Bihar Board 12th Biology Important Questions Long Answer Type Part 2

Bihar Board 12th Biology Important Questions Long Answer Type Part 2 are the best resource for students which helps in revision.

Bihar Board 12th Biology Important Questions Long Answer Type Part 2

Question 1.
Differentiate between them:
(i) Antibodies and antigens
(ii) Interferons and antibodies
(iii) Benign tumour and Malignant tumour
(iv) B-cells and T-cells.
(i) Antibodies and antigens:

Antibodies Antigens
1. Antibody is a molecule synthesized by an animal to combat foreign material. 1. Antigen is usually a foreign material that elicits antibody formation.
2. Each antibody is a protein molecule. 2. Antigen is a protein or polysaccharide molecule.
3. Antibody occurs on the surface of a plasma cell and also in body fluids. 3. Antigen may occur on the surface of a microbe or as a free molecule.
4. Antibody directly joins an antigen to destroy the latter. 4. Antigen binds to a macrophage to reach a helper T-cell to initiate immune response.

(ii) Interferons and antibodies:

Interferons Antibodies
1. Are produced by any microbe infected cell. 1. Are produced by plasma B-cells only.
2. Leave the infected cell and enter a nearby healthy cell to dispose of the microbes. 2. Pass into the circulate in blood any lympth to dispose of antigens.
3. Induce the healthy cell to synthesize antimicrobial proteins that check microbial. 3. Selectively bind to antigens that are immobilised for easy attack by phagocytes.
4. Are quick in action but give a temporary protection against microbes. 4. Are slow in action but give a long-lasting protection against antigens.
5. Act inside the cells. 5. Act outside the cells.
6. Form the body’s second line of defence. 6. Form the body’s third line of defence.

(iii) Benign tumour and Malignant tumour:

Benign tumour Malignant tumour
1. These remain confined in the organ affected. 1. These spread from one organ to other organs of the body.
2. These are enclosed in a connective tissue sheath. 2. These are not enclosed in connective tissue sheath.
3. These have less growth rate. 3. These have rapid growth rate.
4. Cancer cells are more adhesive. 4. Cancer cells are less adhesive.
5. No metastasis. 5. Metastasis is generally present.

(iv) B-cells and T-cells:

B-cells T-cells
1. B-cells form humoral or antimediated immune system (AMIS). 1. T-cells form cell-mediated immune system (CMIS).
2. They are differentiated in bone marrow. 2. They are differentiated in thymus gland.
3. They defend against some virus, encapsulated bacteria and toxins that enter the blood and lymph. 3. They defend against viruses, protists, fungi and some bacteria which enter the cells.
4. They form by division plasma cells. 4. They form by division lymphoblasts of three types: killer, helper and suppressor cells.
5. Plasma cells do not move to the site of infection. 5. Lymphoblasts move to the site of infection.
6. Plasma cells secrete antibodies that pass into the blood and lympth and dispose of the antigens. 6. Killer cells on reaching the site of infection secrete perforin that kills microbes by puncturing their cell membrane.
7. Plasma cells do not react against transplants and cancer cells. 7. Killer cells react against transplants and cancer cells also.
8. Plasma cells have no inhibitory role on immune system. 8. Suppressor cells inhibit the immune system from attacking body’s own cells.

Question 2.
(i) Give a schematic representation of events during inflammatory reaction.
(ii) Write down the role of lymphatic system.
(iii) What is the role of macrophages?
Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 1

(ii) Lymphatic system have following roles:

  • It drains excess tissue fluid from the extracellular spaces back into the blood.
  • Some of the fluid from the digestive tract is absorbed into the lumph. The lymphatic vessels store this fluid temporarily and release it gradually so that the kidneys do not face a sudden pressure of urine excretion.
  • It carries carbon dioxide and nitrogenous waste materials that diffuse into the tissue fluid to the blood.
  • It takes lymphocytes and antibodies from the lymphatic nodes to the blood.
  • It transports fats digested and absorbed in the intestine to the blood in the form of chylomicron droplets.
  • It destroys the invading microorganisms and foreign particles in the lymphatic nodes.
  • It maintains the quality and quantity of the blood by restoring the fluid and solutes that leave it.
  • It brings plasma protein macromolecules synthesized in the liver cells and hormones produced in the endocrine glands to the blood. These molecules cannot pass into the narrow blood capillaries but can diffuse into lymphatic capillaries.

