Class 12 Biology Chapter 12 Ecosystem

Key Concept: Basic Definition

c) Gross primary productivity

[Solution Description] The rate at which plants produce organic matter through photosynthesis is referred to as Gross Primary Productivity (GPP).

Key Concept:Mineralisation Role

b) Releases inorganic nutrients from humus
[Solution Description] Mineralisation involves the microbial degradation of humus, releasing inorganic nutrients into the soil which are essential for plant growth.

Key Concept: Primary Productivity

b) $1500 kcal/m^2/year$

[Solution Description] Net Primary Productivity (NPP) is calculated using the formula $$NPP = GPP – \text{Respiration}$$. Here, $$GPP = 2000 kcal/m^2/year$$ and Respiration = $$500 kcal/m^2/year$$. Thus, $$NPP = 2000 – 500 = 1500 kcal/m^2/year$$.

Key Concept:Trophic Level Function

c) Primary consumers
[Solution Description] Phytoplankton are producers as they convert sunlight into energy through photosynthesis. Zooplankton feed on phytoplankton and are hence considered primary consumers. They occupy the first consumer level. Thus, their role is as primary consumers.

Key Concept: Energy Calculation, Energy Flow Laws

b) 5 units

[Solution Description] The energy flow follows the rule where only 10% of the energy is transferred from one trophic level to the next. We calculate step by step:

$$\text{Energy for Primary Consumers} = 5000 \times 0.10 = 500 \, \text{units}$$

$$\text{Energy for Secondary Consumers} = 500 \times 0.10 = 50 \, \text{units}$$

$$\text{Energy for Tertiary Consumers} = 50 \times 0.10 = 5 \, \text{units}$$

Therefore, the energy available to tertiary consumers is 5 units.

Key Concept: Nutrient Cycling

c) Assertion is true, but Reason is false.

[Solution Description] The assertion accurately describes nutrient cycling, which includes decomposition where detritus is broken down into simpler inorganic forms through various steps like fragmentation, leaching, catabolism, humification, and mineralization. However, the reason provided is incorrect as decomposition does not convert organic matter into nutrients directly; it involves several intermediate processes before becoming available as nutrients for plants.

Key Concept:Comparative Analysis

a) It has a narrower base and wider top
[Solution Description] An upright pyramid of numbers typically shows larger populations at lower trophic levels (producers) and smaller populations at higher levels (carnivores). Conversely, an inverted pyramid occurs when there is a large number of consumers supported by a single or few producers, such as parasites on a host.

Key Concept: Climatic Factors

b) Warm and moist

[Solution Description] Warm and moist conditions typically favor a faster rate of decomposition because they provide an ideal environment for microbial activity and enzymatic processes required to break down organic material.

Key Concept:Energy Flow Direction

b) Producers to consumers
[Solution Description] Energy flows unidirectionally from producers to consumers in a food chain. Producers, like plants, capture energy from the sun and convert it into chemical energy. This energy is then transferred to primary consumers (herbivores), secondary consumers, and so forth. Therefore, the correct direction is from producers to consumers.

Key Concept:Simple Calculation

c) $$1800 \, \text{kcal} \, \text{m}^{-2} \, \text{yr}^{-1}$$
[Solution Description] To find the Net Primary Productivity (NPP), we use the formula:
$$
NPP = GPP – R
$$
Substituting the given values:
$$
NPP = 3000 \text{ kcal m}^{-2} \text{ yr}^{-1} – 1200 \text{ kcal m}^{-2} \text{ yr}^{-1} = 1800 \text{ kcal m}^{-2} \text{ yr}^{-1}
$$
So, the Net Primary Productivity is $$1800 \, \text{kcal} \, \text{m}^{-2} \, \text{yr}^{-1}$$.

Key Concept:Process Explanation

b) Photosynthesis
[Solution Description] Autotrophs convert solar energy into chemical energy through the process of photosynthesis. During this process, they fix carbon dioxide and produce organic compounds like glucose.

Key Concept:Pyramid Shape

d) Because energy is lost as heat, showing decrease.
[Solution Description] The pyramid of energy is always upright because energy decreases progressively at higher trophic levels due to energy being lost as heat during metabolic processes.

