EcosystemClass 12 Biology NCERT Solutions
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Solution 1 of 11
Q1EXERCISES
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 _________ .
Solution
(a) Plants are called as autotrophs because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is spindle-shaped or inverted type.
(c) In aquatic ecosystems, the limiting factor for the productivity is light.
(d) Common detritivores in our ecosystem are earthworms.
(e) The major reservoir of carbon on earth is oceans.
Q2EXERCISES
Which one of the following has the largest population in a food chain?
(a)
Producers
(b)
Primary consumers
(c)
Secondary consumers
(d)
Decomposers
Solution
The correct answer is (d) Decomposers.
Explanation:
Decomposers include organisms like bacteria and fungi. Although they are microscopic, their numbers in an ecosystem are vast, far exceeding the number of producers or consumers. They are present in enormous quantities in the soil and at the bottom of water bodies, playing a crucial role in breaking down dead organic matter. While ecological pyramids of numbers for trophic levels (producers, consumers) are often considered, the sheer number of individual decomposer organisms makes their total population the largest in any ecosystem.
Q3EXERCISES
The second trophic level in a lake is
(a)
Phytoplankton
(b)
Zooplankton
(c)
Benthos
(d)
Fishes
Solution
The correct answer is (b) Zooplankton.
Explanation:
In a lake ecosystem, the trophic levels are as follows:
- First Trophic Level (Producers): Phytoplankton, which produce their own food through photosynthesis.
- Second Trophic Level (Primary Consumers): Organisms that feed on producers. Zooplankton are microscopic animals that feed on phytoplankton.
- Third and Higher Trophic Levels (Secondary/Tertiary Consumers): Fishes and other larger organisms that feed on zooplankton or other fishes.
Therefore, zooplankton occupy the second trophic level.
Q4EXERCISES
Secondary producers are
(a)
Herbivores
(b)
Producers
(c)
Carnivores
(d)
None of the above
Solution
The correct answer is (a) Herbivores.
Explanation:
Primary productivity is the rate of biomass generation by producers (autotrophs). Secondary productivity is the rate of formation of new organic matter by consumers. The term 'secondary producers' refers to the organisms that create this biomass at the first consumer level by feeding on primary producers. These organisms are the primary consumers, or herbivores. While all consumers (including carnivores) contribute to secondary productivity, the term 'secondary producers' is most commonly used to refer to herbivores.
Q5EXERCISES
What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a)
(b)
(c)
(d)
Solution
The correct answer is (b) .
Explanation:
According to the chapter, "Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR)." Photosynthetically Active Radiation (PAR) refers to the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This range constitutes approximately 50% of the total incident solar radiation reaching the Earth's surface.
Q6EXERCISES
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
Solution
(a) Grazing food chain (GFC) and Detritus food chain (DFC)
| Feature | Grazing Food Chain (GFC) | Detritus Food Chain (DFC) |
|---|---|---|
| Starting Point | Starts with living green plants (producers). | Starts with dead organic matter (detritus). |
| Energy Source | Directly dependent on solar energy captured by producers. | Depends on the energy stored in dead organic matter. |
| Trophic Levels | Consists of producers, herbivores (primary consumers), and carnivores (secondary/tertiary consumers). | Consists of decomposers (saprotrophs) and detritivores. |
| Energy Flow | Energy transfer is rapid. | Energy transfer is slower. |
| Major Conduit | It is the major conduit for energy flow in aquatic ecosystems. | It is the major conduit for energy flow in terrestrial ecosystems. |
(b) Production and Decomposition
| Feature | Production | Decomposition |
|---|---|---|
| Process | Synthesis of organic matter from simple inorganic substances. | Breakdown of complex organic matter into simple inorganic substances. |
| Organisms Involved | Producers (autotrophs) like green plants and algae. | Decomposers (saprotrophs) like bacteria and fungi, and detritivores. |
| Energy | Energy is captured and stored in organic matter (anabolic process). | Energy stored in dead organic matter is released (catabolic process). |
| Raw Materials | Uses carbon dioxide, water, and solar energy. | Uses dead organic matter (detritus). |
| Products | Biomass (organic matter) and oxygen. | Carbon dioxide, water, and inorganic nutrients. |
(c) Upright and Inverted Pyramid
| Feature | Upright Pyramid | Inverted Pyramid |
|---|---|---|
| Shape | Has a broad base and progressively narrows towards the apex. | Has a narrow or small base and broadens towards the apex. |
| Indication | Indicates that the biomass, number, or energy decreases at successive higher trophic levels. | Indicates that the biomass or number increases at successive higher trophic levels. |
