BIOM BIOSPHERE BIOGEOCHEMICAL CYCLEFlame Institute

Biome

Definition : Each of the major terrestrial ecosystems or distinctive terrestrial areas with their group of climax plants and associated animals constitutes biomes. A biome is the largest terrestrial community. Rainfall, temperature range, nature of soil, barriers, latitude and altitude determine the nature and extent of biomes.

Major biomes of world : Biomes are often classified in seven categories :

(1) Tropical rain forests : The tropical rain forest, a biome occurs in regions of high temperature (average 25°C) and high rainfall (200-450 cm per year). These tropical rain forests occur in Central America, around Amazon basin in South America, in Africa and in South-East Asia.

(i) This biome is characterized by multistoried vegetation (upto five distinct layers or storeys of vegetation). Further maximum biodiversity on land is shown by this biome and it is estimated that one half to two-thirds of all species of terrestrial plants and insects live in tropical forests.

(ii) Lianas (vascular plants rooted in soil and they only get support of trees for climbing to top) and epiphytes (air plants) are common in this biome due to excess of moisture. Further giant trees of the tropical forest support a rich and diverse community of animals on their branches.

(iii) No one species dominates in this biome.

(iv) The productivity of this biome is maximum.

(v) The trees of this biome possess buttressed trunks and phenomenon of cauliflory (presence of flowers and fruits on main trunk and main branches) is common in this biome.

(2) Savannahs : Like tropical forests, savannahs are found near the equator but in areas having less annual rainfall (90-150 cm/year). Some areas near the equator experience prolonged dry seasons. The heat, periodic dryness and poor soils cannot support a forest but have led to evolution of tropical open grasslands with scattered shrubs and trees.

(i) The vegetation of this biome support large grazing herbivores like buffalo, zebra, etc., which are food for carnivores like lions, tigers, etc. The savannah also supports a large number of plant eating invertebrates like mites, grasshoppers, ants, beetles and termites.

(ii) The termites are one of the most important soil organisms in savannahs.

(iii) Indian tropical grasslands are not true savannahs but these are the result of destruction and modification of tropical deciduous forests by cutting, grazing and fire.

(3) Deserts : These are the biomes that have 25 cm (10 inches) or less of precipitation annually.

(i) Sahara of North Africa, Thar of West Asia and Gobi of Asia are most important deserts.

(ii) Annual plants are abundant in deserts and tide over unfavourable dry season in the form of seeds. Succulent plants are characteristics of deserts. Trees and shrubs present in deserts have deep roots.

(iii) Desert animals have also fascinating adaptations that enable them to adjust with limited water supply.

(iv) Desert plants show phenomenon of Allelopathy, i.e., they secret some chemical substances which inhibit the growth of plants growing in their near vicinity.

(v) Deserts show poor biodiversity and their productivity is minimum.

(4) Temperate grasslands : Temperate grasslands experience a greater amount of rainfall than deserts but a lesser amount than savannahs. They occur at higher latitudes than savannahs but like savannahs are characterized by perennial grasses and herbs of grazing mammals.

Temperate grasslands have different names in different parts of the world, e.g., Prairies of North America, Steppes of Russia, Veldts of South Africa, Pampas of South America, Pusztas of Hungary and Tussocks of New Zealand.

(5) Temperate deciduous forests : Temperate deciduous forests occur in areas having warm summers, cold winters and moderate amount of precipitation (75 – 150 cm annually). The trees of this forest loose their leaves during autumn and remain dormant throughout winter (term ‘deciduous’ derived from Latin word meaning ‘to fall’). These forests are present in Eastern United States, Canada and extensive region in Eurasia.

(i) In temperate forest biome, there is an upper canopy of dominant trees like beech, oak, birch, maple, etc. followed by lower tree canopy and then a layer of shrubs beneath.

(ii) Animal life in this biome is abundant on the ground as well as on the trees.