Key Concept:Exception Identification

c) Aquatic ecosystem
[Solution Description] In terrestrial ecosystems, we generally find upright biomass pyramids where producers have the most biomass and it decreases as we go up trophic levels. However, aquatic ecosystems often show inverted biomass pyramids because phytoplankton, which are primary producers, have less biomass than fish due to their rapid reproduction and consumption rates.

Key Concept:Process Steps

c) Humification
[Solution Description] Decomposition involves several steps such as fragmentation, leaching, catabolism, humification, and mineralization. Humification is an important step where complex organic substances are converted into simpler materials (humus).

Key Concept:Detritus Composition Analysis, Humus Characteristics

c) Presence of specialized microorganisms
[Solution Description] Lignin and chitin are complex organic compounds that are resistant to degradation. Detritus with high lignin and chitin content tends to decompose at a slower rate due to the complexity and stability of these molecules. The presence of microorganisms capable of breaking down such complex structures is crucial for decomposition. Additionally, environmental conditions like moisture and temperature also play significant roles in affecting the decomposition rate.

Key Concept:Omnivore Classification, Trophic Level Dynamics

c) Both primary and secondary consumer, reflecting its diverse diet
[Solution Description] Bears consuming both plant matter (berries) and animal matter (fish) occupy multiple trophic levels. It functions as a primary consumer when eating berries and as a secondary or tertiary consumer when eating fish. This varied diet classifies bears as omnivores, organisms that obtain energy from both plant and animal sources, thus integrating into various points of a food web.

Key Concept:Abiotic and Nutrient Cycling

c) Water
[Solution Description] Water is fundamental for the mineralization process in soil as it facilitates microbial activity necessary for breaking down organic matter into inorganic nutrients, which are then available for plant uptake. Thus, water availability plays a significant role in nutrient cycling.

Key Concept:Producers’ Energy Capture

c) 1600 units
[Solution Description] Phytoplankton capture 8% of the incoming PAR. Thus, the energy captured can be calculated as follows:

$$
\text{Energy captured by phytoplankton} = 20000 \times \frac{8}{100} = 1600 \text{ units}
$$

Hence, the energy captured by the phytoplankton is 1600 units.

Key Concept:Primary vs. Secondary

b) Biomass production by consumers
[Solution Description] Secondary productivity refers specifically to the biomass production by consumers, whereas primary productivity is related to biomass production by producers such as plants.

Key Concept:Trophic Level Dynamics

a) As primary consumers feeding on plankton and as predators of other small fishes.
[Solution Description] Sardines primarily feed on phytoplankton, placing them as primary consumers. However, they also serve as prey for larger carnivorous fish, positioning them as part of the secondary consumer level when considered in terms of biomass dynamics. Hence, they can occupy both primary and secondary consumer levels.

Key Concept:Role Identification

c) Nutrient cyclers
[Solution Description] Decomposers break down dead organic matter, recycling nutrients back into the ecosystem. They are crucial for nutrient cycling as they convert complex organic materials into simpler inorganic forms that can be reused by producers.

Key Concept:Ecosystem Dynamics

c) Increased growth of understory vegetation
[Solution Description] An increase in sunlight reaching the lower layers of a stratified forest due to deforestation can lead to increased growth of understory vegetation as more light becomes available. This could also disrupt existing species interactions and lead to changes in biodiversity.

Key Concept:Chemical Composition

c) Rich in nitrogen
[Solution Description] Decomposition rates are significantly influenced by the chemical composition of detritus. Detritus richer in nitrogen tends to decompose more quickly because nitrogen acts as an essential nutrient for the growth and metabolism of microorganisms involved in decomposition. On the other hand, materials rich in lignin or chitin decompose at slower rates due to their complex molecular structure that resists breakdown.

Hence, detritus rich in nitrogen decomposes faster.

Key Concept:Basic Definition

c) The identification and enumeration of plant and animal species
[Solution Description] Species composition refers to the identification and enumeration of different plant and animal species present in an ecosystem. It involves knowing what species are present and how many individuals of each species exist.

Key Concept:Energy Flow Comparison

d) Terrestrial
[Solution Description] In terrestrial ecosystems, a larger fraction of energy flows through the detritus food chain than the grazing food chain.

