L ON THE E

128FUNDAMENTALS OF PHYSICAL GEOGRAPHYduring photosynthesis and is released in theatmosphere through transpiration andrespiration processes of plants.The Nitrogen CycleNitrogen 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).Figure 15.3 : Nitrogen CycleOther Mineral CyclesOther 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 BalanceEcological 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 a2020-21

129LIFE ON THE EARTHchange 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 onthe 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.EXERCISES1.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 :(iv)2.(a) iron carbonate(c) iron oxides(b) iron nitrites(d) iron sulphateDuring photosynthesis, carbon dioxide combines with water in thepresence of sunlight to form :(a) proteins(c) carbohydrates(b) amino acids(d) vitaminsAnswer 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?2020-21

130FUNDAMENTALS OF PHYSICAL GEOGRAPHY3.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?2020-21

Desert A. Hot and Dry desert B. Semi arid desert C. Coastal desert D. Cold desert Grassland A. Tropical Savannah B. Temperate Steppe A1. 10 N-S A2. 10 -25 N-S B. Eastern North America, N.E. Asia, Western and Central Europe C. Broad belt of Eurasia and NorthAmerica (parts of Siberia, Alaska, Canada