Major Biomes of the World

• Biomes or Biotic Regions is defined as a broad-scale collection of flora and fauna that although different in detail from ecosystem to ecosystem share some commonalities. Ecosystems within a biome are often similar in nutrients and energy available to plants and animals. This leads to similar types of flora and fauna across the biome, even though individual ecosystems within the biome differ in scale, structure, and function.
• The contraction or expansion of biome pattern and distribution is not solely a function of changing temperatures; it also displays changes in atmospheric pressure, humidity, amount of precipitation, wind directions, and other atmospheric factors. Biomes are also strongly controlled by the type of soil and other aspects related to the lithosphere, hydrosphere, and cryosphere.

Ecosystems fall into two major groups

• Terrestrial: Terrestrial ecosystems, which are dominated by land plants spread widely over the upland surfaces of the continents.
• Aquatic: Aquatic ecosystems include marine environments and the freshwater environments of the lands. Marine ecosystems include the Open Ocean, coastal estuaries, and coral reefs. Freshwater ecosystems include lakes, ponds, streams, marshes, and bogs.

Terrestrial Ecosystems

We divide terrestrial ecosystems into biomes. There are four principal biomes:

• Forests
• Grasslands
• Deserts
• Tundra

Forest Biome

Forests are large areas supporting the rich growth of trees. Depending on the climate and type of trees they are generally grouped into:

1. Tropical rain forests
2. Temperate deciduous forests
3. Boreal or north coniferous forests

Tropical Rain Forest

• The tropical rainforest occupies low-altitude areas near the equator in South America, Central and West Africa, and in the Indo-Malay peninsula and New Guinea regions. Although these areas are physically isolated, the forest growing in them shows great similarity in structure and function. These are found in the high rainfall areas on either side of the equator, having high temperatures and high humidity, and receive above 200 cm of rainfall per year. Soil is rich in humus.
• It is a broad-leaved evergreen forest of dense, prolific growth and an extremely diverse fauna and flora. The hot, wet tropical climate is highly conducive to plant growth and there is very little seasonality which means that the growing period extends throughout the year.
• All green plants strive to reach the light so that they either become very tall, or adopt a climbing habit or live as epiphytes (plants living on other plants but not deriving food from them). The dominant trees are extremely varied in species but have similar appearances, typically characterised by buttress roots, dark leaves and a thin bark. The leaves possess thick cuticles for protection against the strong sunlight, and drip tips whose probable function is to shed water rapidly, thereby aiding transpiration.
• These forests have a very rich biodiversity e.g. Brazilian tropical rain forests have more than 300 species of trees in an area of 200 square kilometer. Trees are tall growing up to 50 to 60 m. These forests also support epiphytes, like vines, creepers, woody creepers and orchid etc. These forests are rich in tree dwelling animals such as monkeys, flying squirrels, snails, centipedes, millipedes, and many insect species are common on the forest floor. Many snakes and mammals are adapted to live in the trees because this is where the bulk of the foliage exists.
• Providing the tropical rain-forest is undisturbed it is the most diverse and productive type of forest ecosystem, but if the canopy is depleted the soils soon become infertile. Nutrient cycling is rapid, as the vegetation is demanding, and decomposition is accomplished quickly by bacterial action.