(iii) Macrophages have following roles/functions:

  • Defence: Wandering macrophages (also called histiocytes) dispose of microbes in the connective tissue. Fixed macrophages destroy microbes in the blood and lymph.
  • Giant Cell Formation: Many macrophages may fuse to form a large multinucleate giant cell to capture a foreign body too big for a single macrophage to engulf.
  • Scavenging: Macrophages also dispose of dead tissue cells and noncellular foreign matter.
  • Antigen Display: Macrophages carry the antigens of the engulfed microbes on their surface to display them to the lymphocytes in the lymphatic nodes. This activates the immune system.
  • Secretion of Endogenous Pyrogen: Macrophages secrete interleukin-1 (IL-1), also called endogenous pyrogen. It plays a role in inflammatory response, in causing fever, and in activation of other phagocytic cells as well as the T-cells and B-cells to produce their effect.

Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 2

Question 3.
What is interspecific hybridisation? Explain the term by giving an example.
A cross between two different related species is called interspecific hybridisation. The progeny is different from both the parents. Sometimes the progeny combines the desirable features of both the parents. Examples is production of mule by a cross between female horse (mare) and male donkey.

It inherits size from horse and sure footedness, great endurance to hardship from donkey. It is more useful on high hills. However with all its hybrid vigour, the mule is sterile and unable to reproduce.

Question 4.
Name the ways through which breeders create desired genetic variations in plants. Write short notes on any four of those.
The different ways through, which breeders create genetic variations in plants are (a) Domestication, (b) Germplasm collection, (c) Plant introduction, (d) Selection, (e) Hybridisation, (f) Polyploidy, (g) Mutation, (h) Genetic engineering.

(i) Plant introduction: When a variety of any plant species is taken from one area into a new area (where it was not grown before) it is called as plant introduction. Plant introductions from other countries are valuable sources of germplasm. These provide us with many valuable crops. Example – potato, tomato, cauliflower, grapes, guava etc. are introduced crops. Soyabean is a recently introduced crop.

Introductions have also provided us with improved varieties of the existing crops. Example ‘Sonara-64’ variety of wheat and ‘Taichung Native I’ variety of rice are introductions. These are dwarf varieties and provided the starting material for ‘Green revolution’ in our country.

(ii) Hybridization: process of crossing of two species having desirable characters is known as hybridisation. This method produces genetic variations in the next progeny. It is a scientific improvement of plants by crossing of two varieties. It can also be called as the mating between lines of different genotypes. One parent is male and the other female, when they are crossed together the hybrids are produced. They are called F1 seeds.

In F2 generation there will be segregation, recombination and independent assortment. Hybridization may be between different varieties of same species, then it is termed as intervarietal hybridisation. If a crop species is mated with other different but related species it is called interspecific hybridisation. Such variations are very important for plant breeding.

(iii) Mutation: Mutation is defined as “a sudden and heritable change in a character of an organism.” It may arise due to a change in any of the following: (a) chromosome structure, (b) chromosome number, and (c) base sequence of the concerned gene. Mutations occur in nature spontaneously at a very low rate. They are called spontaneous mutations. Mutant alleles are generally recessive. They produce harmful effects. Only about 0.1% of the mutant alleles are beneficial.

Mutations may be induced by mutagens which may be physical (X-rays, gamma rays, UV-radiations etc.), chemical (ethyl methane sulphonate (EMS) and sodium azide etc.); Mutation breeding is used in plant breeding to develop improved varieties.

(iv) Polyploidy: The condition in an organism, in which the number of complete chromosome sets is higher than the diploid number is called polyploidy. For example monoploids have ‘n’ (sign); diploids have 2n; triploids have 3n (3 chromosome sets) and so on.

