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Q1. List the three parameters used for constructing ecological pyramids. Describe any one instance where the pyramid may look inverted. 

Solution

The three parameters used for constructing ecological pyramids are (i) Number of individuals in a trophic level. (ii) Biomass of individuals in a trophic level. (iii) Rate of flow of energy in a trophic level. The pyramid may look inverted in a tree ecosystem where the numbers of consumers are numerous depending on a single producer. 

Q2. A food chain has only a few trophic levels. Justify.

Solution

In a food chain, about 80-90% of energy available with a trophic level is lost during its transfer from one trophic level to the next, so the amount of energy available decreases. 

Q3. What are seral communities?

Solution

The whole series of changes in community characteristics from the pioneer stage to the climax stage constitute a sere.

Q4. Describe the basic requirements of an ecosystem.

Solution

The basic requirements of an ecosystem are (a) Inorganic nutrients such as carbon dioxide and water. (b) Autotrophs or producers such as plants. (c) Micro-consumers or decomposers such as bacteria and fungi. (d) A source of continuous supply of energy, i.e. Sun.

Q5. Explain how does (i) A primary succession starts on a bare rock and reaches a climax community. (ii) The algal bloom eventually chokes the water body in an industrial area.

Solution

(i) The bare rock is invaded by pioneer species called lichens. Lichens secrete acids to dissolve rock, helping in weathering and soil formation which pave the way for the bryophytes, which hold in a small amount of soil. The bryophytes are succeeded by the bigger plants, leading to forests. Forests are the climax community in this succession. (ii) An algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Algal blooms may occur in freshwater or marine environments. The fertility of a lake in an industrial area increases with inorganic chemicals and sewage, causing an increase of primary productivity and leading to an increased growth of algae. The decomposition of algae leads to reduction in oxygen quantity and death of aquatic life. This excessive richness of nutrients in a lake or other body of water, frequently due to run-off from the land, causes a dense growth of plant life called eutrophication.

Q6. What are starting points of a grazing food chain and a detritus food chain respectively? 

Solution

Producers are the starting point of a grazing food chain, and detritus is the starting point of a detritus food chain.

Q7. Name the dominant producers in a deep aquatic ecosystem. What other name could you give to a primary consumer?

Solution

Phytoplankton are the dominant producers in an aquatic ecosystem. Primary consumers are also known as herbivores.

Q8. (i) Explain primary productivity and the factors which influence it. (ii) Describe how do oxygen and chemical composition of detritus control decomposition.

Solution

(i) Primary productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It varies in different types of ecosystems. It is expressed in terms of weight or energy. It depends on the following factors: (a) Plant species inhabiting a particular area (b) Environmental factors (c) Availability of nutrients (d) Photosynthetic capacity of plants (ii) Decomposition of detritus is slow if it contains lignin, chitin, tannins and cellulose, whereas it is quick if detritus is made up of nitrogenous compounds and water-soluble substances like sugars. This is because the latter are easy to degrade. Oxygen is required for the activity of decomposers and detritivores. Therefore a reduced oxygen amount will slow down the process of decomposition.

Q9. Explain with the help of two examples, how the pyramid of numbers and the pyramid of biomass can look inverted. 

Solution

The pyramid of biomass in a pond is generally inverted because the biomass of fish exceeds that of phytoplankton and the number of big fish eating the small fish is also greater than the small ones. Also, in the pyramid of numbers, the number of insects feeding on a big tree is far greater than the tree. Now the number of small birds depending on the insects and the number of larger birds eating the smaller ones also increases in the order. 

Q10. Mention one similarity between hydrarch and xerarch successions.

Solution

Both hydrarch and xerarch successions result in medium water conditions.

Q11. What could be the reason for the faster rate of decomposition in the tropics? 

Solution

The rate of decomposition is regulated by climatic factors such as temperature and soil moisture as they have an effect on the activities of soil microbes. The tropics with its hot and humid climatic conditions provide an environment which is ideal for the microbes to speed up the process of decomposition.

Q12. State two differences between the first trophic levels of the detritus food chain and the grazing food chain. 

Solution

  First trophic level of detritus food chain First trophic level of grazing food chain (i) Decomposers are the first trophic level. (i) Producers are the first trophic level. (ii) They break down complex organic matter into a simpler form by secreting enzymes. (ii) They prepare complex organic molecules from simple organic material by using sunlight.    

Q13. What are primary consumers?

Solution

The herbivores which depend on plants for their food are called primary consumers.

Q14. What is the role of plants, animals and bacteria in an ecosystem?

Solution

Plants, animals and bacteria are producers, consumers and decomposers in an ecosystem. 

Q15. The climax stage is achieved quickly in secondary succession as compared to primary succession. Why?

Solution

The rate of succession is much faster in secondary succession as the soil is already present as compared to primary succession where the process starts from a bare area.

Q16. Why is a food web formed in nature? 

