LIFE ON THE EARTH

This unit deals with

• Biosphere — importance of plants and other organisms;

ecosystems, bio-geo chemical cycle and ecological balance;

biodiversity and conservation

UNIT

VI

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LIFE ON THE EARTH

By now you might have realised that all

units of this book have acquainted youwith the three major realms of the

environment, that is, the lithosphere, the

atmosphere and the hydrosphere. You know

that living organisms of the earth, constitutingthe biosphere, interact with other environmental

realms. The biosphere includes all the living

components of the earth. It consists of all plants

and animals, including all the micro-

ECOLOGY

You have been reading about ecological andenvironmental problems in newspapers and

magazines. Have you ever thought what

ecology is? The environment as you know, is

made up of abiotic and biotic components. Itwould be interesting to understand how the

diversity of life-forms is maintained to bring a

kind of balance. This balance is maintained in

a particular proportion so that a healthyinteraction between the biotic and the abiotic

components goes on.

The interactions of a particular group of

organisms with abiotic factors within aparticular habitat resulting in clearly defined

energy flows and material cycles on land, water

and air, are called ecological systems.

Life on the earth is found almosteverywhere. Living organisms are foundfrom the poles to the equator, from thebottom of the sea to several km in theair, from freezing waters to dry valleys,from under the sea to underground waterlying below the earth’s surface.

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 theatmosphere. There are also many organisms

that move freely from one realm to the other.

The biosphere and its components are very

significant elements of the environment. Theseelements interact with other components of the

natural landscape such as land, water and

soil. They are also influenced by the

atmospheric elements such as the temperature,rainfall, moisture and sunlight. The

interactions of biosphere with land, air and

water are important to the growth,

development and evolution of the organism.

The term ecology is derived from the Greekword ‘oikos’ meaning ‘house’, combinedwith the word ‘logy’ meaning the ‘scienceof’ or ‘the study of ’. Literally, ecology isthe study of the earth as a ‘household’,of plants, human beings, animals andmicro-organisms. They all live togetheras interdependent components. AGerman zoologist Ernst Haeckel, whoused the term as ‘oekologie’ in 1869,became the first person to use the term‘ecology’. The study of interactionsbetween life forms (biotic) and thephysical environment (abiotic) is thescience of ecology. Hence, ecology can be

defined as a scientific study of the

interactions of organisms with their

physical environment and with each other.

C H A P T E R

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A habitat in the ecological sense is the totalityof the physical and chemical factors thatconstitute the general environment. A systemconsisting of biotic and abiotic components isknown as ecosystem. All these components inecosystem are inter related and interact witheach other. Different types of ecosystems existwith varying ranges of environmentalconditions where various plants and animalspecies have got adapted through evolution.This phenomenon is known as ecological

adaptation.

Types of Ecosystems

Ecosystems are of two major types: terrestrial

and aquatic. Terrestrial ecosystem can befurther be classified into ‘biomes’. A biome is aplant and animal community that covers alarge geographical area. The boundaries ofdifferent biomes on land are determined mainlyby climate. Therefore, a biome can be definedas the total assemblage of plant and animalspecies interacting within specific conditions.These include rainfall, temperature, humidityand soil conditions. Some of the major biomesof the world are: forest, grassland, desert and

tundra biomes. Aquatic ecosystems can beclassed as marine and freshwater ecosystems.Marine ecosystem includes the oceans,estuaries and coral reefs. Freshwater

Figure 15.1 : Structure and functions of ecosystems

ecosystem includes lakes, ponds, streams,marshes and bogs.

Structure and Functions of Ecosystems

The structure of an ecosystem involves adescription of the available plant and animalspecies. From a structural point of view, allecosystems consist of abiotic and biotic factors.Abiotic factors include rainfall, temperature,sunlight, atmospheric humidity, soilconditions, inorganic substances (carbondioxide, water, nitrogen, calcium, phosphorus,potassium, etc.). Biotic factors include theproducers, the consumers (primary,secondary, tertiary) and the decomposers. Theproducers include all the green plants, whichmanufacture their own food throughphotosynthesis. The primary consumers

include herbivorous animals like deer, goats,mice and all plant-eating animals. Thecarnivores include all the flesh-eating animalslike snakes, tigers and lions. Certain carnivoresthat feed also on carnivores are known as topcarnivores like hawks and mongooses.Decomposers are those that feed on deadorganisms (for example, scavengers likevultures and crows), and further breakingdown of the dead matter by other decomposingagents like bacteria and various micro-organisms.