Autopolyploidy: The variation of the number of chromosome sets arising within a species is called autopolyploidy. For example, in sexually reproducing organisms egg may be fertilized by more than one sperm, and resulting failure, mitosis and failure of meiosis during gamete formation. Autopolyploidy variety of tea in India.

Allopolyploidy is produced in two steps – two different species are hybridized to produce F1. Then F1 is doubled in chromosome numbers, e.g. wheat, oat, tobacco.

Question 5.
Describe the main sources of biofertilizers, giving suitable examples.
Biofertilizers are organisms which enrich the soil with nutrients. The main sources of biofertilizers are bacteria, cyanobacteria and fungi. These are discribed as follows:
(i) Legume-Rhizobium symbiosis: Leguminous plants show symbiotic relationship with nitrogen fixing bacteria Rhizobium.

(ii) Azolla-Anabaena symbiosis: Anabaena are cyanobacteria, which lives in symbiosis with fern Azolla. Anabaena fixes atmospheric nitrogen. Azolla and cyanobacteria Nostoc are added to rice fields to increase the yield. Azolla grows rapidly in the fields, finally decays and add nitrogen content to the soil.

(iii) Free living bacteria: These are asymbioitc, aerobic and anaerobic bacteria which fix atmospoheric nitrogen under poor aeration conditions. They require Hydrogen gas to fix nitrogen. For example – Azotobacter, Bacillus, Clostridium etc.

(iv) Cyanobacteria: Cyanobacteria or blue green algae are widely used as biofertilizers. They have thick walled heterocysts which fix atmospheric nitrogen. For example Anabaena, Nostoc, Aulosira. These are capable of photosynthesis and can supply nitrogen to entire rice crop. These are simple to use and very cost effective.

(v) Mycorrhiza: Some fungus live symbiotically in the roots of higher plants. These fix atmospheric nitrogen and are termed as Mycorrhiza. These may be of two types: Ectomycarrbiza, where fungus hyphae penetrate into cortex of roots. This increases the surface area for absorption of water and minerals like N, P and K. These also solubilise some insoluble organic compounds. For example they are found in Eucalyptus, pine, oaks etc. Endomycorrhiza: The fungus lives in the intercellular spaces of the cortical cells of the roots. For example legumes, cereals, grasses, citrus, rubber, tea etc.

(vi) Microphos Biofertilizers: Some microorganisms solubilize bound phosphate of soil and make it available for plant absorption. For example Pseudomonas striata and Bacillus polymyxa and fungi – Aspergillus.

(vii) Loose association of Nitrogen fixing bacteria: Some bacteria live in close contact with the roots of grasses and cereals, called associative symbiosis. They fix nitrogen, which is absorbed by the plant and the bacteria get nourishment in return, For example Azospirillum lipoferum with the roots of Maize and Brazilian grasses.

Question 6.
Write short notes on the following:
(a) Baker’s yeast
(b) Brewer’s yeast
(c) Batch process
(d) Continuous process.
(a) Baker’s yeast: Saccharomyces cerevisiae is used in baking industry for desirable changes in flavour and texture of the baked products, it is called baker’s yeast. The baker’s yeast is prepared by growing the stock strain on molasses and then centrifuged and washed. These yeasts ferment the sugar in the dough, CO2 and alcohol escape during baking so the baked products become soft and porous.

(b) Brewer’s yeast: The yeasts used in brewing industry for making alcoholic drinks are called brewer’s yeast. Beer produced is undistilled product of grain-mesh fermentation brought about by Saccharomyces cerevisiae and S. Carisbergensis. Wine is an undistilled product of fruit-juice fermentation brought by Saccharomyces ellipsoidens.

(c) Batcl process: This is type of fermentation process where a large volume of nutrient medium (volume upto 2,25.000 litres) is inoculated and allowed to proceed when maximum yield is obtained. Then the fermentation is stopped, the product is recovered and the fermentation tank (bioreactor) is cleaned, resterilized and used again.

