Life on Earth [UPSC Geography Notes]

Presenting important notes on relevant topics for the UPSC Civil Services Exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams, and so on. This article talks about life on earth and discusses the concepts of biosphere, biomes, food chain, energy flow, etc.

Biosphere 

The biosphere includes all the living components of the earth. It consists of all plants and animals, including all the microorganisms and their interaction with the surrounding environment. Most of the organisms exist on the lithosphere and/or the hydrosphere as well as in the atmosphere. Many organisms move from one realm to another.

• The word “biosphere” originates from the Greek words “bios” which means life and “sphaira” meaning sphere. There are many reasons which contribute to the existence of life on earth and the most important ones are the earth’s distance from the sun, the presence of oxygen in the atmosphere and the presence of water. These factors along with the existence and interaction of the three spheres of the earth – lithosphere, atmosphere and hydrosphere give rise to the fourth sphere which is called the biosphere (life sphere). The term “biosphere” was coined by Eduard Suess in 1875.
• The biosphere is formed of biotic components – organisms, population, community and ecosystem.

Ecology – Ecology can be defined as a scientific study of the interaction of organisms with their physical environment (abiotic) and with each other. A German Zoologist Ernst Haeckel was the first person to use the term “ecology”. 

Ecosystem – The term “ecosystem” was first proposed by A.G. Tansley in 1935. He defined an ecosystem as the system resulting from the integration of all living (biotic) and abiotic factors of the environment. Different types of ecosystems exist with varying ranges of environmental conditions where various animal and plant species have got adapted through evolution. This phenomenon is called ecological adaptation. 

Major Components of an Ecosystem:

There are two main components of an ecosystem – Abiotic component and Biotic component. 

1. Abiotic components include temperature, sunlight, rainfall, atmospheric humidity, soil conditions, inorganic substances like calcium, phosphorus, nitrogen, hydrogen, carbon dioxide etc. 
2. Biotic components include the producers, the consumers (primary, secondary and tertiary) and the decomposers. 
   1. Producers – The producers include all the green plants, which manufacture their own food through the process of photosynthesis. The green plants are called “autotrophs”, (auto means self and trophs means nourishing) as they manufacture their own food.
   2. Consumers – These are organisms that cannot manufacture their own food and get their food and nutrients from producers directly or from other organisms. They are also called heterotrophs (hetero – others, trophs – nourishing). 
      1. Consumers are further divided into:
         1. Primary consumers – These organisms feed on producers (green plants). They are also called herbivores or plant-eating organisms e.g., grasshopper, sheep, cow, zooplankton, small fish, etc.
         2. Secondary consumers – They are also called carnivores – flesh-eating animals. Animals that kill and eat plant-eating animals, herbivores are called secondary consumers e.g., lions, tigers, foxes, frogs, etc.
         3. Tertiary consumers – They are carnivores at the topmost level in a food chain that feed on other carnivores or secondary consumers e.g., hawks and mongooses. Tertiary consumers that occupy the top trophic level and are not predated by other animals are called apex predators. When they die, their bodies are consumed by scavengers besides decomposers.
         4. Some organisms eat both plants and animals and are called omnivores e.g., humans, seagulls, foxes, etc. 
         5. Detritivores are consumers that feed on detritus – fallen leaves, parts of dead trees and faecal wastes of animals e.g., ants, earthworms, millipedes, termites, sea cucumbers, etc.
   3. Decomposers – These are organisms that help decompose dead or decaying organisms. Decomposers are heterotrophs and are nature’s built-in recycling system. Decomposers return nutrients to the soil by breaking down materials. They create food sources for producers e.g., fungi, moulds, bacteria, mushrooms etc.

Food Chain

Organisms of an ecosystem are linked together through a food chain. The producers are consumed by the primary consumers, which in turn are being eaten by secondary consumers. Also, the secondary consumers are consumed by tertiary consumers. The decomposers feed on the dead at each and every level. This sequence of eating and being eaten and the resultant transfer of energy from one level to another is known as the flow of energy. Each level in a food chain is called a trophic level.