| Example (Biomass) | In most terrestrial ecosystems, the biomass of producers is the highest. | In an aquatic ecosystem, the biomass of phytoplankton (producers) can be less than that of zooplankton (consumers). |
| Example (Number) | In a grassland ecosystem, the number of grass plants is higher than the number of herbivores. | On a single large tree, the number of insects (herbivores) is much larger than the number of producers (one tree). |
(d) Food Chain and Food Web
| Feature | Food Chain | Food Web |
|---|---|---|
| Structure | A single, linear pathway of energy flow. | A complex network of many interconnected food chains. |
| Organism's Role | An organism generally occupies one specific trophic level. | An organism can occupy multiple trophic levels simultaneously (e.g., omnivores). |
| Stability | Less stable; removal of one organism can disrupt the entire chain. | More stable; provides alternative food sources, so the loss of one species has a less drastic effect. |
| Representation | A simplistic representation of feeding relationships. | A more realistic and complex representation of feeding relationships in an ecosystem. |
(e) Litter and Detritus
| Feature | Litter | Detritus |
|---|---|---|
| Definition | Freshly fallen, non-decomposed dead organic material on the soil surface. | All dead organic matter, including litter, dead remains of animals, and fecal matter, in various stages of decomposition. |
| Composition | Consists of recognizable plant parts like leaves, bark, flowers, and twigs. | Consists of litter as well as partially decomposed organic matter. |
| Role | It is the initial stage or raw material for the formation of detritus and subsequent decomposition. | It is the raw material for the entire process of decomposition. |
(f) Primary and Secondary Productivity
| Feature | Primary Productivity | Secondary Productivity |
|---|---|---|
| Definition | The rate of production of biomass or organic matter by producers (autotrophs) during photosynthesis. | The rate of formation of new organic matter by consumers (heterotrophs). |
| Organisms Involved | Producers (e.g., plants, algae, phytoplankton). | Consumers (e.g., herbivores, carnivores, decomposers). |
| Energy Source | Solar energy. | Chemical energy stored in the organic matter consumed from a lower trophic level. |
| Trophic Level | Occurs at the first trophic level. | Occurs at the second trophic level and higher (all consumer levels). |
Q7EXERCISES
Describe the components of an ecosystem.
Solution
An ecosystem is a functional unit of nature, and its structure is formed by the interaction of its living and non-living components. These components are broadly categorized into abiotic and biotic components.
1. Abiotic Components:
These are the non-living physical and chemical factors of an ecosystem. They create the conditions and provide the resources necessary for life. Abiotic components include:
- Climatic Factors: These include solar radiation, temperature, water, day-length, and humidity. These factors regulate the functioning of the entire ecosystem.
- Inorganic Substances: These are essential elements and compounds like carbon, nitrogen, phosphorus, sulphur, water (), and carbon dioxide (). They are involved in the continuous cycling of materials.
- Organic Substances: These include proteins, carbohydrates, lipids, and humus. They are present in the soil, water, and dead organic matter, linking the abiotic and biotic components.
2. Biotic Components:
These are the living organisms within an ecosystem. Based on their nutritional or trophic level, they are classified into three main groups:
-
Producers (Autotrophs): These organisms produce their own food, typically through photosynthesis. They convert inorganic substances into organic compounds using energy from the sun. In a terrestrial ecosystem, producers are primarily herbaceous and woody plants. In an aquatic ecosystem, the major producers are phytoplankton, algae, and various aquatic plants.
-
Consumers (Heterotrophs): These organisms depend on other organisms for their food. They are further divided based on their food source:
- Primary Consumers (Herbivores): They feed directly on producers. Examples include insects, birds, and mammals in terrestrial ecosystems, and zooplankton and molluscs in aquatic ecosystems.
- Secondary Consumers (Primary Carnivores): They feed on herbivores. Examples include frogs, foxes, and some fish.
- Tertiary Consumers (Secondary Carnivores): They feed on secondary consumers (primary carnivores).
-
Decomposers (Saprotrophs): These organisms, mainly bacteria and fungi, break down dead organic matter (detritus) from producers and consumers. They play a vital role in decomposition and mineralisation, releasing inorganic nutrients back into the soil and water, which are then available for producers to use again. This process is essential for nutrient cycling.