Temperate Deciduous Forests

• This type of forest, dominated by broad-leaved deciduous trees, had a great extent in the past when it covered most of the temperate areas of Europe, eastern North America, eastern Asia, and small parts of South America and Australia. The temperate deciduous forest has probably been more modified by human activity than any other type of ecosystem.
• The temperate deciduous forest consists largely of trees that drop their leaves during the cold season. It is characteristic of the marine west coast and moist continental climates.
• There is a longer growing season, higher light intensity, and a moderate amount of precipitation of between 50 and 150 cm per annum. The temperature regime is also characterized by a lack of extremes but there is still a marked cold season which plants and animals must endure. The climatic zone it occupies is less extreme than that of the boreal forest.
• Trees common to the deciduous forest of eastern North America, southeastern Europe, and eastern Asia are oak, beech, birch, hickory, walnut, maple, elm, and ash. Where the deciduous forests have been cleared by lumbering, pines readily develop as second-growth forest.
• In Western Europe, the mid-latitude deciduous forest is associated with the marine west coast climate. Here, the dominant trees are mostly oak and ash, with beech found in cooler and moister areas. In Asia, the mid-latitude deciduous forest occurs as a belt between the boreal forest to the north and steppe lands to the south. A small area of deciduous forest is found in Patagonia, near the southern tip of South America.
• Larger amounts of nutrients are used and their movement is more rapid. There is a bulk return of nutrients from the trees with the leaf fall of autumn. Characteristically the leaf litter is nutrient-rich and decays by the action of bacteria to form mull humus. The soils associated with the temperate deciduous forest are varied but on the whole they are brown earths.

Boreal Forest or North Coniferous Forests

• Boreal forest is the cold-climate needle leaf forest of high latitudes .It occurs in two great continental belts, one in North America and one in Eurasia. These belts span their land masses from west to east in latitudes 45° N to 75° N and they closely correspond to the region of boreal
  forest climate.
• The area occupied by this formation has been subjected to severe glacial or periglacial activity and has much subdued relief and surface water. The conditions for life are harsh because of the adverse climate. The growing season is only of three or four months’ duration and even during this time; the energy input from solar radiation is small because of the high latitude. Temperatures are low throughout the year, although the average temperature of the warmest month of the year is higher than 10° C. In the winter the temperatures fall too many degrees below freezing and permafrost frequently extends into the northern edge of the forest. Precipitation ranges from 40 to 70 cm per annum, mostly falling as snow, the weight of which may cause mechanical damage to the trees.
• Despite the climate, coniferous trees forms dense canopies which intercept a great amount of light and precipitation so that conditions beneath are dark and dry. Consequently there is little opportunity for undergrowth to develop and very few other plants are associated with the coniferous trees.
• The boreal forest of North America, Europe, and western Siberia is composed of such evergreen conifers as spruce and fir, while the boreal forest of north-central and eastern Siberia is dominated by larch. The larch tree sheds its needles in winter and is thus a deciduous needle leaf tree.
• The combination of coniferous dominants which are low in nutrient demand, the lack of diversity, and the climatic conditions, results in slow, impoverished nutrient cycles. Most decomposition is fungal since bacterial activity will be slow in these conditions, and the resulting humus is the mor type. Characteristically the boreal forest is found growing on podzols which tend to become highly acidic.

Monsoon Forest

• Monsoon forest, also called dry forest or tropical deciduous forest. It is typically open, but grades into woodland, with open areas occupied by shrubs and grasses .Monsoon forest of the tropical latitude zone differs from tropical rainforest in that it is deciduous; that is, most of the trees of the monsoon forest shed their leaves due to stress during the long dry season, which occurs at the time of low Sun and cool temperatures.
• This forest develops in the wet-dry tropical climate, where a long rainy season alternates with a dry, rather cool season. They are located in the monsoon climate beyond the equatorial region between 10◦ and 25 ◦ and North and South of the equator. The countries are along the coastal regions of southwest India, Sri Lanka, Bangladesh, Myanmar, Thailand, and Cambodia, South western Africa, French Guiana, and northeast and southeastern Brazil.
• In the monsoon forest of southern Asia, the teakwood tree was once abundant, but it was cut down and the wood widely exported to the Western world to make furniture, paneling, and decking.

Grassland Biome

Grasslands are areas dominated by grasses. They occupy about 20% of the land on the earth’s surface. Grasslands occur in both in tropical and temperate regions where rainfall is not enough to support the growth of trees. Grasslands are known by various names in different parts of the world. Grasslands are found in areas having well defined hot and dry, warm and rainy seasons

Grassland ecosystems contrast with forest ecosystems in several ways. They have a much smaller biomass, of which a large percentage is made up of roots. Grasses are probably not as effective at precipitation interception as trees, except for the period of maximum growth. The grass form facilitates stem flow, and surface run-off is greater from grass-covered than from forested slopes. The annual primary productivity of a grassland ecosystem is only about an eighth or ninth of an adjacent forest area. The smaller standing crop also means that there are more limited nutrient reservoirs in grassland.

