Fundamentals of Physical Geography (Class XI) Chapter-15- LIFE ON THE EARTH

  • The biosphere includes all the living components of the earth. It consists of all plants and animals, including all the micro-organisms that live on the planet earth and their interactions with the surrounding environment.
  • Most of the organisms exist on the lithosphere and/or the hydrosphere as well as in the atmosphere. There are also many organisms that move freely from one realm to the other.
  • Life on the earth is found almost everywhere. Living organisms are found from the poles to the equator, from the bottom of the sea to several km in the air, from freezing waters to dry valleys, from under the sea to underground water lying below the earth’s surface.


  • is the study of the earth as a ‘household’, of plants, human beings, animals and microorganisms. They all live together as interdependent components.
  • ecology is the study of the earth as a ‘household’, of plants, human beings, animals and micro-organisms. They all live together as interdependent components.

Ecological systems

  • The interactions of a particular group of organisms with abiotic factors within a particular habitat resulting in clearly defined energy flows and material cycles on land, water and air.

Ecological adaptation- 

  • Different types of ecosystems exist with varying ranges of environmental conditions where various plants and animal species have got adapted through evolution.

Types of Ecosystems- 

  • Terrestrial ecosystem- can be further be classified into ‘biomes’.
  1. a biome can be defined as the total assemblage of plant and animal species interacting
    within specific conditions. These include rainfall, temperature, humidity and soil conditions. Some of the major biomes of the world are: forest, grassland, desert and tundra biomes.
  • Aquatic ecosystems- can be classed as marine and freshwater ecosystems. Marine ecosystem includes the oceans, estuaries and coral reefs. Freshwater ecosystem includes lakes, ponds, streams, marshes and bogs

Structure and Functions of Ecosystems- 

  • The structure of an ecosystem involves a description of the available plant and animal species.
  • From a structural point of view, all ecosystems consist of abiotic and biotic Factors
  • Abiotic factors include rainfall, temperature, sunlight, atmospheric humidity, soil conditions, inorganic substances (carbon dioxide, water, nitrogen, calcium, phosphorus, potassium, etc.)
  • Biotic factors include the producers, the consumers (primary, secondary, tertiary) and the decomposers. 
  • producers include all the green plants, which manufacture their own food through photosynthesis. 
  • The primary consumers include herbivorous animals like deer, goats, mice and all planteating animals. The carnivores include all the flesh-eating animals like snakes, tigers and lions.
  • Certain carnivores that feed also on carnivores are known as top carnivores like hawks and mongooses.
  • Decomposers are those that feed on dead organisms (for example, scavengers like vultures and crows), and further breaking down of the dead matter by other decomposing agents like bacteria and various micro- organisms.

Two types of food-chains are recognised:

  • Grazing food-chain, the first level starts with plants as producers and ends with carnivores as consumers at the last level, with the herbivores being at the intermediate level. There is a loss of energy at each level which may be through respiration, excretion or decomposition. The levels involved in a food- chain range between three to five and energy is lost at each level.
  • A detritus food-chain is based on autotrophs energy capture initiated by grazing animals and involves the decomposition or breaking down of organic wastes and dead matter derived from the grazing food-chain.
  • Types of Biomes- There are five major biomes — forest, desert, grassland, aquatic and altitudinal biomes

Biogeochemical Cycles

  • The sun is the basic source of energy on which all life depends. This energy initiates life processes in the biosphere through photosynthesis, the main source of food
    and energy for green plants.
  • Out of the total solar insolation that reaches the earth’s surface, only a very small fraction (0.1 per cent) is fixed in photosynthesis
  • Life on earth consists of a great variety of living organisms. These living organisms exist and survive in a diversity of associations. Such survival involves the presence of systemic flows such as flows of energy, water and nutrients.
  • balance of the chemical elements is maintained by a cyclic passage through the tissues of plants and animals. The cycle starts by absorbing the chemical elements by the organism and is returned to the air, water and soil through decomposition.
  • These cycles are largely energised by solar insolation. These cyclic movements of chemical elements of the biosphere between the organism and the environment are referred to as biogeochemical cycles. There are two types of biogeochemical cycles : the gaseous and the sedimentary cycle. In the gaseous cycle, the main reservoir of nutrients is the atmosphere and the ocean.
  • In the sedimentary cycle, the main reservoir is the soil and the sedimentary and other rocks of the earth’s crust.

