Biomes

Biomes (terrestrial and aquatic ecosystems) are distributed throughout the Earth’s surface. Terrestrial biomes occupy the landmass from North Pole to South Pole. Aquatic biomes occupy the bodies of water on Earth.

Background

Biomes are natural habitats for bacteria, protists, fungi, plants, and animals. Biomes maintain the natural life cycle of these organisms and preserve the products of geological processes on Earth. A is a source of shelter, rocks and minerals, and food and fiber for human needs.

Technical Definition

A terrestrial biome is a large characterized by a particular type of and soils with defined groups of highly adapted living organisms. Biome formation is influenced by warm temperature and heavy precipitation in the tropics and extreme cold and low precipitation near the poles. Most ecologists do not consider aquatic ecosystems as biomes and refer to them as “aquatic biomes,” which are classified based on the concentration of dissolved salts: less than 0.1 percent in freshwater biomes, 0.1 to 1.0 percent in estuaries, and more than 1.0 percent in marine biomes.

Climate and Biomes

Climate shapes terrestrial biomes. Climate is predominantly driven by the solar energy and atmospheric circulation. Air circulation is initiated at the equator, because the equator receives the greatest solar energy with the warmest air near the ground. Because of different air densities, warm air in the troposphere rises into the stratosphere and cools. Cool air in the stratosphere descends into troposphere and warms. This rise and fall pattern of circulating air starts at 0° (equator) to 30° latitude, then continues at 30° to 60° latitude, and ends at 60° to 90° latitude (poles).

There are six major atmospheric circulations: Three move from the equator to the North Pole; the other three move from the equator to the South Pole. At 0° latitude, the ascending warm, humid air from the troposphere cools and condenses as it reaches the stratosphere, releasing heavy rain to or near the equator. That the dominant biomes formed at the equator are the tropical rainforests is no accident. After releasing rain, the cool, dry air moves poleward and descends at 30° latitude. The descending cool, dry air becomes warm as it reaches the troposphere and then absorbs all the available moisture. Not surprisingly, the dominant biomes at 30° latitude are the deserts, where the warm, humid air splits. One air mass moves equatorward to recirculate at 0° latitude. The other moves poleward and rises at 60° latitude, releasing rain or snow while at the stratosphere. As a result, the dominant biomes at 60° latitude are the temperate forests and temperate grasslands. The cool, dry air at the stratosphere divides again 60° latitude. One air mass moves toward 30° latitude to descend and recirculate in the desert. The other moves poleward, then descends and releases the remaining moisture near the poles, where the arctic tundra biomes are formed.

Terrestrial Biomes

The are nine major terrestrial biomes.

Arctic Tundra. Arctic tundra is located in the Northern Hemisphere near the North Pole and covers 10 percent of Earth’s landmass. It has extremely long, freezing, and harsh winters, with very short (six-to eight-week) summers. It is considered “cold desert,” because it receives less than fifteen inches of rain or snow per year. Melting snow creates bogs in summer, but there are frozen layers of subsoil (permafrost) at least a meter deep that exist throughout the year. Soil is nutrient-poor. Only the low-growing grasses and dwarf woody shrubs adapted to extreme cold and a short growing season are found. No trees survive. Their roots cannot penetrate the permafrost. Few animal species live in tundra. In winters, ptarmigans, musk oxen, snowy owls, lynxes, arctic foxes, and snowshoe hares are found. Polar bears are common in the coastal regions. In summers, few migrating animals from taiga move to tundra. No reptiles are found, but mosquitoes survive.

Taiga. Taiga, also called boreal coniferous forest, exists south of tundra and covers approximate 3 percent of the Earth’s land surface. It is found in the northern parts of North America and Eurasia and along the Pacific coast of northern North America to Northern California. It has patchy and shallower permafrost than tundra, and has acidic, nutrient-poor soil. It has short summers and long, cold winters and receives as much as twenty-five inches of precipitation per year in high elevations and as little as three inches of precipitation annually in other areas. Evergreen conifers are adapted to these conditions, with low-lying mosses and lichens beneath the forest canopy. Seeds of conifers attract birds. Bears, deer, moose, beavers, muskrats, wolves, mountain lions, and wolverines inhabit the taiga.

