Maharashtra Board Class 12 Biology Chapter 14 Ecosystems and Energy Flow Solutions

Biology Class 12 Chapter 14 Exercise Solutions

1. Multiple Choice Questions

Question 1.Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
Answer: (a) Producers
In simple words: In most food chains, producers form the base and are the most numerous organisms, supporting all other trophic levels.

🎯 Exam Tip: Understanding the base of a food pyramid is crucial; producers are always at the bottom and are the most abundant.

Question 2.The second trophic level in a lake is ..................
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Answer: (b) Zooplankton
In simple words: In an aquatic ecosystem like a lake, phytoplankton are producers (first trophic level), and zooplankton feed on them, thus occupying the second trophic level.

🎯 Exam Tip: Remember the specific examples of organisms at each trophic level in different ecosystems, especially aquatic ones.

Question 3.Secondary consumers are ..................
(a) Herbivores
(b) Producers
(c) Carnivores
(d) Autotrophs
Answer: (c) Carnivores
In simple words: Secondary consumers are organisms that feed on primary consumers (herbivores), making them carnivores.

🎯 Exam Tip: Clearly distinguish between different consumer types (primary, secondary, tertiary) based on their diet.

Question 4.What is the % of photosynthetically active radiation in the incident solar radiation?
(a) 100%
(b) 50%
(c) 1-5%
(d) 2-10%
Answer: (b) 50%
In simple words: Roughly 50% of the total incident solar radiation reaching the Earth's surface is Photosynthetically Active Radiation (PAR), which plants use for photosynthesis.

🎯 Exam Tip: Know the approximate percentage of solar radiation that is useful for photosynthesis, as this is a common factoid.

Question 5.Give the term used to express a community in its final stage of succession?
(a) End community
(b) Final community
(c) Climax community
(d) Dark community
Answer: (c) Climax community
In simple words: The climax community is the stable, mature, and self-sustaining ecological community that develops at the end of ecological succession.

🎯 Exam Tip: The "climax community" is the key term for the stable, mature stage of ecological succession.

Question 6.After landslide which of the following type of succession occurs?
(a) Primary
(b) Secondary
(c) Tertiary
(d) Climax
Answer: (a) Primary
In simple words: A landslide creates a new, barren area where no soil exists, leading to primary succession where life starts from scratch.

🎯 Exam Tip: Differentiate between primary and secondary succession; primary succession occurs on newly formed or exposed land without existing soil, like after a landslide or volcanic eruption.

Question 7.Which of the following is most often a limiting factor of the primary productivity in any ecosystem?
(a) Carbon
(b) Nitrogen
(c) Phosphorus
(d) Sulphur
Answer: (c) Phosphorus
In simple words: Phosphorus is frequently a limiting nutrient for primary productivity, especially in aquatic ecosystems, as its availability often restricts the growth of producers.

🎯 Exam Tip: Recall that nitrogen and phosphorus are common limiting factors for plant growth, with phosphorus often being the most critical in many natural systems.

2. Very Short Answer Question.

Question 1.Give an example of ecosystem which shows inverted pyramid of numbers.
Answer: Number of insects dependent on a single tree, is an example of ecosystem having inverted pyramid of numbers.
In simple words: An ecosystem with a single large producer (like a tree) supporting many small primary consumers (insects) demonstrates an inverted pyramid of numbers.

🎯 Exam Tip: Be able to provide specific, clear examples for each type of ecological pyramid, especially inverted ones.

Question 2.Give an example of ecosystem which shows inverted pyramid of biomass.
Answer: Oceanic ecosystem has inverted pyramid of biomass.
In simple words: In an oceanic ecosystem, the small biomass of fast-reproducing phytoplankton (producers) supports a larger biomass of zooplankton and fishes, creating an inverted biomass pyramid.

🎯 Exam Tip: An inverted pyramid of biomass is characteristic of aquatic ecosystems where producers have short lifespans and high turnover rates.

