12.7 Exercise Problems

Q1. Fill in the blanks.
(a) Plants are called as_________because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.
(c) In aquatic ecosystems, the limiting factor for the productivity is_________.
(d) Common detritivores in our ecosystem are_________.
(e) The major reservoir of carbon on earth is_________.

Answer: (a) Plants are called as autotrophs because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is of inverted type.
(c) In aquatic ecosystems, the limiting factor for productivity is light.
(d) Common detritivores in our ecosystem are earthworms.
(e) A major reservoir of carbon on Earth is oceans.

Q2. Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers

Answer:  (d) Decomposers: Decomposers are microorganisms including fungi and bacteria. These break down the remains of dead plants and animals. Decomposers form the largest community of organisms in a food chain.

Q3. The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes

Answer: (b) Zooplankton: In a lake, the first trophic level is formed by phytoplankton while the second trophic level is formed by zooplankton.

Q4. Secondary producers are:
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Answer: (d) None of the above. In a food chain, plants are the only producers, there are no other producers. Herbivores and carnivores are primary and secondary consumers respectively.

Q5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100 %
(b) 50 %
(c) 1-5 %
(d) 2-10 %

Answer: (b) 50 %, Out of total incident solar radiation, \(50 \%\) is formed by photosynthetically active radiation

Q6. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity

Answer: (a) Grazing food chain and detritus food chain

\(
\begin{array}{|l|l|}
\hline \text { Grazing food chain } & \text { Detritus food chain } \\
\hline \text { Solar energy is utilized } & \begin{array}{l}
\text { Biomass energy stored in the bodies of plants and } \\
\text { animals is utilized }
\end{array} \\
\hline \text { The grazing food chain starts from producers } & \begin{array}{l}
\text { The detritus food chain starts from detritus i.e., dead } \\
\text { plants and animals }
\end{array} \\
\hline \text { It includes a number of trophic levels } & \begin{array}{l}
\text { Detritus food chain includes a lesser number of trophic } \\
\text { levels }
\end{array} \\
\hline
\end{array}
\)

Answer: (b) Production and decomposition

\(
\begin{array}{|l|l|}
\hline \text { Production } & \text { Decomposition } \\
\hline \begin{array}{l}
\text { It is the process of formation of organic matter by } \\
\text { producers by means of photosynthesis }
\end{array} & \begin{array}{l}
\text { It is the process of breakdown of complex organic matter } \\
\text { into simpler organic molecules }
\end{array} \\
\hline \text { It requires sunlight } & \text { It can take place in the absence of sunlight also } \\
\hline
\end{array}
\)

Answer: (c) Upright and inverted pyramid

\(
\begin{array}{|l|l|}
\hline \text { Upright pyramid } & \text { Inverted pyramid } \\
\hline \begin{array}{l}
\text { Upright pyramids show a decrease in number } \\
\text { and biomass from producer level to the } \\
\text { tertiary consumer level }
\end{array} & \begin{array}{l}
\text { Inverted pyramids depict an increase in number and } \\
\text { biomass from producer level to tertiary consumer level }
\end{array} \\
\hline \text { Pyramid of energy is upright } & \text { Pyramid of biomass and number can be inverted } \\
\hline
\end{array}
\)

Answer: (d) Food chain and Food web

\(
\begin{array}{|l|l|}
\hline \text { Food chain } & \text { Food web } \\
\hline \begin{array}{l}
\text { A series of living organisms present at } \\
\text { different trophic levels and related to food } \\
\text { habits }
\end{array} & \text { An interconnected web of a number of food chains. } \\
\hline \begin{array}{l}
\text { The members at higher trophic level feed } \\
\text { upon the members of lower trophic levels. }
\end{array} & \begin{array}{l}
\text { Different organisms have more than } \\
\text { one food sources }
\end{array} \\
\hline
\end{array}
\)

Answer: (e) Litter and detritus

\(
\begin{array}{|l|l|}
\hline \text { Litter } & \text { Detritus } \\
\hline \text { The term litter refers to any kind of waste generated } & \begin{array}{l}
\text { The term detritus refers to the dead waste of plants and } \\
\text { animals’ bodies }
\end{array} \\
\hline \begin{array}{l}
\text { Litter can be biodegradable as well as non- } \\
\text { biodegradable }
\end{array} & \text { Detritus is biodegradable } \\
\hline
\end{array}
\)

Answer: (f) Primary and secondary productivity

\(
\begin{array}{|l|l|}
\hline \text { Primary productivity } & \text { Secondary productivity } \\
\hline \begin{array}{l}
\text { Primary productivity refers to the amount of organic } \\
\text { matter produced by producers per unit area over a } \\
\text { period of time. }
\end{array} & \begin{array}{l}
\text { Secondary productivity refers over a period of time to the } \\
\text { rate of production of organic matter by consumers }
\end{array} \\
\hline
\end{array}
\)

Q7. Describe the components of an ecosystem.