(d) Continuous process: In this type of fermentation process, the fresh nutrient medium is added either continuously or at intervals along with continuous or at intervals withdrawl of a swall portion of fermentation product from the medium.
Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 3

Question 7.
Give a brief explanation of population growth pattern.
There are mainly two primary reasons for increase in human population. These are:
(i) Decline in death rate,
(ii) Increase in longevity.

However, in the civilized world of today, it is unfortunate that man himself is an enemy of man. He harms his fellow men through hoarding, adulteration, deprivation, exploitation, subjugation, violence and war.

World human population growth rate is about 2% on the basis of 1973 census. It was 0.8% per year for the developed countries and nearly 2.591 of developing countries. At this current rate of growth human population is doubling itself every 35 years. If the 6.0 billion people in the world today (1997) continue to increase at the present rate, their combined mass of more than 338 million tonnes will exceed the mass of the earth (which is about 6.5 billion trillion tonnes) in a period of about 1550 years. Thus, the human population explosion as shown in Fig. has become a serious global concern.

Whether our earth planet will have the carrying capacity of such a large mass of human population is a big question. Earth will soon become severely overcrowded and uninhabitable. Will the human beings then take the extraterrestrial migration and colonies other planets? About 56% of the total world population resides in. Asia alone.

Japan is the most thickly populated country and Australia is the most thinly populated country in the world. In India, Kerala is the most thickly populated state.

According to 1991 census, population of India is 843 million. The sex ratio is 929 females per 1,000 males. The literacy rate is 52.11% population growth rate in India is 2.14%.

Population beyond certain limit experiences acute food storage and many individuals face death from starvation or malnutrition. Accordingly if the present trend of human population growth continues, all available natural resources will be too meagre to meet the needs. When all these sources fall far below the need of a common man, a severe competition is bound to ensue, which may lead to large number of socio-economic problems such as energy crisis, ecodegraddation, hunger, poverty, famine, soil erosion, floods, deforestation, unemployment, shortage of raw materials, medical aid, housing, drinking water, clothing and lack of educational facilities. Danger of epidemic diseases will increase. These conditions may result into misery, poor health and increase of urban slums. Overpopulation is the main cause of economic backwardness in our country.
Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 4

Question 8.
How can be know whether a population is flourishing or declining?
The size of a population keeps changing in time, depending on various factors including food availability, predation pressure and reduce weather. These changes in the population density can give us some idea what is happening to the population – whether it is flourishing or declines. However, the density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes. These are as follows:

  • Natality: Number of births during a given period in the population that are added to the initial density.
  • Mortality: The number of deaths in the population during a given period.
  • Immigration: Number of individuals of the same species that have come into from elsewhere during the time period under consideration.
  • Emigration: Number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.

Question 9.
Describe a biotic community (near your home or school) stating the relationships that exist between the common species comprising it.
Biotic community: It includes all the living things present in a particular area having various food relationships. A biotic community is organised into three kinds of living things producers, consumers and decomposers. All the three interact with each other in relation to their food and dependent upon each other for their food. Now let us take the example of a biotic community i.e., pond found near the vicinity of the school. In the pond we can identify all the three components mentioned below:

1. Autotrophs: These are green plants which grow and live in water. The common examples of autotrophs (green plants) found in a freshwater pond are: Hydrilla, Vallisneria, water weeds, lotus and other species of aquatic plants. These plants capture the sun’s energy (high energy) and with the help in their green parts produce food with the help of CO2 and H2O. This process is called photosynthesis. These plants manufacture their food themselves, so they are termed as autotrophs.

2. Consumers: Consumers present in the pond are small animals, insect larvae, Cyclops, protozoan, snails. These organisms feed on aquatic plants and get their energy. On these small animals, feed bigger fishes which form consumer No. 3, finally birds, mammals and even man feed on them.

3. Decomposers: These are microorganisms such as bacteria and fungi which help in the decay of dead living things (organic matter) These help in the recycling of the materials in the bioshpere.