(6) Taiga : The taiga or northern coniferous forests or boreal forests consist of evergreen, cone bearing trees like spruce, hemlock and fir and extend across vast areas of Eurasia, and North America.

(i) The taiga is characterized by long, cold winters with little precipitation.

(ii) The harsh climate limits productivity of the taiga community. The cold temperatures, very wet soil during the growing season and acids produced by fallen conifers needles and Sphagnum inhibit full decay of organic matter, due to which thick layers of semidecayed organic material called peat is formed, which acts as energy source.

(7) Tundra : The tundra encircles the top of the world. This biome is characterised by desert like levels of precipitation (less than 25 cm annually), extremely long and cold winters and short warmer summers.

(i) Tundra is uniform in appearance and is dominated by scattered patches of grasses, sedges and lichens. Some small trees do grow but are confined to margins of streams and lakes (In general treeless).

(ii) Tundra is a biome of low diversity and low productivity.

(iii) The precipitation that falls remains unavailable to plants for most of the year because it freezes. During the brief arctic summer, some of the ice melts and permafrost (or permanent ice) found about a meter down from the surface, never melts and is impenetrable to both water and roots. However, the alpine tundra found at high elevation in temperate or tropical regions does not have this layer of permafrost.

Indian biomes : Indian forests are classified into three major types based on temperature are tropical, temperate, alpine.

(1) The marine environment : It is characterized by its high concentration of salt (about 3.5 percent in open sea) and mineral ions (mostly sodium and chloride followed by sulphur, magnesium and calcium).

(i) The vertical zones of the ocean are determined on the basis of availability of light for photosynthesis. The lighted upper 200 metres form the photic or euphotic zone. The next zone upto the depth 200–2000 metres gets less light which is insufficient for photosynthesis form the aphotic zone. Below 2000 metre is the area of perpetual darkness, the abyssal zone.

(ii) Three major environments may be recognized in the ocean basin

(a) The littoral zone : The sea floor from the shore to the edge or the continental shelf.

(b) The benthonic zone : The sea floor along the continental slope and the aphotic and abyssal zone.

Marine life : It can be grouped into three main categories :

(i) Plankton : These are passively drifting or floating organisms. Most of these minute organisms, plankton includes photosynthesizing organisms like diatoms (phytoplankton) as well as heterotrophic organisms like small crustaceans (zooplanktons).

(ii) Nektons : These consist of actively moving organisms with well developed locomotory organs.

(iii) Benthonic organisms : These are found along the floor of the sea bed and include creeping, crawling or sessile organisms.

(2) Other (Lakes and Ponds) : Lakes and ponds are stagnant fresh water bodies and are found practically in every biome. Many lakes are direct or indirect result of glaciation. Others are natural or man made depression filled with water. The relatively shallow lakes, called eutrophic lakes, have a rich accumulation of organic products e.g., Dal lake of Kashmir.

Generally deep lakes, often with the steep and rocky sides, are poor in circulating nutrients like phosphates. These are called oligotrophic lakes. Some of the lakes contain a saline or brackish water (Sambhar lake of Rajasthan).

Biosphere

All the thousands of ecosystems together constitute the biosphere, which exists as a thin envelope around the earth’s surface. The global environment consists of three main sub division :

(1) Hydrosphere : All the water (liquid) component of the oceans, seas, rivers and other island water bodies.

(2) Lithosphere : The solid components of the earth crust, rocks, soil and minerals.

(3) Atmosphere : The gaseous cover which envelops the hydrosphere and the lithosphere and the atmosphere. The entire inhabited part of the earth and its atmosphere (including the living and the non-living components) forms the biosphere.

As a result of manipulation by man, the biosphere has become transformed into a human dominated environment of noosphere mind).