Solution

Many organisms occupy positions in different food chains, and several food chains become interconnected to form a food web. 

Q17. The gradual and predictable change in the species composition of a given area is called ecological succession. What do you understand with the pioneer and climax communities in this context? 

Solution

The first community invading the habitat and starting ecological succession is called a pioneer community. Examples: Phytoplankton in hydrosere and lichens in lithosere The most stable community developed at the end of ecological succession is called a climax community. Example: Forest stage

Q18. Name the pioneer and the climax species in a water body. Mention the changes observed in the biomass and biodiversity of the successive seral communities developing in the water body.  

Solution

Pioneer species in a hydrosere in a water body is formed of plankton like diatoms, phytoplankton, cyanobacteria etc., while the climax community in it is forest trees depending on the nature of the climate. In a hydrosere, there is successive increase in biomass as well as biodiversity in the seral communities.

Q19. (i) What are the shortcomings of ecological pyramids in the study of an ecosystem? (ii) ‘Pyramid of energy is always upright’. Explain. (iii) What is meant by saying that the energy flow in an ecosystem is unidirectional? 

Solution

(i) The ecological pyramid is like a simple food chain and does not accommodate food webs. Thus, it does not take into account the fact that species may belong to two or more trophic levels at a time. Also, the saprophytes in an ecosystem are given no place in the ecological pyramids despite their vital role. (ii) Pyramid of energy is always upright because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step. (iii) The Sun is an ultimate source of all energy. The energy passes from producers to primary consumers, secondary consumers, tertiary consumers and ultimately to the quaternary consumers, and the flow is in one direction, i.e. energy cannot pass back in a reverse direction. 

Q20. Why nutrient cycles in nature are called biogeochemical cycles?

Solution

Nutrient cycles in nature are called biogeochemical cycles because the elements move cyclically from the environment to living organisms and back to the environment.

Q21. Mention the role of pioneer species in primary succession on rocks. 

Solution

In primary succession on rocks, the pioneer species invade a bare area and give way for other species to grow.

Q22. Why is the rate of assimilation of energy at the herbivore level called secondary productivity? 

Solution

The rate of assimilation of energy at the herbivore level is called secondary productivity because the biomass available to the consumer for consumption is a resultant of the primary productivity from plants.

Q23. What does secondary productivity in an ecosystem indicate? List any two factors by which productivity is limited in an aquatic ecosystem. 

Solution

The rate of assimilation and formation of new organic matter by consumers is called secondary productivity. The factors which affect productivity in an aquatic ecosystem are (a) Nitrogen in a marine ecosystem (b) Light, which decreases with increasing depth of water

Q24. What is stratification in an ecosystem? Explain with an example.  

Solution

The vertical distribution of different species occupying different levels in an ecosystem is called stratification. Trees occupy the topmost vertical layer of a forest, shrubs occupy the second layer and herbs and grasses occupy the bottommost or base layers.

Q25. Why is the pyramid of biomass inverted in a pond ecosystem? 

Solution

The pyramid of biomass is inverted in a pond ecosystem because the biomass of fish is higher as compared to producers. 

Q26. Why is the length of a food chain in an ecosystem generally limited to 3 to 4 trophic levels? 

Solution

The amount of energy flow decreases with successive trophic levels as only 10% of energy is transferred from one trophic level to the other. The energy is lost in the form of respiration and other vital activities to maintain life. If more trophic levels are present, then the residual energy will be limited and decreases to such an extent that it cannot further support any trophic level by the flow of energy. 

Q27. Explain the function of ‘reservoir’ in a nutrient cycle. List the two types of nutrient cycles in nature.

Solution

The function of the ‘reservoir’ is to meet the deficit which occurs due to an imbalance in the rate of influx and efflux. Two types of nutrient cycles are gaseous cycles (nitrogen and oxygen cycles) and sedimentary cycles (sulphur and phosphorus cycles).

Q28. Explain the response of all communities to environment over time. 

Solution

Environmental factors like temperature, water and light may influence the members of communities in varying degrees. Organisms, in response to these factors, try to adapt according to their capacities. In this process, they may try to maintain a constant internal environment through homeostasis or migrate to a less stressful environment or may even suspend activities till favourable conditions return.

Q29. (i) Name the kind of organisms which constitute the pioneer community of xerarch and hydrarch succession. (ii) Differentiate between pioneer community and climax community.

Solution

(i) Xerarch succession is seen in lichens and hydrarch succession is seen in phytoplankton. (ii) Pioneer Community Climax Community (a) The species which invade a bare land is called pioneer community. (a) The final stage in a succession constitutes the climax community. (b) The pioneer species have high reproductive rate. (b) The climax species have low reproductive rate. (c) Pioneer community is replaceable. (c) It is stable and not replaced.  

Q30. All the primary productivity is not available to a herbivore. Give one reason. 

Solution

All the primary productivity is not available to a herbivore because a considerable amount is utilised by the plant in respiration.