Question 3.Which mineral acts as limiting factor for productivity in an aquatic ecosystem?
Answer: Phosphorus acts as limiting factor for productivity in an aquatic ecosystem.
In simple words: In water bodies, the availability of phosphorus often restricts the growth of algae and aquatic plants, thereby limiting overall productivity.

🎯 Exam Tip: Remember phosphorus is a crucial limiting nutrient in aquatic systems, often leading to eutrophication when in excess.

Question 4.Name the reservoir and sink of carbon in carbon cycle.
Answer: Atmosphere is the reservoir of carbon cycle, while fossil fuels embedded in ocean and oceanic waters are the sink of carbon in carbon cycle.
In simple words: The atmosphere acts as the main active reservoir for carbon, while long-term storage areas like fossil fuels and deep oceanic waters are considered carbon sinks.

🎯 Exam Tip: Distinguish between a reservoir (active, cycling pool) and a sink (long-term storage) in biogeochemical cycles.

3. Short Answer Questions.

Question 1.Upright and inverted pyramid of biomass.
Answer:

🎯 Exam Tip: When comparing types of pyramids, use clear, concise points of contrast and provide examples where possible.

Question 2.Food chain and Food web.
Answer:

🎯 Exam Tip: Highlighting the linearity vs. interconnectedness and the stability implications are key differences to score well.

4. Long Answer Questions

Question 1.Define ecological pyramids and describe with examples, pyramids of number and biomass.
Answer:1. Ecological Pyramids : Ecological Pyramids are the representation of relationships between different components of ecosystem at successive trophic levels. 2. Pyramid of numbers: • Pyramid of numbers is the diagrammatic representation which shows the relationship between producers, herbivores and carnivores at successive trophic levels in terms of their numbers. • As we go up the trophic levels, the interdependent organisms keep on reducing in their numbers. • For example, the number of grasses are more than the number of herbivores which eat them. The number of herbivores such as rabbits would be lesser than grass but greater than the carnivores that are dependent upon the population of rabbits. • Thus, the producers would be more than primary consumers and primary consumers would be more than secondary consumers. The top level consumers would be least in their numbers. This pyramid shows upright nature. 3. Pyramid of biomass: (1) Pyramid of biomass are constructed by taking into consideration the different biomass in every successive trophic level. (2) Pyramid of biomass in seas in inverted as the biomass of fishes is more than the biomass of phytoplankton.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह एक सीधा बायोमास पिरामिड दर्शाता है जहाँ उत्पादकों की बायोमास सबसे अधिक (100%) होती है, और जैसे-जैसे आप ऊपर के पोषी स्तरों (प्राथमिक उपभोक्ता 10%, द्वितीयक उपभोक्ता 1%, तृतीयक उपभोक्ता 0.1%) में बढ़ते हैं, बायोमास लगातार कम होता जाता है। यह एक विशिष्ट स्थलीय पारिस्थितिकी तंत्र में ऊर्जा प्रवाह और द्रव्यमान वितरण का प्रतिनिधित्व करता है।
ℹ️ चित्र व्याख्या (Diagram Explanation): यह दूसरा चित्र एक सीधा बायोमास पिरामिड दिखाता है जिसमें विभिन्न पोषी स्तरों पर जीवों का द्रव्यमान दर्शाया गया है। सबसे नीचे 900 किलोग्राम घास (उत्पादक) हैं, उसके ऊपर 100 किलोग्राम चूहे (द्वितीयक पोषी स्तर), फिर 50 किलोग्राम साँप (तृतीयक पोषी स्तर), और सबसे ऊपर 10 किलोग्राम उल्लू (चौथा पोषी स्तर) हैं। यह दर्शाता है कि खाद्य श्रृंखला में ऊपर जाने पर बायोमास कैसे घटता जाता है।
In simple words: Ecological pyramids graphically represent trophic level relationships (number, biomass, or energy), with pyramids of number showing population counts and pyramids of biomass showing the total dry weight of organisms at each level, which can be upright or inverted.

🎯 Exam Tip: When describing ecological pyramids, always define them first, then elaborate on specific types with clear examples, and mention their typical shapes (upright, inverted).