Role of atmosphere in metabolism : Of much greater significance to metabolism, however, are the biogeochemical cycles of the atmosphere. The air consists mainly of oxygen (20.95 per cent), carbon dioxides (about 0.03 per cent), nitrogen (78.08 per cent), water vapour and minute traces of inert gases. Except the inert gases, all these components of air serve as metabolites; each circulates through a cycle in which the organisms play an important role. As all gases are dissolved in natural waters, the hydrosphere maintains an equilibrium with the atmosphere.

Biogeochemical cycle

Organisms are built up on chemical substances. They require certain chemicals like N₂, O₂, H₂, P, C, etc. continuously for their survival. These chemicals enter the organisms from the environment and come out after undergoing changes or without changes. Thus these elements tend to circulate in a characteristic path from the environment to the organism and back to the environment. This cyclical path of the elements from the abiotic system to the biotic system and back is called biogeochemical cycles (Bio = living organism; Geo = water, air, earth). As these chemicals form the components of food, these cycles are also called nutrient cycles.

Phases of biogeochemical cycles : Each biogeochemical cycle has two phases, namely the biotic phase (organic phase) and the abiotic phase.

(1) Biotic phase : It refers to the flow of chemicals in the living organisms through food chain.

(2) Abiotic phase : It refers to the distribution and flow of chemicals in the non-living environment.

Types of biogeochemical cycles : The biogeochemical cycles are classified into two types, namely gaseous cycles and sedimentary cycles.

(1) Gaseous cycles : In gaseous cycles the main reservoirs of chemicals are the atmosphere and ocean. e.g., Carbon cycle, N₂ cycle, O₂ cycle, etc.

(2) Sedimentary cycle : In sedimentary cycles the main reservoirs are soil and rocks. e.g., Sulphur cycle, phosphorus cycle, etc.

Important biogeochemical cycles

(1) Carbon Cycle : The cycling of carbon between biotic and abiotic systems is called carbon cycle. It is a gaseous cycle. The main source of carbon is the carbon dioxide (CO₂). CO₂ is present in the air and water. Air is the main reservoir. CO₂ content of air is 0.03%. Its amount remains constant.

(i) Flow of Carbon into the biotic system : Carbon flows into the biotic system in two ways :

(a) Photosynthesis : Carbon enters the biotic system through photosynthesis. In photosynthesis green plants utilize CO₂ and incorporate the carbon of CO₂ in glucose. Glucose is used for the synthesis of other types of carbohydrates, proteins and lipids. These compounds, containing carbon, are stored up in the plant tissues. When plants are eaten up by herbivores, the carbon flows into the body of herbivorous animals through food chain. When herbivores are eaten by carnivores, the carbon enters the body of carnivorus animals.

6CO₂ + 6H₂O ® C₆H₁₂O₆ + 6O₂.

(b) Formation of shell : The CO₂ dissolved in sea water is utillized by the marine animals like protozoans, corals, molluscs, algae, etc., for the construction of shell. In these animals CO₂ is converted into calcium carbonate (CaCO₃) which is used for the construction of shells.

CO₂ + H₂O ® H₂CO₃ (Carbonic acid)

H₂CO₃ ® H⁺ + HCO₃ (Bicarbonate)

HCO₃ + Ca⁺ ® H⁺ + CaCO₃ (Calcium carbonate)

(ii) Flow of Carbon into the abiotic system : The carbon of the biotic system flows into the abiotic system in five ways :

(a) Respiration : Plants and animals release CO₂ by respiration (biological oxidation).

C₆H₁₂O₆ ® CO₂ + H₂O + Energy

(b) Decomposition : When plants and animals die, the dead bodies are decomposed into CO₂ by decomposers like bacteria, algae, etc.

(c) Shells : After the death of marine animals, CaCO₃ stored in the shells is either deposited as sedimentary rocks or dissolved in water to release CO₂ by the reversion of the above said reactions.

(d) Coal : A certain proportion of carbon from plants is deposited as coal. Carbon from coal returns to air in the form of CO₂ through combustion and weathering.