Temperate Rain Forest. Temperate rainforest, a coniferous forest, stretches along the west coast of Canada and the United States, the southeast of Australia, and the south of South America. It has dense fog, mild winters, cool summers, and high annual precipitation of eleven to over thirty-five inches. With abundant rain and nutrient-rich soil, the temperate rainforests have retained some of the tallest conifers (such as coastal redwoods) and oldest trees, some as old as eight hundred years. Moisture-loving plants (mosses and ferns) grow on the tree trunks of evergreen conifers. Temperate rainforest is a habitat for squirrels, lynxes, and several species of amphibians, reptiles, and birds (such as the spotted owl).

Temperate Deciduous Forest.Temperate deciduous forest is located south of the taiga in eastern North America, eastern Asia, and much of Europe. Temperate deciduous forests have a moderate climate, with occasional hot summers and cold winters and high annual precipitation of thirty to sixty inches. They have long growing seasons ranging from 140 to 300 days. The soil is rich in minerals. The dominant trees are deciduous (oak, beech, sycamore, and maple), which shed their broad leaves in the fall and grow them in the spring. Under the forest’s canopy, understory trees and shrubs are found. Layers of growth in the forest are home for several insects and birds. Ground animals include rabbits, squirrels, woodchucks, chipmunks, turkeys, beavers, and muskrats.

Temperate Grasslands. Temperate grasslands include the South American pampas, the Russian steppes, and the North American prairies. Tall-grass prairies are found between Illinois and Indiana, whereas short-grass prairies extend from Texas to Montana and North Dakota. They have hot and dry summers and bitterly cold winters, with annual precipitation of ten to thirty-five inches. Grasses in these biomes produce a deep, dark, mineral-rich soil. Herbivore mammals (bison, pronghorn antelope, mice, prairie dogs, and rabbits) dominate the temperate grasslands. Hawks, snakes, badgers, coyotes, and foxes are the predators in this biome.

Shrubland. Shrubland, or chaparral, is composed of thickets of small-leaf evergreen shrubs (shorter than trees and without main trunks). Shrublands, with frequent fires in dry summers and winters of eight to forty inches of rain annually, are found along the cape of South Africa, the western coast of North America, the southwestern and southern shores of Australia, around the Mediterranean Sea, and in central Chile. The shrubland in California is called chaparral, because it lacks understory. Shrubs are fire-adapted and highly flammable. The seeds of many species require the scarring action of fire to induce germination. Other shrubs resprout from the roots after fire. Mule deer, rodents, scrub jays, and lizards inhabit the shrublands.

Deserts. Deserts exist near or at 30° north and south latitudes and cover approximately one-third of the Earth’s land surface. The dry air that descends in this region absorbs most of the available moisture, then moves away to the equator and to 60° latitude. Deserts receive less than ten inches of rain annually. The Sahara Desert of Africa and the Arabian Peninsula and the deserts of North America (Mojave, Chihuahuan, and Sonoran) have little or no vegetation. Organisms with specialized water-conserving adaptations survive, including cactus, agave, Joshua trees, and sagebrush plants. Hawks prey on lizards, snakes, roadrunners, and kangaroo rats.

Tropical Grasslands. Tropical grasslands, or savannas (such as African savannas), characterized by widespread growth of grasses with few interspersed trees, are found in areas with seasonal low rainfall and prolonged dry periods. Other savannas occur in South America and northern Australia. Savanna has an annual precipitation of twenty to thirty-five inches. Savanna soil is nutrient-poor. Acacia trees survive the severe dry season. Hoofed herbivore mammals (giraffes, elephants, zebras, and rhinoceroses) feed on tree vegetation and on grasses. Carnivores such as hyenas, lions, cheetahs, and leopards prey on herbivores.