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The producers are consumed by theprimary consumers whereas the primaryconsumers are, in turn, being eaten by thesecondary consumers. Further, the secondaryconsumers are consumed by the tertiaryconsumers. The decomposers feed on the deadat each and every level. They change them intovarious substances such as nutrients, organicand inorganic salts essential for soil fertility.Organisms of an ecosystem are linked togetherthrough a foodchain (Figure 15.1). Forexample, a plant eating beetle feeding on apaddy stalk is eaten by a frog, which is, in turn,eaten by a snake, which is then consumed bya hawk. This sequence of eating and beingeaten and the resultant transfer of energy fromone level to another is known as the food-chain.Transfer of energy that occurs during theprocess of a foodchain from one level toanother is known as flow of energy. However,food-chains are not isolated from one another.For example, a mouse feeding on grain maybe eaten by different secondary consumers(carnivores) and these carnivores may be eatenby other different tertiary consumers (topcarnivores). In such situations, each of thecarnivores may consume more than one typeof prey. As a result, the food- chains getinterlocked with one another. This inter-connecting network of species is known as foodweb. Generally, two types of food-chains arerecognised: grazing food-chain and detritusfood-chain. In a grazing food-chain, the firstlevel starts with plants as producers and endswith carnivores as consumers at the last level,with the herbivores being at the intermediatelevel. There is a loss of energy at each levelwhich may be through respiration, excretionor decomposition. The levels involved in a food-chain range between three to five and energyis lost at each level. A detritus food-chain isbased on autotrophs energy capture initiatedby grazing animals and involves thedecomposition or breaking down of organicwastes and dead matter derived from thegrazing food-chain.

Types of Biomes

In the earlier paragraphs, you have learnt themeaning of the term ‘biome’. Let us now try toidentify the major biomes of the world. Thereare five major biomes — forest, desert, grassland,

aquatic and altitudinal biomes. Some featuresof these biomes are given in Table 15.1.

Biogeochemical Cycles

The sun is the basic source of energy on whichall life depends. This energy initiates lifeprocesses in the biosphere throughphotosynthesis, the main source of food andenergy for green plants. During photosynthesis,carbon dioxide is converted into organiccompounds and oxygen. Out of the total solarinsolation that reaches the earth’s surface, onlya very small fraction (0.1 per cent) is fixed inphotosynthesis. More than half is used for plantrespiration and the remaining part istemporarily stored or is shifted to otherportions of the plant.

Life on earth consists of a great variety ofliving organisms. These living organisms existand survive in a diversity of associations. Suchsurvival involves the presence of systemic flowssuch as flows of energy, water and nutrients.These flows show variations in different partsof the world, in different seasons of the yearand under varying local circumstances. Studieshave shown that for the last one billion years,the atmosphere and hydrosphere have beencomposed of approximately the same balanceof chemical components. This balance of thechemical elements is maintained by a cyclicpassage through the tissues of plants andanimals. The cycle starts by absorbing thechemical elements by the organism and isreturned to the air, water and soil throughdecomposition. These cycles are largelyenergised by solar insolation. These cyclicmovements of chemical elements of thebiosphere between the organism and theenvironment are referred to as biogeochemicalcycles. Bio refers to living organisms and geoto rocks, soil, air and water of the earth.

There are two types of biogeochemicalcycles : the gaseous and the sedimentary cycle.In the gaseous cycle, the main reservoir ofnutrients is the atmosphere and the ocean. Inthe sedimentary cycle, the main reservoir is thesoil and the sedimentary and other rocks ofthe earth’s crust.

The Water Cycle

All living organisms, the atmosphere and thelithosphere maintain between them a

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Table 15.1 : World Biomes

Biomes

Forest

Subtypes RegionsClimatic

CharacteristicsSoil Flora and Fauna

A. Tropical1. Equatorial2. DeciduousB. TemperateC. Boreal

Desert A. Hot and Drydesert

B. Semi ariddesert

C. Coastaldesert

D. Cold desert

Grassland A. TropicalSavannah

B. TemperateSteppe

A1. 10° N-SA2. 10°-25° N-SB. Eastern North

America, N.E.Asia, Westernand CentralEurope

C. Broad belt ofEurasia andNorth America(parts ofSiberia,Alaska,Canada andScandinavia)

A. S a h a r a ,K a l a h a r i ,Marusthal i ,Rub-el-Khali

B. Marginal areasof hot deserts

C. AtacamaD. Tundra climatic

regions

A. Large areasof A f r i c a ,Australia,SouthAmerica andIndia

B. Par t s ofEurasia andNorth America

A1. Temp. 20-25°C,evenly distributed

A2. Temp. 25-30°C,Rainfall, ave. ann.1,000mm, seasonal

B. Temp. 20-30° C,Rainfall evenlydistributed 750-1,500mm, Well-defined seasonsand distinct winter.

C. Short moist moder-ately warmsummers and longcold dry winter;very lowtemperatures.Precipitation mostlysnowfall400 -1,000mm