Two main types of grassland are normally distinguished: Temperate grasslands, in which woody growth is absent or negligible, and Tropical grassland (savanna) in which scattered trees are much more common.

Temperate Grasslands

• These include the prairies of North America, the steppes of Eurasia, the pampas of South America, and the veldt of South Africa. Smaller tracts occur in Australia and New Zealand. Precipitation in these areas ranges from 25 to 100 Cm per annum, and the grasslands extend over a wide range of soil conditions. Trees only occur on steep slopes or near water. The geographical isolation of these areas from each other has led to some species differentiation, but most other features are similar.
• The animals of the grassland are distinctive and feature many grazing mammals. The grassland ecosystem supports some rather unique adaptations to life. Animals such as jackrabbits and jumping mice have learned to jump or leap, to gain an unimpeded view of their surroundings.
• Tall grass prairie is a ground cover of tall grasses along with some broad-leafed herbs, named forbs. Steppe, or short-grass prairie, consists of sparse clumps of short grasses. Steppe grades into semi-desert in dry environments and into prairie where rainfall is higher. Steppe grassland is concentrated largely in the mid-latitude areas of North America and Eurasia.
• Prairie grasslands are associated with the drier areas of a moist continental climate, and steppe grasslands correspond well with the semiarid subtype of the dry continental climate. The Pampa region falls into the moist subtropical climate with mild winters and abundant precipitation.
• This grassland biome includes tall grass and short grass prairie (steppe). Tall grass prairie provides rich agricultural land suited to cultivation and cropping. Short-grass prairie occupies vast regions of semi-desert and is suited to grazing.

Tropical grasslands (Savannas)

• Tropical grasslands are commonly called Savannas. They occur in eastern Africa, South America, Australia and India. Savannas form a complex ecosystem with scattered medium size trees in grasslands.
• The savanna biome is usually associated with the tropical wet-dry climate of Africa and South America. Its vegetation ranges from woodland to grassland. In savanna woodland, the trees are spaced rather widely apart because there is not enough soil moisture during the dry season to support a full tree cover. The woodland has an open, park-like appearance. Savanna woodland usually lies in a broad belt adjacent to equatorial rainforest.
• Savanna biome vegetation is described as rain-green. Fires occur frequently in the savanna woodland during the dry season, but the tree species are particularly resistant to fire. The much greater diversity of tropical as opposed to temperate grasslands is often a function of the added variety afforded by wooded plants. In some cases the tree cover may be as much as 50 percent; in others, it may be nil. Marked contrasts exist in the appearance of the savanna during the year: the brown and withered short grasses of the dry season give way rapidly to tall lush growth with the arrival of the summer rains. The Ferralsolic soils of savanna areas frequently include near-surface lateritic crusts, creating an impermeable surface soil layer in which nutrients, especially phosphates and nitrates, are markedly lacking.
• As in the case of prairies, tropical grasslands tend to show little ecotone development, especially on margins adjacent to tropical rainforests. Overall, savanna boundaries on all continents reveal only a poor correlation with precipitation amounts or the duration of the rainy season.
• The African savanna is widely known for the diversity of its large grazing mammals. With these grazers come a large variety of predators— lions, leopards, cheetahs, hyenas, and jackals. Elephants are the largest animals in the savanna and adjacent woodland regions.