Q8EXERCISES
Define ecological pyramids and describe with examples, pyramids of number and biomass.
Solution
Ecological Pyramids
An ecological pyramid is a graphical representation designed to show the relationship between organisms at different trophic levels in an ecosystem. The shape is pyramidal, with the base representing the producers (first trophic level) and the successive levels towards the apex representing herbivores (second trophic level), carnivores (third trophic level), and top carnivores. This relationship can be expressed in terms of number of individuals, biomass (dry weight), or energy content at each trophic level.
1. Pyramid of Number
This pyramid represents the total number of individual organisms at each trophic level in an ecosystem.
-
Upright Pyramid of Number: In most ecosystems, like a grassland or pond ecosystem, the number of producers is the largest, followed by a decreasing number of primary consumers, secondary consumers, and so on. For example, a large number of grass plants support a smaller number of grasshoppers, which in turn support an even smaller number of frogs, which are eaten by a very small number of snakes. This results in an upright pyramid (See Figure 12.4 a in the textbook).
-
Inverted/Spindle-shaped Pyramid of Number: In some cases, the pyramid can be inverted or spindle-shaped. For example, a single large tree (producer) can support thousands of herbivorous insects (primary consumers). These insects may support a smaller number of predatory birds (secondary consumers). Here, the base (producer level) is very small in number, and the subsequent level of primary consumers is much larger, creating an inverted or spindle shape.
2. Pyramid of Biomass
This pyramid represents the total amount of living organic matter (biomass) at each trophic level in an ecosystem. Biomass is usually measured as dry weight per unit area (e.g., ).
-
Upright Pyramid of Biomass: In most terrestrial ecosystems, the total biomass of producers is significantly greater than the biomass of the herbivores that they support. The biomass of herbivores is, in turn, greater than that of the carnivores. This results in a broad base and a narrowing top, forming an upright pyramid (See Figure 12.4 b in the textbook).
-
Inverted Pyramid of Biomass: This is a characteristic feature of many aquatic ecosystems. The standing crop (biomass) of producers, which are mainly phytoplankton, is very small at any given time. However, they have a very high rate of reproduction and a short life span. They are consumed rapidly by zooplankton and fish (primary consumers), which have a larger biomass and longer life span. Thus, the biomass of consumers can exceed the biomass of producers at any given moment, resulting in an inverted pyramid (See Figure 12.4 c in the textbook).
Q9EXERCISES
What is primary productivity? Give brief description of factors that affect primary productivity.
Solution
Primary Productivity
Primary productivity is defined as the rate at which energy is converted by photosynthetic and chemosynthetic autotrophs (producers) to organic substances. In simpler terms, it is the amount of biomass or organic matter produced per unit area over a specific time period by plants during photosynthesis. It is the primary source of energy for all organisms in an ecosystem.
Primary productivity is expressed in terms of weight (e.g., ) or energy (e.g., ). It is divided into two types:
- Gross Primary Productivity (GPP): This is the total rate of production of organic matter or capture of energy during photosynthesis by producers.
- Net Primary Productivity (NPP): A considerable amount of energy fixed during GPP is used by the plants for their own metabolic activities, primarily respiration (R). The remaining biomass, which is available for consumption by heterotrophs (herbivores and decomposers), is called Net Primary Productivity. The relationship is expressed as:
Factors Affecting Primary Productivity
The rate of primary productivity varies among different ecosystems and depends on several factors:
-
Solar Radiation (Light): The sun is the ultimate source of energy. The amount, intensity, and duration of sunlight available determine the rate of photosynthesis. Of the total incident solar radiation, only a small fraction (less than 50% is PAR, and plants capture only 2-10% of PAR) is utilized.
-
Temperature: Photosynthetic enzymes function optimally within a certain temperature range. Both very low and very high temperatures can inhibit photosynthesis and thus reduce productivity.
-
Availability of Nutrients: The presence of essential nutrients like nitrogen, phosphorus, potassium, and other micronutrients in the soil or water is crucial for plant growth and metabolism. A scarcity of any essential nutrient can become a limiting factor for productivity.
-
Availability of Water: Water is a key raw material for photosynthesis and is essential for all metabolic activities in plants. Its scarcity can severely limit primary productivity, as seen in deserts.
-
Photosynthetic Capacity of Plants: The type of plant species in an area is important. Different species have different photosynthetic efficiencies and adaptations to the local environment, which affects the overall productivity of the ecosystem.
