only 10 percent of the world’s ocean area but contain 90 percent of all ocean species. These are the regions where most commercial fishing is done. The vast open oceans contain only 10 percent of all oceanic species. Vegetation is mostly limited to free-floating plankton. Exotic bottom fauna exist on deep hydrothermal vents. Animal life includes whales, dolphins, tuna, sharks, flying fish, and squids.

A number of transitional biomes also exist at the ocean-land interface. The intertidal biome can be composed of sandy beaches or more rocky zones that are covered by water only during periods of high tide. A variety of crustaceans and mollusks are found on wet sandy beaches, whereas rock tidal pools contain kelp, Irish moss, and rockweed, all of which compete for space with snails, barnacles, sea urchins, and starfish.

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