• Generally, two types of food chains are recognised – grazing food chain and detritus food chain.
  • Grazing food chain – Here, 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. The levels involved in a food chain varies between 3 & 5 and energy is lost at each level through excretion, respiration or decomposition.
  • Detritus food chain – It begins with detritus such as dead bodies of animals or plants, fallen leaves, which are then eaten by decomposers or detritivores. These detritivores are in turn consumed by their predators. It is comparatively smaller than the grazing food chain.

Food Chain

Food Web

• It is a complex network of interconnected food chains. Food chains show a direct transfer of energy between organisms. A food chain might involve a mouse eating some seeds on the forest floor, a snake eating the mouse and the mouse being eaten by an eagle.
• In a food web, the mouse might eat seeds, but it also might eat some grains or even some grass. The mouse might be eaten by a snake, or an eagle or even a fox. The snake could be eaten by the eagle but also might be eaten by a fox in the forest. 
• A food web is a more realistic scheme of the transfer of energy within an ecosystem as each organism can eat multiple organisms and could be eaten by multiple organisms.
• Organisms in a food chain or a food web are linked and dependent on each other for survival. If one organism in one trophic level becomes threatened, this affects the organisms in other levels. It is important that an ecosystem remains balanced containing an appropriate proportion of producers and consumers.

Food Web

Energy Flow in an Ecosystem and Energy Pyramid 

• Energy in an ecosystem flows from producers to consumers. The available energy in a food chain decreases with each step or trophic level up in the food chain. As such, there is less energy available to support organisms at the top of the food chain. This supports the fact that tertiary and quaternary consumers are far less in number in an ecosystem than organisms in the lower level.
• Most of the energy in an ecosystem is available at the producer level. The amount of available energy decreases significantly as we move up in the pyramid. It is estimated that only 10% (Ten Percent Law) of the energy available at one trophic level gets transferred to the next level of the energy pyramid. The remaining 90% of energy is either utilised by the organisms within that level for respiration and other metabolic activities or lost to the environment as heat.

Biogeochemical Cycles

The cyclic movements of chemical elements of the biosphere between the organism and the environment are referred to as biogeochemical cycles (bio means life and geo refers to rocks, air, soil and water) of the earth. These cycles start with the absorption of chemical elements by the organisms and through decomposition is returned to air, soil and water. These cycles are largely energised by solar insolation. Biogeochemical cycles are of two types:  

1. The gaseous cycle – In this cycle, the main reservoir of nutrients is the atmosphere and the ocean.
2. The sedimentary cycle – In this cycle, the main reservoir is the soil and the sedimentary and other rocks of the earth’s crust.

Carbon Cycle

Carbon forms the basic constituent of all organic compounds. It is one of the basic elements of all living organisms. The carbon cycle is primarily concerned with atmospheric carbon dioxide, its incorporation into organic matter by photosynthesis and its subsequent release by the respiration of all the biota. Decomposers like bacteria and fungi break down dead plants and animal tissues thereby releasing some carbon into the air, water and soil. In a carbon cycle, carbon moves between reservoirs – the atmosphere, the oceans, vegetation, rocks and soils. 

Carbon Cycle

Oxygen Cycle

In the biosphere, the main processes in the oxygen cycle are respiration and photosynthesis. During respiration, animals and humans breathe by consuming oxygen to be used in the metabolic processes and exhaling carbon dioxide. Photosynthesis is just the opposite of this process, wherein carbon dioxide is taken up by algae and terrestrial green plants and converted into carbohydrates through the process of photosynthesis and oxygen is the main by-product. In the lithosphere, oxygen is fixed and trapped in minerals such as silicates and oxides. A portion of the oxygen is freed by chemical weathering.

Know more about the oxygen cycle in the link.

Nitrogen Cycle

Nitrogen is a main constituent of the atmosphere comprising about 78% of the atmospheric gases. It is also an essential component of organic compounds like amino acids, proteins, vitamins and pigments like chlorophyll. 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. Plants convert atmospheric nitrogen to nitrites, nitrates and ammonia compounds by a process called the nitrogen cycle.