Desert Biome

• The desert is a highly evolved ecosystem that supports a multitude of plants and animals. The desert biome includes semi-desert and dry deserts and occupies the tropical, subtropical, and mid-latitude dry climates. Desert plants vary widely in appearance and in adaptation to the dry environment. Deserts are hot and low rain areas suffering from water shortage and high wind velocity. Annual rainfall is very little. It may be less than 25 cm per annum. At some places, if it is high it is unevenly distributed. They show extremes of temperature. Globally deserts occupy about 1/7th of the earth‘s surface.
• The desert biome includes several formation classes that are transitional from grassland and savanna biomes into the vegetation of the arid desert.
• Semi-desert is a transitional formation class found in a wide latitude range, from the tropical zone to the mid-latitude zone. Semi-desert consists primarily of sparse xerophytic shrubs. One example is the sagebrush vegetation of the middle and southern Rocky Mountain region and the Colorado Plateau.
• Dry desert is a formation class of plants that are widely dispersed over the ground. It consists of small, hard-leafed, or spiny shrubs, succulent plants (such as cactus), and/or hard grasses. Many species of small annual plants appear only after rare and heavy downpours.
• Desert plants around the world look very different from each other. In the Mojave and Sonoran deserts of the southwestern United States, for example, plants are often large, giving the appearance of woodland.
• Desert animals are insects, reptiles, and burrowing rodents. Desert shrew, fox, kangaroo, wood rat, rabbit; armadillo are common mammals in the desert. Camel is known as the ship of the desert as it can travel long distances without drinking water for several days.

Adaptations:

Desert plants are in hot and dry conditions.

• These plants conserve water by following methods:
  • They are mostly shrubs.
  • Leaves are absent or reduced in size.
  • Leaves and stems are succulent and water storing.
  • In some plants, even the stem contains chlorophyll for photosynthesis.
  • The root system is well-developed and spread over a large area.
• The animals are physiologically and behaviorally adapted to desert conditions.
  • They are fast runners.
  • They are nocturnal in the habit to avoid the sun‘s heat during day time.
  • They conserve water by excreting concentrated urine.
  • Animals and birds usually have long legs to keep the body away from the hot ground.
  • Lizards are mostly insectivorous and can live without drinking water for several days.
  • Herbivorous animals get sufficient water from the seeds which they eat.

Tundra Biome

• The word tundra means a ―barren land since they are found in those regions of the world where environmental conditions are very severe. There are two types of tundra- arctic and alpine.
• Permanently frozen subsoil called permafrost is found in the Arctic and Antarctic tundra. The summer temperature may be around 15°C and in winter it may be as low as –57°C in arctic tundra.A very low precipitation of less than 400 mm per year.A short vegetation period of generally less than 50 days between spring and autumn frost. Productivity is low.
• Typical vegetation of arctic tundra is cotton grass, sedges, dwarf heath, willows birches, and lichens. Animals of tundra are hurepian reindeer, musk ox, arctic hare, caribous, lemmings, and squirrel. Their body is covered with fur for insulation; Insects have short life cycles which are completed during favourable periods of the year.
• Most of them have long life e.g. Salix arctica which is arctic willow has a life span of 150 to 300 years. They are protected from chill by the presence of thick cuticles and epidermal hair. Mammals of the tundra region have large body size and small tails and ear to avoid the loss of heat from the surface.

Aquatic Ecosystem

An Aquatic Ecosystem refers to plant and animal communities ‘occurring in water bodies. Aquatic ecosystems are classified on the basis of salinity into the following two types:

Freshwater Ecosystem

Water on land that is continuously cycling and has low salt content is known as fresh water and its study is called limnology.

• Static or still water (Lentic) e.g. pond, lake, bogs, and swamps.
• Running water (Lotic) e.g. springs, mountain brooks, streams, and rivers.

Physical characteristics

Freshwaters have a low concentration of dissolved salts. The temperature shows diurnal and seasonal variations. In tropical lakes, surface temperature never goes below 400C, in temperate fresh waters, never goes above or below 40C and in polar lakes never above 40C.

• In temperate regions, the surface layer of water freezes but the organisms survive below the frozen surface.
• Light has a great influence on freshwater ecosystems. A large number of suspended materials obstruct the penetration of light in water.
• Certain animals float up to the water’s surface to take up oxygen for respiration. Aquatic plants use carbon dioxide dissolved in water for photosynthesis.
• Lakes and ponds are inland depressions containing standing water. The largest lake in the world is Lake Superior in North America. Lake Baikal in Siberia is the deepest. Chilka Lake of Orissa is the largest lake in India.