Answer: An ecosystem refers to be the functional unit of nature in which living organisms interact among themselves and also with their surrounding physical environment in order to perform nutrient cycling, energy flow, decomposition and productivity. There are many types of ecosystems such as pond ecosystem, forest ecosystem etc.
Components of ecosystem: The components of the ecosystem can be divided into abiotic components and biotic components.

• 1. Abiotic components- These include the non-living components of nature such as light, temperature, water, soil, air, inorganic nutrients etc.
• 2. Biotic components- The biotic components of an ecosystem refer to the living organisms present in that ecosystem. The biotic components are divided as producers, consumers and decomposers. Producers are plants, consumers include animals which can be herbivores or carnivores and decomposers include microorganisms like fungi and bacteria.

Q8. Define ecological pyramids and describe with examples, pyramids of number and biomass.

Answer: Ecological pyramid- The graphical representation of an ecological parameter such as number, biomass or energy, sequence wise in various trophic levels of a food chain in which producers are at the base, herbivores in the middle and carnivores at the top level. Ecological pyramids can be upright, inverted, or spindle-shaped. The three common types of ecological pyramids include the pyramid of number, pyramid of biomass and pyramid of energy.

• 1. Pyramids of number- It is the number of individuals per unit area at various trophic levels. It is generally upright, however, the pyramid of number in case of a big tree is generally inverted because number of insects that feed on tree generally exceeds in number.
• 2. Pyramids of biomass- It represents the biomass in various trophic levels. A pyramid of biomass is upright except in an aquatic food chain. A pyramid of biomass in the sea is generally inverted because the biomass of fishes is generally more than that of phytoplankton.
• 3. Pyramids of energy- It is the graphic representation of the amount of energy trapped at different trophic levels per unit area. Pyramid of energy is always upright.

Q9. What is primary productivity? Give brief description of factors that affect primary productivity.

Answer: Primary productivity refers to the amount of organic matter or biomass produced by producers per unit area over a period of time. The primary productivity of an ecosystem depends upon a number of factors such as light, temperature, water, precipitation, availability of nutrients etc.

Q10. Define decomposition and describe the processes and products of decomposition.

Answer: Decomposition is the process of breaking down complex organic matter of detritus into inorganic substances such as carbon dioxide, water and nutrients. Dead remains of plants and animals constitute detritus. The process of decomposition involves steps like fragmentation, leaching, catabolism, humification and mineralization.

• 1. Fragmentation of Detritus- In this step, the detritus is broken down into small fragments by earthworms
• 2. Leaching- In leaching the water-soluble nutrients, seep down into the soil and become unavailable salts.
• 3. Catabolism- The small fragments of decomposed by decomposers like fungi and bacteria with the help of action of enzymes.
• 4. Humification- In this step, humus is formed. Humus is a dark colored, amorphous solid substance that acts as a nutrient reserve.
• 5. Mineralization- In this step by the action of microbes, the inorganic nutrients are released from the humus.

Q11. Give an account of energy flow in an ecosystem.

Answer: Energy Flow in an ecosystem :
All living organisms are dependent for their food on producers, directly or indirectly. There is a unidirectional flow of energy from the sun to producers and then to consumers. Photosynthetically active radiation (PAR) is responsible for the synthesis of food by plants. Animals obtain their food from plants, so they are called consumers. The process of eating and being eaten is called a food chain in which energy flows from producers to consumers. For example, In Grazing food chain, the grass is eaten by goats and Goats are further eaten by man. Similarly, in the detritus food chain, the sequence begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms (fungi and bacteria). These are also known as saprotrophs. Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them. Natural interconnection of food chain forms the food web. Thus, in an ecosystem, energy flow occurs through food chains and food webs.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Name an organism found as secondary carnivore in an aquatic ecosystem.

Answer: Catfish/water snake etc.

Q2. What does the base tier of the ecological pyramid represent?

Answer: Producers

Q3. Arrange the following as observed in vertical stratification of a forest: Grass, Shrubby plants, Teak, Amaranths.