Question 10.
Briefly describe tropical forest biomes.
Tropical forest are of two types: Tropical rain forests, and Tropical deciduous forests.

Tropical rain forests or evergreen forests occur near the equator. In India they are along the Western Ghats in the north-eastern region. The climate is warm and moist, and with no seasonal variations. Rainfall is high (200 cm or more) and uniform with no dry periods, sunlight is strong, soil is rich in minerals and humus. They have excellent conditions for plant growth throughout the year. Life is abundant with about 71 to 80% of world’s insect species and 80-85% of birds species. The dense vegetation is vertically stratified into 4 to 5 strata, each storey in inhabited by different animals. The soil is highly leached. Nutrients are stored in tall vegetation and soil storage is low. Tropical rain forests have highest standing crop biomass among all the biomes.

Tropical deciduous forests: These are distributed in northern and southern parts of India. The climate is warm, with alternate wet and dry periods. Rainfall is high around 100 to 200 cm per annum. Sunlight is strong, soil is rich in mineral and humus. In wet period, the forest is green with dense foliage and thick herbaceous layer. The epiphytes and climbers are fewer than tropical rain forests. The trees shed their leaves during dry seasons. They have trees like sal, teak, mahua, amla, jamun, senwl etc. These forests are rich in animal population too. The common animals are deer, elephants, pigs, lion, birds, reptiles, insects and worms.

Question 11.
Diagrammatically show how energy flows from one trophic level to another trophic level.
Energy is lost as heat from each successive trophic level. This can be shown as follows:
Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 5

Sun serves as a source of energy in the biosphere. Producers trap a small amount of this energy and transfer it to other members of food chain (consumers and decomposers). Thus producers have highest energy level. The pyramid of energy is always straight i.e., it decreases from bottom to top.

Question 12.
Give an account of factors affecting rate of decomposition.
Factors affecting decomposition – Rate of decomposition of detritus is regulated by climatic factors and chemical quality of detritus. The key role is played by temperature and soil moisture through their regulatory effect on the activities of soil microbes. Detritus decomposes are rapidly, within a few weeks or month, in a climate characterized by higher temperature (>25°C) and moist conditions (e.g. in humid tropical regions.

Low temperature (<10°C) sharply reduces decomposition rate even if moisture is in plenty. In regions of high latitude (or altitude) complete decomposition of detritus can take several years or decades. The composition rate is low under prolonged soil dryness even if the temperature remains high as in tropical deserts.

Accumulation of some substance in the detritus increases decomposition rate. Chemical quality of detritus is determined by the relative proportions of water soluble substances, sugars, polyphenols, lignin and nitrogen. Under same climatic conditions decomposition rate is slower if the detritus is rich in substances such a lignin and chitin. The nitrogen-rich detritus (having low amounts of lignin) decomposes relatively, quickly. The actual rate of decomposition of detritus in natural conditions depends upon the integrated effect of environmental conditions and detritus quality also.

Question 13.
Give major characteristics of desert biome.
Deserts are located in rain shadow areas, where dry air from the equator falls upper atmosphere. Their main characteristics are:

  • Prolonged scarcity of rainfall
  • Period of dryness and evaporation from soil higher
  • The annual rainfall is below 70mm to 120mm
  • There is variation of temperature during day and night
  • There is no permanent or temporary flowing water. The sunlight is strong and abundant
  • The soil is sandy or rocky
  • The desert biome has less life and organisms which have adaptation to extreme temperature and arid conditions can only survive
  • The desert plants nuclide ephemeral annual herbs which grow during rainy season, succulent xerophytes like cactus, euphorbias, thorny shrubs, small trees like Prosopis, Salvador, Tamarix. The most common grass in deserts in Cenchrus
  • Animals in desert biome are arthropods, scorpions, spiders, insects, rattlesnake, lizards, camels, jackrabbit, desert rat, cactus woodpecker, roadrunners, burrowing owl. Mostly animals are small and nocturnal
  • The biomass and primary productivity levels in deserts are low.