Question 2.What is primary productivity? Give brief description of factors that affect primary productivity.
Answer:(1) Primary Productivity : The rate of generation of biomass in an ecosystem which is expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day (g/m²/day) is called primary productivity. (2) Primary productivity is described as gross primary productivity (GPP) and net primary productivity (NPP). (3) The rate of production of organic matter during photosynthesis is called gross primary productivity of an ecosystem. Of this the amount of energy lost through respiration of plants is called respiratory losses. (4) Gross primary productivity minus respiratory losses gives the net primary productivity (NPP). (5) Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores, carnivores and decomposers). (6) Factors affecting primary productivity: Gross primary productivity (GPP) depends on the following factors: • Plant species inhabiting a particular area. • Variety of environmental factors such as temperature, sunlight, salinity, oxygen and carbon dioxide content, etc. • Availability of nutrients and • Photosynthetic capacity of plants.
In simple words: Primary productivity is the rate at which producers create organic matter from inorganic sources, measured as gross (total) or net (remaining after respiration) productivity, and is influenced by plant type, environmental conditions, nutrient availability, and photosynthetic efficiency.

🎯 Exam Tip: Clearly define GPP and NPP, explain their relationship, and provide a comprehensive list of biotic and abiotic factors influencing primary productivity.

Question 3.Define decomposition and describe the processes and products of decomposition.
Answer:1. Decomposition is the process carried out by the decomposer organisms. 2. Most of the bacteria, actinomycetes and fungi are decomposers. They convert the dead and decaying organic matter into simpler compounds. These simpler inorganic substances return back to the environment. 3. Decomposition takes place through detritus food chain. It starts from the dead organic matter. Detritus eating organisms called detritivores like earthworm, etc. breakdown the detritus into smaller fragments. Therefore, this first step of decomposition is called fragmentation. 4. Water soluble inorganic nutrients seep into the soil after fragmentation. These nutrients get precipitated as salts. Therefore, this second step of decomposition is called leaching. 5. The third step of decomposition is called catabolism. In this step, fungal and bacterial enzymes degrade the detritus into simple inorganic substances. 6. The partially decomposed organic matter is called humus which is formed by the process of humification. Humus is a dark coloured amorphous substance which is the reservoir of nutrients. 7. Humus too undergoes decomposition by bacterial action at a very slow rate and ultimately releases inorganic matter. This process is therefore called mineralization. 8. Decomposition requires oxygen in greater amount. The rate of decomposition is dependent upon the temperature and the humidity of the environment.
In simple words: Decomposition is the breakdown of complex organic matter by decomposers into simpler inorganic substances, involving steps like fragmentation, leaching, catabolism, humification, and mineralization, with its rate influenced by oxygen, temperature, and humidity.

🎯 Exam Tip: List and explain each step of the decomposition process (fragmentation, leaching, catabolism, humification, mineralization) in order, and mention the factors affecting its rate.

Question 4.Write important features of a sedimentary cycle in an ecosystem.
Answer:1. Reservoir of sedimentary cycles is earth's crust. 2. The nutrients such as phosphorus which show sedimentary cycle, moves through hydrosphere, lithosphere and biosphere. 3. There is no respiratory release of nutrients into the atmosphere which show sedimentary cycle. 4. Natural reservoir of such nutrients are usually in the form of rocks. The rocks upon weathering release such nutrients into circulation. 5. Sedimentary cycles are very slow in their reactions.
In simple words: Sedimentary cycles, like the phosphorus cycle, involve nutrients primarily stored in the Earth's crust and rocks, circulate through water, land, and living organisms, do not involve atmospheric gaseous phases, and generally proceed at a very slow rate.

🎯 Exam Tip: Emphasize the Earth's crust/rocks as the main reservoir, the absence of an atmospheric gaseous phase, and the slow pace as key characteristics of sedimentary cycles.