Answer: Grass, Amaranths, Shrubby plants, Teak

Q4. Name an omnivore which occurs in both grazing food chain and the decomposer food chain. 

Answer:  Sparrow/crow

Q5. Justify the pitcher plant as a producer.

Answer: Justify the pitcher plant as a producer.

Q6. Name any two organisms which can occupy more than one trophic level in an ecosystem.

Answer: Man and sparrow ete.

Q7. What is the ultimate source of energy for the ecosystems?

Answer: Sunlight (solar radiation)

Q8. Is the common edible mushroom an autotroph or a heterotroph? 

Answer: Heterotroph (as it does not have chlorophyll)

Q9. Why are oceans least productive?

Answer: Oceans are least productive because they do not receive sufficient sunlight which is necessary for 1° productivity.

Q10. Why is the rate of assimilation of energy at the herbivore level called secondary productivity?

Answer: It is because the biomass available to the consumer for consumption is a resultant of the primary productivity from plants.

Q11. What is common to earthworm, mushroom, soil mites and dung beetle in an ecosystem.

Answer: They are all detritivores i.e., decomposing organisms which feed on dead remains of plants and animals.

SHORT ANSWER TYPE QUESTIONS

Q1. Organisms at a higher trophic level have less energy available. Comment.

Answer: The transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. So, higher trophic level have less energy as- compared to lower trophic level.

Q2. The number of trophic levels in an ecosystem are limited. Comment.

Answer: In a food chain, only 10% of the total amount of energy is passed on to the next trophic level from the previous trophic level. So, there is a decrease in the amount of energy available at the successive trophic levels. As we move higher up in the food chain the amount of energy diminishes to a level at which it cannot sustain any trophic level, thereby limiting the number of trophic levels.

Q3. Is an aquarium a complete ecosystem?

Answer:  Aquarium has all the biotic and abiotic components of an ecosystem, so, aquarium is a complete ecosystem.

Q4. What could be the reason for the faster rate of decomposition in the tropics?

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

Q5. Flow of energy through various trophic levels in an ecosystem is unidirectional and non-cyclic. Explain.

Answer: The energy from the sun reaches the food chain through the primary producers (plants). This energy is passed on through successive trophic levels in the food chain. The energy transfer in the food chain follows the 10 percent law where in only 10% of the energy is transferred from one trophic level to the next successively. So, the movement of energy is only in one direction from lower to higher trophic level.

Q6. Apart from plants and animals, microbes form a permanent biotic component in an ecosystem. While plants have been referred to as autotrophs and animals as heterotrophs, what are microbes referred to as? How do the microbes fulfil their energy requirements?

Answer: Microbes are referred to as heterotrophs and saprotrophs. They fulfil their energy requirement by feeding on dead remains of plants and animals through the process of decomposition.

Q7. In relation to energy transfer in ecosystem, explain the statement “10kg of deer’s meat is equivalent to 1 kg of lion’s flesh”.

Answer: Transfer of energy in an ecosystem follow 10% law, i.e., only 10% energy is transferred from one trophic level to higher trophic level. So, if lion feeds on deer than 10 kg of deer’s meat will form 1 kg of lion’s flesh.

Q8. Primary productivity varies from ecosystem to ecosystem. Explain?

Answer: Primary productivity varies from ecosystem to ecosystem because it depends on the plant species inhabiting the area and their photosynthetic activity. It also depend on various environmental factors.

Q9. What is an incomplete ecosystem? Explain with the help of suitable example.

Answer: An ecosystem is a functional unit with biotic and abiotic factors interacting with one another resulting in a physical structure. Absence of any component will make an ecosystem incomplete as it will hinder the functioning of the ecosystem. Examples of such an ecosystem can be a fish tank or deep aphotic zone of the oceans where producers are absent.

Q10. What are the shortcomings of ecological pyramids in the study of ecosystem?

Answer: The ecological pyramid assumes a simple food chain and does not accommodate food webs. Thereby,’ it does not take into account the fact that species may belong to two or more trophic levels at a time. Also saprophytes despite their vital role in ecosystem are given no place in the ecological pyramids.

Q11. How do you distinguish between humification and mineralisation?

Answer: Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients . The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.

Q12. Fill in the trophic levels (1, 2, 3 and 4) in the boxes provided in the figure.

Answer:

Q13. The rate of decomposition of detritus is affected by the abiotic factors like availability of oxygen, pH of the soil substratum, temperature etc. Discuss.