Q10EXERCISES
Define decomposition and describe the processes and products of decomposition.
Solution
Definition of Decomposition
Decomposition is a fundamental ecological process where complex organic matter from dead organisms and their waste products (collectively known as detritus) is broken down into simpler inorganic substances like carbon dioxide, water, and nutrients. This process is primarily carried out by microorganisms such as bacteria and fungi, which are called decomposers.
Processes of Decomposition
The process of decomposition involves several key steps that often occur simultaneously on the detritus:
-
Fragmentation: Detritivores (e.g., earthworms, termites) break down the detritus into smaller particles. This increases the surface area of the detritus, making it more accessible to microbial action.
-
Leaching: As water percolates through the soil, water-soluble inorganic nutrients are washed out from the decomposing detritus and move down into the soil horizon. These leached nutrients may precipitate as unavailable salts.
-
Catabolism: This is the core process of decomposition where decomposers (bacteria and fungi) secrete digestive enzymes onto the detritus. These enzymes degrade the complex organic molecules into simpler inorganic substances. This enzymatic conversion is known as catabolism.
-
Humification: During decomposition, a dark-colored, amorphous, and colloidal substance called humus is formed. Humus is highly resistant to microbial action and decomposes at an extremely slow rate. It is rich in nutrients and improves the soil's water-holding capacity and aeration.
-
Mineralisation: In this final step, the humus is further degraded by some microbes, which leads to the release of inorganic nutrients (minerals) like , , , etc., into the soil. This process is called mineralisation, and it makes the nutrients available for re-uptake by plants.
Products of Decomposition
The main products resulting from the complete process of decomposition are:
- Inorganic Nutrients: Essential minerals such as nitrates, phosphates, calcium, potassium, etc., are released into the soil or water. These are then reused by producers, completing the nutrient cycle.
- Carbon Dioxide (): Released into the atmosphere through the respiration of decomposers.
- Water (): Another simple inorganic product of decomposition.
- Humus: A stable organic matter that improves soil fertility and acts as a reservoir of nutrients.
Q11EXERCISES
Give an account of energy flow in an ecosystem.
Solution
Energy flow in an ecosystem describes the path of energy from its initial source through the various living organisms. The flow of energy is a fundamental principle of ecosystem functioning.
1. Source of Energy:
Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR), which plants can use.
2. Unidirectional Flow:
The flow of energy in an ecosystem is unidirectional, or one-way. It flows from the sun to producers, from producers to consumers, and then to decomposers. Energy is not recycled in the same way nutrients are; it cannot flow back from a higher trophic level to a lower one.
3. Energy Capture by Producers:
Producers (green plants and photosynthetic bacteria) are the only organisms that can trap solar energy. They capture a very small fraction of the available PAR (about 2-10%) and convert it into chemical energy in the form of organic molecules (food) through photosynthesis. This stored energy forms the basis for the entire ecosystem.
4. Transfer through Trophic Levels:
Energy is transferred from one trophic level to the next when an organism is eaten by another.
- First Trophic Level: Producers (plants).
- Second Trophic Level: Primary Consumers (herbivores) that eat producers.
- Third Trophic Level: Secondary Consumers (carnivores) that eat herbivores.
- Fourth Trophic Level: Tertiary Consumers (top carnivores) that eat other carnivores.
5. The 10 Per Cent Law:
The transfer of energy between trophic levels is highly inefficient. According to the 10 per cent law, only about 10% of the energy from a lower trophic level is transferred and stored in the biomass of the next higher trophic level. The remaining 90% is either:
- Used by the organism for its own metabolic processes (like respiration, growth, reproduction) and lost as heat.
- Not consumed and becomes part of the detritus upon death. This loss of energy at each step limits the number of trophic levels in a food chain, which is usually restricted to 4 or 5 levels.
6. Food Chains and Food Webs:
Energy flows through ecosystems via food chains. There are two main types:
- Grazing Food Chain (GFC): Starts with producers and moves to herbivores and then to carnivores.
- Detritus Food Chain (DFC): Starts with dead organic matter (detritus), which is consumed by decomposers and detritivores. In nature, these food chains are interconnected, forming a complex food web, which represents the multiple pathways of energy flow.
In summary, the flow of energy in an ecosystem follows the laws of thermodynamics. It is a continuous, one-way process that starts with the capture of solar energy by producers and is characterized by a progressive decrease in energy at each successive trophic level.