Key Concept:Role of Decomposers

b) Inorganic substances
[Solution Description] Decomposers play a crucial role in breaking down detritus and releasing inorganic nutrients back into the environment through mineralisation. Thus, they convert detritus into inorganic substances.

Key Concept:Humification and Mineralisation

c) Formation of humus
[Solution Description] Humification results in the formation of humus, which is a dark organic material that enriches soil quality and retains nutrients.

Key Concept:Detritus Types

c) Rich in nitrogen
[Solution Description] Detritus rich in nitrogen tends to decompose faster because nitrogen serves as a nutrient source for decomposers, enhancing microbial growth and activity.

Key Concept:Comparative Analysis

c) A tropical rainforest
[Solution Description] Terrestrial ecosystems typically host a wider variety of plant species compared to aquatic ecosystems. This diversity can enhance resource use efficiency, resilience to environmental changes, and overall ecosystem productivity. Hence, terrestrial ecosystems are likely to exhibit higher primary productivity due to their greater plant species diversity.

Key Concept:Standing Crop

b) Total biomass
[Solution Description] Standing crop in an ecological pyramid represents the total amount of living material or biomass present at each trophic level at a given time. It can be measured in terms of fresh or dry weight. This measurement helps in understanding the distribution of energy within an ecosystem.

Key Concept:Energy Flow

c) Due to energy loss as heat
[Solution Description] The pyramid of energy is always upright because energy decreases at each trophic level due to the loss of energy as heat and other forms during transfer, following the 10% law.

Key Concept:PAR Percentage

d) Less than 50%
[Solution Description] Less than 50% of incident solar radiation is photosynthetically active radiation. PAR refers to the portion of sunlight spectrum that is usable by plants for photosynthesis.

Key Concept:Humification vs. Mineralisation, Decomposition Rate Factors

b) Focus on mineralisation; increase oxygen supply
[Solution Description] Mineralisation is the process through which humus is further degraded to release inorganic nutrients. To increase inorganic nutrient availability rapidly, the mineralisation process should be accelerated. This can be achieved by optimizing environmental conditions like increasing oxygen availability and maintaining moisture balance, as well as ensuring the presence of microorganisms conducive to mineralisation.

Key Concept:Energy Flow Direction

b) Unidirectional
[Solution Description] Energy flows through an ecosystem in a unidirectional manner. It starts from producers like plants and algae which capture solar energy and convert it into chemical energy, then moves to consumers and ultimately to decomposers before being lost as heat.

Key Concept:Humus Formation, Climate and Decomposition

b) Decrease due to enhanced aerobic decomposition
[Solution Description] Draining and drying of a wetland lead to reduced soil moisture and potentially increased oxygen availability. Such changes disrupt the anaerobic conditions favorable for slow humus accumulation. With prolonged exposure to aerobic conditions, organic matter decomposition accelerates, reducing the formation and stability of humus. As microbial action intensifies, humus breaks down faster than it forms, leading to decreased humus content in the soil.

Key Concept:Energy Transfer

b) 15 units
[Solution Description] According to the 10% law, each trophic level receives only 10% of the energy from the previous level. The primary consumer will get 10% of 1500 units as energy, which is calculated as follows:

$$
\text{Energy to Primary Consumer} = \frac{10}{100} \times 1500 = 150 \text{ units}
$$

Now, for the secondary consumer, 10% of this 150 units will be transferred, calculated as:

$$
\text{Energy to Secondary Consumer} = \frac{10}{100} \times 150 = 15 \text{ units}
$$

Therefore, 15 units of energy are transferred to the secondary consumer.

Key Concept:Comparative Analysis

b) Leaching removes nutrients; humification retains them
[Solution Description] Humification is the process where organic matter undergoes transformation to become humus, enhancing soil fertility, while leaching is the movement of inorganic nutrients dissolved in water down through the soil. Humification improves nutrient retention, whereas leaching results in the loss of nutrients.

Key Concept:Unit Recognition

a) $gm^{-2} or kcal m^{-2}$
[Solution Description] Gross Primary Productivity (GPP) is typically expressed in terms of weight or energy. The common units are grams per square meter $(gm^{-2})$ or kilocalories per square meter $(kcal\ m^{-2})$.