Tropical Rainforests. Tropical rainforests are located in South America, Africa, Southeast Asia, and the Indo-Malayan region on or near the equator. Wet and dry seasons are warm year-round. Average annual rainfall is from 50 to 260 inches. Tropical rainforest soil is typically nutrient-poor, but plentiful rain supports the growth of diverse groups of woody and herbaceous plants. Some of the rains come from recycled water released by forest trees by transpiration. Of all the biomes, tropical rainforest is the richest, based on species diversity, productivity, and abundance of all organisms. Tropical rainforest has three levels: the canopy (the highest layer of the forest), the understory (middle layers of small trees and shrubs), and forest floor (ground layers of herbaceous plants). Epiphyte plants (such as bromeliads, orchids, ferns, and Spanish moss) gain access to sunlight by growing on trunks and branches of tall trees. Lemurs, sloths, and monkeys are tree-dwelling primates that feed on fruits. The largest carnivores in the tropical rainforest are the jaguars in South America and the leopards in Africa and Asia.

Aquatic Biomes

All aquatic biomes share three ecological groups of organisms: the plankton, nekton, and benthos. Plankton are classified into microscopic phytoplankton and large zooplankton. Phytoplankton are producers and include photosynthetic cyanobacteria and free-floating algae, which provide oxygen and food for heterotrophic organisms. Zooplankton are consumers, heterotrophic, nonphotosynthetic organisms that include protozoa, small crustaceans, and larvae of aquatic animals. Nekton are larger swimming animals such as turtles, fishes, and whales. Benthos are bottom-dwelling animals that attach themselves to a substratum (sponges, oysters, and barnacles), burrow themselves into soil (clams, worms, and echinoderms) or simply swim or walk on the bottom (crayfish, crabs, lobsters, insect larvae, and brittle stars).

Based on salt contents, the three major aquatic ecosystems are the freshwater, estuary, and marine ecosystems. Freshwater ecosystems, which contain less than 0.1 percent dissolved salts and occupy about 2 percent of the Earth’s surface, include flowing waters (streams and rivers), standing waters (ponds and lakes), and freshwater wetlands (marshes and swamps). While all freshwater habitats provide homes for animal species, greater vegetations are found in marshes (grasslike plants) and in swamps (trees and shrubs) than in flowing- and standing-water ecosystems. Estuaries occur where and meet, with salt concentrations of 0.1 to 1.0 percent. Temperate estuaries called salt marshes are dominated by salt-tolerant grasses. Tropical estuaries are called mangrove forests. Marine ecosystems, which contain more than 1.0 percent dissolved salts, dominate, occupying about 70 percent of the Earth’s surface. Marine biomes have three zones: the intertidal, pelagic, and benthic zones. The intertidal zone is the shoreline area between low and high tide. The pelagic zone is the ocean water (shallow or deep), where plankton and swimming marine organisms are found. The benthic zone is the ocean floor, where marine animals burrow. Coral reefs, kelp forests, and seagrass beds are part of the benthic zone.

History

The existence of aquatic and terrestrial ecosystems was discovered through fossil records. Aquatic biomes emerged before the terrestrial biomes. Approximately 542 million years ago, during the Cambrian period, organisms in marine biomes became diversified and included bacteria, cyanobacteria, algae, fungi, marine invertebrates, and first chordates. The first terrestrial biome existed when the first forest and gymnosperm appeared about 416 million years ago, during the Denovian period. About 359 million years ago, during the Carboniferous period, the formation of much more diversified forest occurred, which consisted of ferns, clubmosses, horsetails, and gymnosperms and which housed many insects, amphibians, and first reptiles. Flowering plants (angiosperms) later evolved and became the dominant organisms of most major biomes.

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