Three main zones can be differentiated in a lake:

1. Peripheral zone (littoral zone) with shallow water.
2. Open water beyond the littoral zone where water is quite deep.
3. Benthic zone (bottom) or the floor of the lake.
4. Aquatic organisms can be floating in the water or free-swimming or sedentary (fixed), depending on their size and habit. Microscopic floating organisms such as algae, diatoms, protozoans, and larval forms are called plankton. Rooted aquatic plants, fish, mollusk, and echinoderms are bottom dwellers.
5. Wetlands are areas that periodically get inundated with water and support a flourishing community of aquatic organisms including frogs and other amphibians. Wetlands are between aquatic and terrestrial ecosystems. They show an edge effect and form an ecotone. Ecotone is a transitional zone between two ecosystems. Swamps, marshes, and mangroves are examples of wetlands.

Marine Ecosystem

• Oceans cover 70 percent of the surface area of the world, they are habitable throughout and support total biomass probably as much as ten times that on land. In many ways, the marine environment is much more favorable to life than land areas; it is more equable, and the two most essential gases for life, oxygen and carbon dioxide, are readily available in water, provided it is not polluted. In addition, many of the nutrient minerals found in the Earth’s crust are dissolved in the sea in varying amounts. The main environmental gradients in the sea are related to temperature, salinity, and light intensity.
• The most saline conditions occur where temperatures, and hence evaporation, are highest. Many marine organisms have very narrow tolerance ranges to particular salinity concentrations, which may therefore localize them considerably in terms of depth or area. In the open water of the major oceans, the range is much less, from 37%o in the tropics to 33%0 in polar seas.
• Temperature variations in the sea are much less than those on land. The difference between the surface temperature of the warmest seas (32°C) and the coldest (-2°C) gives a range far less than that of land (about 90°C). Both vertical and horizontal ocean currents play a major role in equalizing variations of temperature, salinity, and dissolved gases in the oceans, as well as being important factors in the global energy budget.
• The availability of light exercises as much fundamental control on the basic process of photosynthesis in the sea as it does on the land. The amount of light reaching the surface varies with latitude and with the season; much is lost by reflection from the water surface in high latitudes and when the sea is rough.
• Marine plants are confined to the euphotic zone by the light factor. They are far less diverse than land plants, being dominated by algae, with only a few angiosperms present, most of which are found in the nearshore zone. The most obvious and visible types of marine algae are seaweeds, but about 99 percent of marine vegetation is made up of microscopic floating algae (phytoplankton). These are one-celled organisms containing chlorophyll and include diatoms and dinoflagellates.
• Near-shore areas additionally receive nutrients from rivers. Coastal and estuarine areas therefore have a high productivity and great diversity of plant life, making them among the most fertile parts of the marine ecosystem.
• The biodiversity of marine ecosystems is very high compared to terrestrial ecosystems. Almost every major group of animals occurs in the sea. Insects and vascular plants are completely absent in the marine ecosystem. The maximum diversity of marine organisms is found in the tidal zone that is near the shore. Diatoms, algae, dinoflagellates, and jellyfish are some of the free-floating life forms in oceans. Large crustaceans, mollusks, turtles, and mammals like seals, porpoises, dolphins, and whales are free-swimming animals that can navigate. Bottom dwellers are generally sessile (fixed) organisms like sponges, corals, crabs, and starfish.

Adaptations

• Lightweight animals and plants float in water and move with the water currents.
• Animals and plants in the ocean are tolerant to the high concentrations of salts (Osmoregulation). Osmoregulation is the process by which a constant osmotic pressure is maintained in the blood.
• Swimming animals have streamlined bodies. Their body is laterally compressed.
• Deep-sea forms show bioluminescence (they emit light).
• They are dependent on their food in the upper sea zones.

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