Key Concept: Thermodynamics Laws, Energy Transfer Limitation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] The assertion correctly states the implication of the second law of thermodynamics, which highlights the inevitability of energy loss during transfers due to increased entropy. The reason provided aligns with this, explaining that only 10% of energy is successfully transferred to the next trophic level, with the rest lost primarily as heat during metabolic activities. This explanation is directly related to the assertion about energy loss dictated by thermodynamic principles.

Key Concept:Units of Measurement

c) $gm^{-2} yr^{-1} or kcal m^{-2} yr^{-1}$
[Solution Description] The units commonly used to express Net Primary Productivity (NPP) reflect both mass and energy over a specific area and time period. These are represented in terms of grams per square meter per year or kilocalories per square meter per year. These units allow scientists to compare productivity across different ecosystems efficiently.

Thus, the units for NPP are:

Key Concept:Factors Affecting Rate

c) Warm and moist environment
[Solution Description] The rate of decomposition is affected by the physical and chemical conditions surrounding the organic material. A warm and moist environment provides optimal conditions for microorganisms responsible for decomposition, as it supports their enzymatic activities. Hence, such an environment boosts the decomposition rate.

Key Concept:Formula Application

c) $ NPP = GPP – R $
[Solution Description] Net Primary Productivity (NPP) can be calculated using the formula $ NPP = GPP – R $, where GPP is Gross Primary Productivity and R is Respiration loss.

Key Concept:Detritus Food Chain

c) Dead organic matter
[Solution Description] The detritus food chain begins with dead organic matter as its primary energy source. Decomposers, such as bacteria and fungi, break down this material to obtain energy and recycle nutrients back into the ecosystem.

Key Concept:Environmental Impact

d) Cold and anaerobic
[Solution Description] Low temperature and anaerobic conditions inhibit the decomposition process because these conditions slow down microbial activity, which is essential for the breakdown of organic matter.

Key Concept:Detritus Composition Impact

c) Lignin
[Solution Description] Lignin, a complex polymer found in plant cell walls, is highly resistant to breakdown by microbial enzymes. Its presence in detritus thus slows down the decomposition process compared to other compounds like sugars or proteins that are more readily degraded.

Key Concept:Decomposition Factors

c) High moisture levels
[Solution Description] The rate of decomposition is influenced by factors such as temperature, moisture, and the chemical composition of detritus. High temperatures often increase enzymatic activity, accelerating decomposition rates. Similarly, sufficient soil moisture facilitates microbial activities essential for decomposition.

Key Concept:Real-world Application, Complex Interactions

c) It would likely decrease because plant biomass decreases.
[Solution Description] An increase in the population of primary consumers can lead to greater consumption of producers (plants). This may reduce the biomass of producers available for growth and reproduction, leading to a decrease in net primary productivity (NPP). Net primary productivity (NPP) is defined as the gross primary productivity (GPP) minus the energy used by producers for respiration. Hence, with increased herbivory pressure, the energy available in the form of plant biomass will decline, thereby reducing NPP.

Key Concept:Energy Capture

b) 8%
[Solution Description] Plants capture only about 2-10% of the photosynthetically active radiation (PAR) available to them. Therefore, the correct answer falls within this range.

Key Concept:Definition Focus

c) Such chains hardly exist in real ecosystems
[Solution Description] Simple food chains rarely exist in nature because actual ecosystems consist of multiple interconnected food chains forming food webs.

Key Concept:Abiotic Components

c) Water
[Solution Description] Abiotic components of an ecosystem refer to the non-living physical and chemical elements present in that system. In a pond ecosystem, water is considered an abiotic component because it is a non-living element essential for the functioning of the ecosystem.

Key Concept:Process Identification

b) Inorganic nutrients
[Solution Description] Mineralisation involves the conversion of organic substances in humus into inorganic nutrients which are then available for plant uptake.

Key Concept:Components of Productivity

b) Respiration losses
[Solution Description] To determine Net Primary Productivity (NPP), we subtract respiration losses (R) from Gross Primary Productivity (GPP). The equation for this is given by $$GPP – R = NPP$$. Respiration losses account for the energy used by plants for maintenance and metabolic processes.