A. Temp. 20 - 45°C.B. 21 - 38°C.C. 15 - 35°C.D. 2 - 25°CA-D Rainfall is less than

50 mm

A. Warm hotclimates, Rainfall500-1,250 mm

B. Hot summers andcold winter.Rainfall 500 -900 mm

A1. Acidic,poor innutrients

A2. Rich innutrients

B. Fertile,en-richedwithdecayinglitter

C. Acidic andpoor innutrients,thin soilcover

Rich innutrients withlittle or noorganic matter

A. Porous withthin layer ofhumus.

B. Thin floccu-lated soil,rich in bases

A1. Mult i - layeredcanopy tall andlarge trees

A2. Less dense, treesof medium height;many varieties co-exist. Insects,bats, birds andmammals arecommon speciesin both

B. Moderately densebroad leaved trees.With less diversityof plant species.Oak, Beach, Mapleetc. are somecommon species.Squirrels, rabbits,skunks, birds,black bears,mountain lions etc.

C. Evergreen coniferslike pine, fur andspruce etc. Woodpeckers, hawks,bears, wolves,deer, hares andbats are commonanimals

A-C. Scanty vege-tation; few largem a m m a l s ,insects, reptilesand birds

D. Rabbits, rats,antelopesand groundsquirrels

A. Grasses; treesand large shrubsabsent; giraffeszebras, buffalos,leopards, hyenas,elephants, mice,moles, snakesand worms etc.,are commonanimals

B. Grasses; occ-asional treessuch as cotton-woods, oaks andwillows; gazelles,zebras, rhin-

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Figure 15.2 : Carbon Cycle

circulation of water in solid, liquid or gaseousform referred to as the water or hydrologic cycle(Chapter 13 of this book).

The Carbon Cycle

Carbon is one of the basic elements of all livingorganisms. It forms the basic constituent ofall the organic compounds. The biospherecontains over half a million carbon compoundsin them. The carbon cycle is mainly theconversion of carbon dioxide. This conversionis initiated by the fixation of carbon dioxidefrom the atmosphere through photosynthesis.Such conversion results in the production ofcarbohydrate, glucose that may be convertedto other organic compounds such as sucrose,starch, cellulose, etc. Here, some of thecarbohydrates are utilised directly by the plantitself. During this process, more carbon dioxideis generated and is released through its leavesor roots during the day. The remainingcarbohydrates not being utilised by the plantbecome part of the plant tissue. Plant tissuesare either being eaten by the herbivorousanimals or get decomposed by the micro-organisms. The herbivores convert some of theconsumed carbohydrates into carbon dioxidefor release into the air through respiration. Themicro-organisms decompose the remainingcarbohydrates after the animal dies. Thecarbohydrates that are decomposed by themicro-organisms then get oxidised into carbon

dioxide and are returned to the atmosphere(Figure 15.2).

Aquatic A. FreshwaterB. Marine

Altitudinal ———

A. Lakes, streams,rivers andwetlands

B. Oceans, coralreefs, lagoonsand estuaries

Slopes of highmountain rangeslike the Himalayas,the Andes and theRockies

A-B Temperatures varywidely with cooler airtemperatures andhigh humidity

A. Water, swampsand marshes

B.Water, tidalswamps andmarshes

Algal and other aquaticand marine plantcommunities withvarieties of waterdwelling animals

oceros, wildhorses, lions,varieties of birds,worms, snakesetc., are commonanimals

Temperature andprecipitation varydepending uponlatitudinal zone

Regolith overslopes

Deciduous to tundravegetation varyingaccording to altitude

The Oxygen Cycle

Oxygen is the main by-product ofphotosynthesis. It is involved in the oxidationof carbohydrates with the release of energy,carbon dioxide and water. The cycling ofoxygen is a highly complex process. Oxygenoccurs in a number of chemical forms andcombinations. It combines with nitrogen toform nitrates and with many other mineralsand elements to form various oxides such asthe iron oxide, aluminium oxide and others.Much of oxygen is produced from thedecomposition of water molecules by sunlight

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during photosynthesis and is released in theatmosphere through transpiration andrespiration processes of plants.

The Nitrogen Cycle

Nitrogen is a major constituent of theatmosphere comprising about seventy-eightper cent of the atmospheric gases. It is also anessential constituent of different organiccompounds such as the amino acids, nucleicacids, proteins, vitamins and pigments. Onlya few types of organisms like certain species ofsoil bacteria and blue green algae are capableof utilising it directly in its gaseous form.Generally, nitrogen is usable only after it isfixed. Ninety per cent of fixed nitrogen isbiological. The principal source of free nitrogenis the action of soil micro-organisms andassociated plant roots on atmospheric nitrogenfound in pore spaces of the soil. Nitrogen canalso be fixed in the atmosphere by lightning andcosmic radiation. In the oceans, some marineanimals can fix it. After atmospheric nitrogenhas been fixed into an available form, greenplants can assimilate it. Herbivorous animalsfeeding on plants, in turn, consume some of it.Dead plants and animals, excretion ofnitrogenous wastes are converted into nitritesby the action of bacteria present in the soil.Some bacteria can even convert nitrites intonitrates that can be used again by green plants.There are still other types of bacteria capableof converting nitrates into free nitrogen, aprocess known as denitrification (Figure 15.3).