Question 5.Describe carbon cycle and add a note on the impact of human activities on carbon cycle.
Answer:I. Carbon cycle: (1) The entire carbon cycle has following basic processes viz. Photosynthesis, Respiration, Decomposition, Sedimentation and Combustion. (2) Carbon is an important element as it forms 49% of the dry weight of all organisms. 71% of global carbon is present in the oceans. Therefore, ocean is the major reservoir of carbon. Carbon is also present in all fossil fuels. This is long term storage places or sinks for carbon which is in the form of coal, natural gas, etc. (3) Respiration and photosynthesis are the two events that keep the carbon in cyclic circulation. During respiration, oxygen is used for combustion of carbohydrates as a result of which carbon dioxide and water are formed with the release of energy. The process of photosynthesis utilizes carbon dioxide and water vapour liberating oxygen and producing carbohydrates at the same time. (4) Solar energy is stored in the carbon-carbon bonds of carbohydrates during photosynthesis whereas respiration releases the same stored energy. (5) The main reservoirs for carbon dioxide are in the oceans and in rocks. Carbon dioxide is highly soluble in water and forms mild carbonic acid upon dissolving. This dissolved carbon dioxide precipitate as a solid rock or limestone which is calcium carbonate. This reaction in the seas is aided by corals and algae which in turn builds the coral reefs made up of limestone. (6) Carbon moves through food chains. Autotrophic green plants on land and in water take up carbon dioxide and manufacture carbohydrates by the process of photosynthesis. The carbon stored in plants has three different fates, viz. liberation into atmosphere, consumption by animals upon feeding, storage in the plant till the plant dies. (7) Animals get their carbon requirement through their food. When autotrophs are consumed, the heterotrophs obtain carbon. Carbon in animals also has three fates, viz. release back into the atmosphere in the process of respiration, release of stored carbon from the body by the action of decomposers or conversion into fossil fuels if buried intact. (8) Fossil fuels such as coal, oil, natural gas, etc. can be mined and burned for energy purposes. This burning releases carbon dioxide back into the atmosphere. (9) Carbon from limestone can also be released if pushed to the surfaces and slowly weathered away. Subducting and volcanic eruptions can also release the stored carbon from sediments.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र कार्बन चक्र का विस्तृत आरेख दर्शाता है। यह वातावरण में कार्बन डाइऑक्साइड, वर्षा/मिट्टी द्वारा समुद्र में घुला हुआ कार्बन, और हरे पौधों, भूमि जानवरों, समुद्री जानवरों व पौधों, और मिट्टी में कार्बन के प्रवाह को दिखाता है। इसमें अपक्षय, दहन, श्वसन, प्रकाश संश्लेषण, अपघटन और जीवाश्म ईंधन (कोयला, तेल, चूना पत्थर) के रूप में कार्बन के भंडारण जैसी प्रक्रियाएं शामिल हैं, जो दर्शाती हैं कि कार्बन विभिन्न भंडारों और प्रक्रियाओं के माध्यम से कैसे घूमता है। II. Impact of human activities on carbon cycle: (1) Excessive burning of fossils fuels for power plants, industrial processes and vehicular traffic, adds excessive carbon dioxide into atmosphere. When fossil fuels burn to run factories, power plants, motor vehicles, most of the carbon quickly enters the atmosphere as carbon dioxide gas. (2) Each year, 5.5 billion tonnes of carbon is released through combustion of fossil fuels. Of this massive amount, 3.3 billion tonnes stays in the atmosphere. (3) Rapid deforestation also increases carbon dioxide. Since plants absorb carbon dioxide for their photosynthesis, they always reduce the concentration of CO2. But deforestation upsets this balance. (4) Massive burning of fossil fuel for energy and transport, have significantly increased the rate of release of carbon dioxide into the atmosphere which is causing global warming and resultant climate change.
In simple words: The carbon cycle involves the movement of carbon through photosynthesis, respiration, decomposition, and combustion, with major reservoirs being oceans and the atmosphere; human activities like burning fossil fuels and deforestation significantly disrupt this balance by increasing atmospheric carbon dioxide, leading to global warming.

🎯 Exam Tip: For the carbon cycle, list all key processes, major reservoirs/sinks, and then clearly explain how human activities (deforestation, fossil fuel burning) unbalance the cycle and their consequences.

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