Question 14.
(a) How is biodiversity distributed along major environmental gradients?
(b) What are the main strategies for conservation of the wildlife?
(a) Gradients of biodiversity: The biodiversity varies with change in latitude or altitude. The biodiversity increases from the poles to the equator. In the temperate region the climate is severe with short growing period for plants. In topical rain forest the conditions are favourable for growth throughout the year. Speciation occurs during favourable conditions.

Mean number of vascular species per 0.1 ha sample area in tropical rain forests varies from 118 – 236. This is only in the range of 21 – 48 species in the temperate zones. Such correlation between diversity and latitude exists for a wide variety of taxonomic groups, like ants, birds, butterflies, and moths.

There is a decrease in species, diversity from lower to higher altitudes on a mountain. A 1000m increase in altitude results in a temperature drop of about 6.5°C. Drop in temperature and greater seasonal variability at higher altitudes are a major factor to reduce biodiversity. The latitudinal and altitudinal gradients of species diversity are two master gradients. More complex and diverse flora and fauna will be in complex and heterogenous physical environment.
Bihar Board 12th Biology Important Questions Long Answer Type Part 2, 6

(b) Strategies for conservation of wild life:

  • Wild life protection in natural as well as artificial habitats.
  • Endangered, vulnerable and rare species should be given preference over others in the conservation programmes.
  • To provide proper habitat to the wild animals, air, water and land should be properly managed.
  • Wild varieties of plants and animals should be preserved for various breeding programmes.
  • The feeding areas, breeding grounds, resting sites etc. of wild animals should be kept intact for animals safe growth and reproduction.
  • Instead of a single species, whole ecosystem should be safeguarded.
  • Areas should be protected for migratory animals.
  • National parks and Sanctuaries should be set up to take care of wild animals.
    Hunting should be prohibited, especially during breeding season.
  • People should be educated and children should be inspired about the need and modes of conservation of wild life.
  • International trade on wild plants, animals and their products should be regulated by law.

Question 15.
Write short notes on the following:
(a) Wild life sanctuary
(b) Biosphere reserves
(c) Genetic diversity
(d) India’s effort in biodiversity conservation.
(a) Wild Life Sanctuary: In a sanctuary, protection is given only to the fauna and operations such as harvesting of timber, collection of minor forest products and private ownership rights are permitted so long as they do not interfere with the well-being of animals. There are presently 368 Sanctuaries in India covering over 1,07,310,13 square kilometres, amounting to 3.2 percent of India’s total geographical area.

(b) Biosphere reserves: Biosphere reserves are a special category of protected areas of land and/or coastal environments wherein people are an integral component of the system. The concept of biosphere reserve was launched in 1975 as a part of the UNESCO’s MAB (man and biosphere) programme. There are 13 biosphere reserves in India which are notified as National Parks. Each biosphere reserve has a core, buffer and a transition zone.

(c) Genetic diversity: It is the genetic variations found amongst members of the same population and geographically separated population of the species. On the basis of genetic differences, all the individuals of the same species become different from one another. Environment plays major role in imparting diversity of living organisms. For eg. there are various races of human beings like Mongoloid, Negrito etc. The differences can be in alleles, in entire genes or in chromosomal structures.

(d) India’s effort in biodiversity conservation: India has contributed a lot to the global biodiversity. India is a homeland of 167 cultivated species and 320 wild relatives of crop plants. It is the centre of diversity of animal species (Zebu, Mithun, chicken, water buffalo, camel); crop plants (rice, sugarcane, banana, tea, millet); fruit plants and vegetables (mango, jackfruit, cucurbits), edible diasporas, alocasia, colocasia, spices and condiments (cardamom, black pepper, ginger, turmeric); bamboos, brassicas, and tree cotton.

The in situ conservation of biodiversity is being, carried out through Biosphere reserves, National parks, Wild Life Sanctuaries and other protected areas by the Ministry of Environment and Forests. The joint forest management systems involve forest departments and local communities.

National Bureau of Plant, Animal and Fish Genetic Resources has a number of programmes to collect and conserve the germplasm of plants and animals in seed gene banks, and field, gene banks for in vitro conservation.