Answer: The decomposition of detritus is due to activities of micro-organisms. The rate of growth of microbes is affected by temperature. The pH of substratum affects the composition of microbes (acidophiles / basophiles) which degrade the dead organic matter. If oxygen is present, aerobic degradation occurs. In the absence of oxygen anaerobiosis sets in and there will be incomplete degradation. Also, the degradation is due to activity of exoenzymes secreted by the microbes and the activity of enzyme is affected by factors such as temperature etc.

LONG ANSWER TYPE QUESTIONS

Q1. A farmer harvests his crop and expresses his harvest in three different ways.
a. I have harvested 10 quintals of wheat.
b. I have harvested 10 quintals of wheat today in one acre of land.
c. I have harvested 10 quintals of wheat in one acre of land, 6 months after sowing.
Do the above statements mean one and the same thing. If your answer is yes, give reasons. And if your answer is ‘no’ explain the meaning of each expression.

Answer:  No, the meaning of above statements is different
(a) In this statement farmer gives information of only the quantity of the wheat but area and time period is not given.
(b) In this statement farmer gives information of primary production that is quantity and area but not the time period.
(c) In this statement farmer gives information of primary productivity that is quantity, area and time period all are given.

Q2. Justify the following statement in terms of ecosystem dynamics. “Nature tends to increase the gross primary productivity, while man tends to increase the net primary productivity”.

Answer: Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).

• Nature tends to increase the gross primary productivity so that it can utilise much of the solar energy and convert it into food.
• Man tends to increase the net primary productivity so that sufficient food is provided to the growing human population.

Q3. Which of the following ecosystems will be more productive in terms of primary productivity? Justify your answer. A young forest, a natural old forest, a shallow polluted lake, alpine meadow.

Answer: Among these a young forest will be more productive because primary productivity depends on the plant species inhabiting a particular area. A young forest grows more rapidly than old forest hence it has more primary productivity. A shallow polluted lake and alpine meadow has lower number of producers hence shows lower primary productivity.

Q4. What are the three types of ecological pyramids. What information is conveyed by each pyramid with regard to structure, function and energy in the ecosystem.

Answer: The three ecological pyramids that are usually studied are (a) pyramid of number (b) pyramid of biomass and (c) pyramid of energy.

• Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level. No generalisations we make will be true if we take only a few individuals at any trophic level into account. Also a given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
• In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also energy at a lower trophic level is always more than at a higher level.

Q5. Write a short note on pyramid of numbers and pyramid of biomass.

Answer: Write a short note on pyramid of numbers and pyramid of biomass.

Q6. Given below is a list of autotrophs and heterotrophs. With your knowledge about food chain, establish various linkages between the organisms on the principle of ‘eating and being eaten’. What is this inter-linkage established known as?

Algae, hydrilla, grasshopper, rat, squirrel, crow, maize plant, deer, rabbit, lizard, wolf, snake, peacock, phytoplankton, crustaceans, whale, tiger, lion, sparrow, duck, crane, cockroach, spider, toad, fish, leopard, elephant, goat, Nymphaea, Spirogyra.

Answer: Primary Producer’s (First trophic level): Algae, Hydrilla, maize plant, Phytoplankton, Nymphaea, Spirogyra.
Primary Consumer (Second trophic level): Crustaceans, grasshopper, deer, mouse, squirrel, rabbit, elephant, goat.
Secondly Consumer (Third trophic level): Spider, cockroach, lizard, wolf, snake, toad, fish, crane.
Top carnivore (Fourth trophic level): Lion, Tiger.

Q7. “The energy flow in the ecosystem follows the second law of thermodynamics.” Explain.

Answer: Ecosystems need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness. Hence, ecosystems are not exempt from the Second Law of thermodynamics.

Q8. What will happen to an ecosystem if:
a. All producers are removed;
b. All organisms of herbivore level are eliminated; and
c. All top carnivore population is removed

Answer: (a) Reduction in primary productivity. No biomass available for consumption by higher trophic levels / heterotrophs
(b) Increase in primary productivity and biomass of producers.
Carnivores population will subsequently dwindle due to food shortage.
(c) Increase in number of herbivores

• Overgrazing by herbivores
• Desertification

Q9. Give two examples of artificial or man made ecosystems. List the salient features by which they differ from natural ecosystems.

Answer: Crop fields and an aquarium considered as man-made ecosystems. In artificial ecosystems the biotic and abiotic components of ecosytems are maintained artificially by regular feeding and clearing of aquarium and regular irrigation and sowing of seeds in crop field ecosystem. Natural ecosystem self sustains biotic and abiotic components of an ecosystem.