(e) Forest fire : Combustion of wood in the forest, releases carbon from plants in the form of CO₂.

(2) Nitrogen cycle : The cycling of nitrogen between abiotic and biotic systems is called nitrogen cycle. It is a gaseous cycle. The main source of N₂ is air which contains 79% N₂.

(1) Flow of Nitrogen into the biotic system : Nitrogen is an important nutrient of plants. But plants cannot utilize free N₂ of air. They obtain N₂ from ammonium salts, nitrites and nitrates. These compounds are formed from atmospheric N₂ by a process called nitrogen fixation.

Nitrogen fixation is a process by which atmospheric free N₂ is converted into soluble salts like nitrites and nitrates. It occurs in two ways namely electrochemical fixation and biological fixation.

(a) Electrochemical fixation : A certain amount of free N₂ is fixed by the action of lightning. The amount of nitrate formed by this method is about 35 mg/m²/year.

(b) Biological fixation : It refers to the conversion of free N₂ into soluble salts by the activity of certain organisms. These organisms are called N₂ fixing organisms. The amount of nitrate formed by this method is about 140 to 700 mg/m²/year, and in a fertile area it exceeds 20000 mg/m. The N₂ fixing organisms are bacteria, blue green algae, fungi and other micro-organisms. e.g., Rhizobium, Azotobacter, Closteridium, Bacillus, Nitrosomonas, Nitrococcus, Nitrobacter, Anabena, Nostoc, etc.

The fixed N₂ is absorbed by plants through the root system and is incorporated into the proteins. When herbivores feed on these plants, the N₂ flows on the carnivores through food chain.

(2) Flow of Nitrogen into the abiotic system : The nitrogen of the biotic system flows into the abiotic system by four methods, namely decomposition, excretion, denitirfication and sedimentation.

(a) Decomposition : Plants and animals contain nitrogen in their body protein. After death, the proteins of dead bodies are decomposed by decomposers into amino acids and ammonia. The convertion of protein from dead bodies into ammonia by decomposition is called ammonification. This ammonia may be converted into nitrates or free nitrogen.

Protein in dead bodies

(b) Excretion : Animals excrete nitrogenous waste products in the form of ammonia, urea and uric acid. These compounds are decomposed to release N₂.

(c) Denitrification : The conversion of nitrate into ammonia or free nitrogen is called denitrification. This is done by denitrifying bacteria. e.g., Pseudomonas.These bacteria utilize the O₂ present in the nitrate for the oxidation of carbohydrate.

(d) Sedimentation : Some amount of nitrate is lost from the ecosystem by sedimentation.

(3) Oxygen cycle : The cycling of O₂ between biotic and abiotic systems is called O₂ cycle. It is a gaseous cycle. Air is the reservoir for O₂. O₂ enters the biosphere through respiration. The O₂ taken into the body is used for oxidation of carbohydrates, proteins and fats. Certain amount of O₂ in atmospheric air is converted into ozone (O₃) the ozone forms an umbrella-like layer in the outer atmosphere. This layer prevents the ultraviolet radiations from reaching the earth’s surface.

Energy

Carbon monoxide is released from volcanoes. This CO is unstable. It combines with O₂ to form CO₂.

O₂ combines with a variety of elements to form compounds. For example, it forms CO₂ with carbon, water with hydrogen, nitrates with N₂ ferric oxide with iron etc. O₂ returns to air by two main methods, namely photosynthesis and photodissociation.

(i) Photosynthesis : Green plants synthesize carbohydrate by photosynthesis. During photosynthesis water molecules break up into hydrogen and oxygen. O₂ is released into the atmosphere and H₂ is trapped and turned into carbohydrates.

(ii) Photodissociation : Water vapour is dissociated to release H₂ and O₂, in presence of light.