The Water Cycle 

  • All living organisms, the atmosphere and the lithosphere maintain between them a circulation of water in solid, liquid or gaseous form referred to as the water or hydrologic cycle

The Carbon Cycle

  • Carbon basic elements of all living organisms. 
  • The carbon cycle is mainly the conversion of carbon dioxide. This conversion is initiated by the fixation of carbon dioxide from the atmosphere through photosynthesis. 
  • Such conversion results in the production of carbohydrate, glucose that may be converted to other organic compounds such as sucrose, starch, cellulose, etc. Here, some of the carbohydrates are utilised directly by the plant itself.
  • During this process, more carbon dioxide is generated and is released through its leaves or roots during the day. The remaining carbohydrates not being utilised by the plant become part of the plant tissue. Plant tissues are either being eaten by the herbivorous animals or get decomposed by the micro- organisms.
  • The herbivores convert some of the consumed carbohydrates into carbon dioxide for release into the air through respiration.
  • The micro-organisms decompose the remaining carbohydrates after the animal dies. The carbohydrates that are decomposed by the micro-organisms then get oxidised into carbon.

The Oxygen Cycle

  • Oxygen is the main by-product of photosynthesis. It is involved in the oxidation of carbohydrates with the release of energy, carbon dioxide and water. 
  • The cycling of oxygen is a highly complex process. Oxygen occurs in a number of chemical forms and combinations. It combines with nitrogen to form nitrates and with many other minerals and elements to form various oxides such as the iron oxide, aluminium oxide and others.
  • Much of oxygen is produced from the decomposition of water molecules by sunlight
    during photosynthesis and is released in the atmosphere through transpiration and respiration processes of plants.

The Nitrogen Cycle

  • Nitrogen is a major constituent of the atmosphere comprising about seventy-nine per cent of the atmospheric gases.
  • It is also an essential constituent of different organic compounds such as the amino acids, nucleic acids, proteins, vitamins and pigments. 
  • Only a few types of organisms like certain species of soil bacteria and blue green algae are capable of utilising it directly in its gaseous form.
  • Generally, nitrogen is usable only after it is fixed. Ninety per cent of fixed nitrogen is biological.
  • The principal source of free nitrogen is the action of soil micro-organisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil.
  • Nitrogen can also be fixed in the atmosphere by lightning and cosmic radiation. In the oceans, some marine animals can fix it.
  • After atmospheric nitrogen has been fixed into an available form, green plants can assimilate it. 
  • Herbivorous animals feeding on plants, in turn, consume some of it.
  • Dead plants and animals, excretion of nitrogenous wastes are converted into nitrites by the action of bacteria present in the soil.
  • Some bacteria can even convert nitrites into nitrates that can be used again by green plants. There are still other types of bacteria capable of converting nitrates into free nitrogen, a process known as denitrification

Other Mineral Cycles 

  • phosphorus, sulphur, calcium and potassium.
  • They usually occur as salts dissolved in soil water or lakes, streams and seas.
  • Mineral salts come directly from the earth’s crust by weathering where the soluble salts enter the water cycle, eventually reaching the sea.
  • Other salts are returned to the earth’s surface through sedimentation, and after weathering, they again enter the cycle.
  • All living organisms fulfill their mineral requirements from mineral solutions in their environments. Other animals receive their mineral needs from the plants and animals they consume. 
  • After the death of living organisms, the minerals are returned to the soil and water through decomposition and flow.

Ecological Balance 

  • Ecological balance is a state of dynamic equilibrium within a community of organisms in a habitat or ecosystem. It can happen when the diversity of the living organisms remains relatively stable.
  • Gradual changes do take place but that happens only through natural succession. It can also be explained as a stable balance in the numbers of each species in an ecosystem. This occurs through competition and cooperation between different organisms where population remains stable.
  • This balance is brought about by the fact that certain species compete with one another determined by the environment in which they grow. This balance is also attained by the fact that some species depend on others for their food and sustenance. Such accounts are encountered in vast grasslands where the herbivorous animals (deer, zebras, buffaloes, etc.) are found in plenty. On the other hand, the carnivorous animals (tigers, lions, etc.) that are not usually in large numbers, hunt and feed on the herbivores, thereby controlling their population.
  • In the plants, any disturbance in the native forests such as clearing the forest for shifting cultivation usually brings about a change in the species distribution.
  • This change is due to competition where the secondary forest species such as grasses, bamboos or pines overtakes the native species changing the original forest structure. This is called succession.

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