The land races and diverse food and medicinal plants are also being conserved.

Question 16.
With an example explain the importance of organic farming.
Organic farming is a cyclical, zero-waste procedure, where waste products from one process are cycled in as nutrients for other processes. This enchances the maximum ultilization of resource and increases the efficiency of production. For example, a farmer of Sonipat, Haryana, Ramesh Chandra Dagar, includes bee-keeping, dairy management, water harvesting, compositing and agriculture in a chain of processes, which support each other and allow an extremely economical and sustainable venture.

Cattle excreta (dung) are used as dung in place of chemical fertilizer for crops. Crop waste is used to create compost, which can be used as a natural fertilizer or can be used to generate natural gas for satisfying the energy needs of the farm.

Question 17.
What are the major cause of species losses in a geographical region?
Or, What kinds of threats to the biodiversity may lead to its loss?
These are:
(i) Habitat loss and fragmentation: The destruction of habitat is the primary cause of loss of diversity. It occurs due to cutting down of trees, filling a wet land, ploughing a grassland or by burning a forest.

This causes the change in the natural habitat and kills or forced out many plants, animals and micro-organisms. A forest patch surrounded by croplands, orchards, plantations or ubran areas is an example of fragmented habitats. With the fragmentation of a large forest tract, the species occupying the deeper parts of forests get disappeared.

(ii) Disturbance and pollution: Communities are largely affected by natural disturbance such as fire, tree fall and defoliation as well as by man made disturbances. Man disturbs the community by using large number of synthetic compounds, releasing of radiation or spilling over of oil in the sea. The lead poisoning and eutrophication cause mortality of species and reduce the species diversity.

(iii) Introduction of exotic species: New species that enter a geographical region are called exotic or alien species. Introduction of such species may cause disappearance of native species through changed biotic interactions e.g, Nile perch, an exotic predatory fish introduced into lake victoria threatens the entire ecosystem of the lake by eliminating several native species of the small Cichlid fish species that were endemic to this freshwater aquatic system. Similarly, Lantana Camara has invaded many forest lands in different parts of India and strongly competes with the native species.

(iv) Co-extinctions: When a species becomes extinct, the plant and animal species associated with it also become extinct. When a fish species become extinct, its assemblage of parasites also meets the same fate.

Question 18.
Why should we conserve biodiversity?
Or, What are the uses of biodiversity?
(i) Source of food and improved varieties: Biodiversity has a prime role in agriculture as a source of new crops, material for breeding improved varieties and new biodegradable pesticides. The wheat, com and rice are the three main major carbohydrate crops which yield two third of the food substaining the human population.

The commercial domestic species are crossbreed with their wild relatives to improve their traits. The genes of wild species are used to confer new properties such as diseases resistance or improve yield in domesticated species e.g., rice grown in Asia is protected from diseases by genes received from a single wild rice species (Oryza nivara) from India.

(ii) Drugs and medicines: Several important pharmaceuticals are obtained from plant based substances and these substances are developed into variable drugs like Morphine, Quinine and Taxol etc. Morphine (Papaver somniferum) used as an analgesic; Quinine (Chinchona ledgeriana) used for the treatment of malaria and Taxol, an anti-cancer drug obtained from the bark of the yew tree (Taxus brevifolia, T. baccata). 25% of the drugs in the pharmacy are derived from a mere 120 species of plants.

(iii) Asthetic and cultural benefits: Biodiversity has great aesthetic value. Examples of aesthetic rewards include ecotourism, bird watching, wildlife, pet keeping and gardening etc. In majority of Indian villages and towns, plants like Ocimum sanctum (Tulsi) and Ficus religiosa (Peepal) are considered sacred and worshipped by the people. Several birds and even snakes have been considered scared.

(iv) Ecosystem services: Biodiversity is essential for maintenance of gaseous composition of the atmoshphere, climate control by forests and oceanic systems, natural pest control, pollination of plants by insects and birds, formation and protection of soil, conservation and purification of water and nutrient cycling etc.