(4) Phosphorus cycle : The cycling of phosphorus between biotic and abiotic system is called phosphorus cycle. It is a sedimentary cycle. Phosphorus is an important mineral nutrient. The main source of phosphorus is rocks. Through erosion and weathering phosphorus is made available in the soil. Plants absorb ionic phosphate through roots. In plants it is incorporated into the protoplasmic components like DNA, RNA, AMP, ADP, ATP, GDP, GTP, NADP, phospholipids etc. from plants, it passes into herbivores and animals, the organic molecules containing phosphate are decomposed and phosphate is liberated as inorganic ion phosphate. It is again used by plants.

The excess of phosphate in the bodies of animals is excreted out through faces. The bird guano (excreta) contains a large amount of phosphate. Phosphate is also released to the soil through the combustion of forest trees and grasses. A large amount of phosphate is lost in the sea by sedimentation. A certain amount of phosphorus gets locked in bones and teeth.

(5) Sulphur cycle : The cycling of sulphur between biotic and abiotic systems is called sulphur cycle. It is a sedimentary cycle. Sulphur is an important component of proteins and amino acids.

Sulphur exists in a number of states. Of these, three are important. They are elemental sulphur, sulphides and sulphates. Sulphur is present in rocks. It is made available for plants in the form of inorganic sulphate by weathering and erosion. Sulphur passes into the animals through food chain. By the death of plants and animals, the decomposers again bring the sulphur to the soil for the use of plants.

Some sulphur in dead bodies is released into the air as hydrogen sulphide (H₂S) by the bacteria called Escherichia coli under anaerobic combustion. Similarly incomplete combustion of fossil fuel releases sulphur dioxide (SO₂) into the air.

Certain bacteria (green and purple photosynthetic bacteria) oxidise H₂S of air to sulphate which can be used by plants.

H₂S + 2O₂ ® SO^–₄ + 2H⁺

Certain amount of sulphur is lost in the sediments. If iron is present in the sediments, sulphur combines with it to form iron sulphide. Fe + S ® FeS

? Number of biomes on a mountain range decreases with the increase in the latitude of the mountain.

? Muskegs : Water filled depressions in tundra biome.

? Latossols : Red coloured nutrient-rich highly fertile soil of tropical rain forests.

? Tropical deciduous forests are also called monsoon forests.

? Chaparral of winter rain areas is called machhie.

? Black soils of tall grass prairies are richest in nutrients and so are most fertile in the world.

? Some latitudinal lines :

(a) Tropic of cancer at 23.5°N of equator.

(b) Arctic circle at 66.72°N

(d) Antarctic circle at 66.72°S.

? Biotic Zones :

(a) Tropical Zone : Between 23°N and 23°S.

(b) Subtropical Zone: Between 23° and 40°N.

(d) Arctic Zone : Between 60° and 70°N.

? Dystrophic lakes. Lakes rich in undecomposed organic matter. e.g., marshy lakes.

? Brackish salts are always oligotrophic e.g., sambhar lake of Rajasthan.

? Major estuaries of India are : (Hooghly-Malta estuarine system, Adyar estuary and chilka lake (largest brackish water lake).

? Winogradsky (1891) discovered nitrogen fixation.

? Guano : Excreta from sea birds and others which can be used as fertilizer due to being rich in phosphate and uric acid.

? Carbon : It constitutes 49% of organic matter.

? Matter occupies a space. It can be seen, smelled, tasted and touched while energy does not occupy space and it can be felt through specific receptors e.g., heat, sound, light.

? Reservoirs Pool : It is the reservoir of biogenetic nutrients from which the latter are slowly transferred to cycling pool e.g., phosphates in rocks.

? Cycling pool : Pool of biogenetic nutrients which is being emptied and filled repeatedly by exchange between biotic and abiotic components of biosphere.

? In mature ecosystems, the amount of nutrient uptake is equal to amount of recycled nutrient.

? In young and growing ecosystems, nutrients uptake is more so that a lot of nutrients are retained by the growing biomass of biota.