• Nitrogen fixation is the first step of the nitrogen cycle. It involves the conversion of atmospheric nitrogen into ammonia (NH₃). Bacteria like azotobacter and rhizobium present in the roots of the leguminous plants help in the conversion of nitrogen gas to ammonia. Nitrogen can also be fixed by lightning and cosmic radiation. Once the nitrogen is fixed, green plants can assimilate it.
• Ammonification – Dead remains of plants and animals are broken down in the soil by some bacteria to release ammonia in the soil.
• Nitrification – The bacteria Nitrosomonas and nitrococcus present in the soil convert ammonia/ammonium (NH₄⁺) to nitrites (NO₂⁻). Nitrobacter converts nitrites to nitrates (NO₃⁻). The nitrites and nitrates released into the soil helps in enriching the soil with nutrients needed for cultivation.
• Denitrification – The conversion of nitrates into nitrogen and other gaseous compounds like NO₂ by the bacteria present in the soil is called denitrification. The process of denitrification releases the free nitrogen back into the atmosphere.

Other mineral cycles

• A good number of minerals are essential for biological life processes.
• They are either used as major or minor essential elements.
• These mineral elements required by living organisms are obtained initially from inorganic sources like phosphorus, potassium, sulphur and calcium.
• They mostly occur as salts dissolved in soil water or lakes, streams and seas.
• All living organisms fulfil their mineral needs from mineral solutions in their environments.
• Other animals receive their mineral requirements from the plants and animals they consume.
• After the death of living organisms, these minerals are returned to the soil and water through decomposition and flow. 

Ecological Balance

It is a state of dynamic equilibrium within a community of organisms in which genetic, species and ecosystem diversity remain relatively stable, subject to gradual changes through natural succession. It can be defined as a stable balance in the numbers of each species in an ecosystem. Ecological balance may get disturbed due to human interference, the introduction of new species or natural hazards. Ecological imbalances have brought many natural calamities like diseases, landslides, floods, climatic changes, etc.

Biomes

A biome is a plant and animal community that covers a large geographical area. According to I.G. Simmons (1982), the most extensive ecosystem unit which is convenient to designate is called a biome. A biome can be defined as the total assemblage of plant and animal species interacting within specific conditions. These include temperature, rainfall, humidity and soil conditions. There are five major biomes in the world – forest, desert, grassland, aquatic and altitudinal biomes.

Forest Biomes

Subtypes  of forest biomes are as under:

• Tropical Forests –
  • Tropical Equatorial Rainforest –
    • Tropical equatorial forests extend between 10° north and south of the equator.
    • The average annual temperature is 20°C – 25° C.
    • This biome is found in the Amazon basin of South America, Congo basin of Africa and the Indo-Malaysian region of south-east Asia (Java, Sumatra, Malaysia, Borneo and Guinea).
    • Characterised by multilayered canopy tall and large trees. 
  • Tropical Deciduous Forest/Monsoon Forest –
    • It extends between 10° – 25°  north and south of the equator.
    • This biome is found in south and south-east Asia (parts of India, Vietnam, Thailand, Cambodia, Myanmar and southern coastal China), also found in smaller parts in south and central America, West Indies, south-eastern Africa and northern Australia.
    • The temperature varies from one season to another. In summer the temperature ranges between 38° C- 48° C, summers are warm and humid. In winters, temperature varies between 10°C-27°C. 
    • The annual rainfall ranges between 75- 150 cm. 
• Temperate Forests –
  • This biome is found in eastern North America, north-east Asia, Western and Central Europe.
  • These biomes have well defined seasons (winter, spring, summer and fall). Winters are cold and summers are warm. The soil is very fertile in these biomes.
  • These forests are also known as broad-leaved forests as the trees have wide flat leaves. Oak, beech, maple, chestnut, etc. are some common trees found here. Animals living in this biome adapt to changing seasons. Some animals migrate or hibernate in winter. 
• Boreal Forests/Taiga Forests – 
  • It is the largest terrestrial biome and extends across Europe, North America and Asia.
  • This biome is also known as the coniferous forest or taiga forest biome.
  • This biome has short wet summers and long cold winters. It receives plenty of snow during winter.
  • It consists mainly of evergreen coniferous forests. The important ones are pines, cedars, spruces and firs.
  • During the short summer season snow melts and this helps lichens, mosses and short grasses to grow and cover the ground. These are called meadows.