Other Mineral Cycles

Other than carbon, oxygen, nitrogen andhydrogen being the principal geochemicalcomponents of the biosphere, many otherminerals also occur as critical nutrients forplant and animal life. These mineral elementsrequired by living organisms are obtainedinitially from inorganic sources such asphosphorus, sulphur, calcium and potassium.They usually occur as salts dissolved in soilwater or lakes, streams and seas. Mineral saltscome directly from the earth’s crust byweathering where the soluble salts enter thewater cycle, eventually reaching the sea. Othersalts are returned to the earth’s surface throughsedimentation, and after weathering, they againenter the cycle. All living organisms fulfill theirmineral requirements from mineral solutionsin their environments. Other animals receivetheir mineral needs from the plants and animalsthey consume. After the death of livingorganisms, the minerals are returned to the soiland water through decomposition and flow.

Ecological Balance

Ecological balance is a state of dynamicequilibrium within a community of organismsin a habitat or ecosystem. It can happen whenthe diversity of the living organisms remainsrelatively stable. Gradual changes do takeplace but that happens only through naturalsuccession. It can also be explained as a stablebalance in the numbers of each species in anecosystem. This occurs through competitionand cooperation between different organismswhere population remains stable. This balanceis brought about by the fact that certain speciescompete with one another determined by theenvironment in which they grow. This balanceis also attained by the fact that some speciesdepend on others for their food andsustenance. Such accounts are encounteredin vast grasslands where the herbivorousanimals (deer, zebras, buffaloes, etc.) are foundin plenty. On the other hand, the carnivorousanimals (tigers, lions, etc.) that are not usuallyin large numbers, hunt and feed on theherbivores, thereby controlling theirpopulation. In the plants, any disturbance inthe native forests such as clearing the forestfor shifting cultivation usually brings about aFigure 15.3 : Nitrogen Cycle

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change in the species distribution. This changeis due to competition where the secondaryforest species such as grasses, bamboos orpines overtakes the native species changingthe original forest structure. This is calledsuccession.

Ecological balance may be disturbed dueto the introduction of new species, naturalhazards or human causes. Human interferencehas affected the balance of plant communitiesleading to disturbances in the ecosystems.Such disturbances bring about numeroussecondary successions. Human pressure onthe earth’s resources has put a heavy toll on

EXERCISES

1. Multiple choice questions.

(i) Which one of the following is included in biosphere?

(a) only plants (c) only animals

(b) all living and non-living organisms (d) all living organisms

(ii) Tropical grasslands are also known as :

(a) the prairies (c) the steppes

(b) the savannas (d) none of the above

(iii) Oxygen combines with iron found in the rocks to form :

(a) iron carbonate (c) iron oxides

(b) iron nitrites (d) iron sulphate

(iv) During photosynthesis, carbon dioxide combines with water in thepresence of sunlight to form :

(a) proteins (c) carbohydrates

(b) amino acids (d) vitamins

2. Answer the following questions in about 30 words.

(i) What do you understand by the term ‘ecology’?

(ii) What is an ecological system? Identify the major types of ecosystems inthe world.

(iii) What is a food-chain? Give one example of a grazing food-chain identifyingthe various levels.

(iv) What do you understand by the term ‘food web’? Give examples.

(v) What is a biome?

the ecosystem. This has destroyed its originalityand has caused adverse effects to the generalenvironment. Ecological imbalances havebrought many natural calamities likefloods, landslides, diseases, erratic climaticoccurrences, etc.

There is a very close relationship betweenthe plant and animal communities withinparticular habitats. Diversity of life in aparticular area can be employed as anindicator of the habitat factor. Properknowledge and understanding of such factorsprovide a strong base for protecting andconserving the ecosystems.

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3. Answer the following questions in about 150 words.

(i) What are bio-geochemical cycles? Explain how nitrogen is fixed in theatmosphere.

(ii) What is an ecological balance? Discuss the important measures neededto prevent ecological imbalances.

Project Work

(i) Show the distribution of the different biomes on the outline map of theworld with a note highlighting the important characteristics of each biome.

(ii) Make a note of trees, shrubs and perennial plants in your school campusand devote half a day to observe the types of birds which come to thegarden. Can you describe the diversity of birds?

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