Desert Biomes

It is further classified into:

1. Hot and dry desert –   For example Sahara Desert (world’s largest hot desert), Kalahari Desert (South Africa), Marusthali (The Great Indian Desert), Rub- al- Khali (Arabian Peninsula). The temperature ranges between 20°- 45°C.
2. Semi-Arid desert – found in the marginal areas of hot deserts. Temperature varies between 21°-38°C.
3. Coastal desert – The Atacama desert in South America is considered to be the driest coastal desert in the world. Temperature varies between  15°-35°C.
4. Cold desert – Temperature varies between 2°- 25°C. Ladakh is a cold desert in India.

Grassland Biomes

• Tropical Grassland Biome or Savanna Biome – 
  • It features vast open grasslands scattered with small shrubs and isolated trees.
  • Found mainly in South America, Australia and Africa. Tropical grasslands are called llanos in Colombia and Venezuela, the Savannas in Africa and Campos in Brazil (South America).
  • Tropical grasslands experience warm, hot climates.
  • Most of the animals in this biome have long legs like the giraffe and kangaroo. Animals like lions, cheetahs, jackals, hyenas, leopards are found in this biome. Zebras and elephants also live here.
• Temperate Grassland Biome or Steppe Biome –
  • The temperate grasslands are known as Prairies in North America (U.S.A and Canada), Pampas in South America, Steppes in Europe and Asia, Downs in Australia, Veldts in South Africa and Puszta in Hungary.
  • Temperate grassland experiences hot summers and cold winters.
  • Grasses form a major part of the vegetation in the temperate grasslands. The height of the grass depends upon the amount and distribution of rainfall.
  • The animals include the bison, wolves of the Prairies of North America. Prairie dogs, foxes, rabbits, wild horses, weasel, grasshoppers, quails are some common species found in temperate grasslands.

Aquatic Biomes

Aquatic biome is further classified into freshwater biome and marine biome.

• Freshwater Biome – It includes lakes, rivers, streams and wetlands. These biomes are spread over all parts of the earth and have different set of species depending on their location and climate. A wetland is a distinct ecosystem that is flooded by water, either permanently or seasonally. Wetlands serve as home to a wide range of plant and animal species. Wetlands play important functions in the environment such as water purification, flood control, carbon sink and shoreline stability. Wetlands can be freshwater, brackish or salt water. 
• Marine Biome – 
  • It is a saltwater biome occupying seas and oceans of the world. Marine biome includes fishes, whales, molluscs – soft-bodied organisms like octopuses, clams, oysters, etc., crustaceans – aquatic arthropods like crabs, lobsters, etc., sea anemones, bacteria and fungi. Marine biome plants like sea grasses and macro-algae provide shelter and nutrients for many animals. Marine plants provide nutrients for the corals and help corals to build up reefs.
  • Corals are marine invertebrates which thrive in compact colonies. They occur in tropical seas and oceans, as they cannot survive in water below 20°C. Coral reefs are marine ecosystems which are held together by structures made of calcium carbonate secreted by the corals. Coral reefs are mainly of three types:
    • Fringing reefs – They are the most common types of reefs. They grow seaward from the shore along the coast forming a fringe.
    • Barrier reefs – These reefs border the shoreline but are separated from the coast by an expanse of water or lagoon.
    • Atolls – These reefs are circular in shape encircling a lagoon with the absence of an island in the centre.

Altitudinal Biomes

This type of biome is found in the slopes of high mountain ranges like the Himalayas, the Andes (South America) and the Rockies (North America). Depending upon the altitude, deciduous to tundra vegetation is found in the altitudinal biome.

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