12. Ecosystem

Class 12 Biology Chapter 12 Ecosystem

Chapter 12 : Ecosystem

Class 12 biology Chapter 12 : Ecosystem Exercise Questions and Answers :

1. 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.

(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 "inverted" type.

(d) Common detritivores in our ecosystem is "earthworms '' .

(e) The major reservoir of carbon on earth is the "oceans" .

2. Which one of the following has the largest population in a food chain?

(a) Producers

(b) Primary consumers

(d) Decomposers

Answer : (d) Decomposers

Decomposers have the largest population in a food chain as they break down organic matter, recycling nutrients and supporting ecosystem health.

3. The second trophic level in a lake is

(a) Phytoplankton

(b) Zooplankton

(d) Fishes

Answer : (b) Zooplankton.

Zooplankton are the primary consumers in a lake ecosystem, feeding on phytoplankton (first trophic level). They are an important link between primary producers and higher trophic levels like fish.

4. Secondary producers are

(a) Herbivores

(b) Producers

(d) None of the above

Answer : (d) None of the above.

Secondary producers do not exist as a standard ecological term. The primary producers are plants, and herbivores are primary consumers.

5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?

(a) 100%

(b) 50 %

(d) 2-10%

Answer : (b) 50%.

The majority of incident solar radiation consists of non-photosynthetically active wavelengths, with only around 50% falling within the photosynthetically active radiation (PAR) range, which plants use for photosynthesis.

6. Distinguish between

(a) Grazing food chain and detritus food chain

(b) Production and decomposition

(d) Food chain and Food web

(e) Litter and detritus

(f) Primary and secondary productivity

Answer : (a) The difference between grazing food chain and detritus food chain :

Grazing Food Chain	Detritus Food Chain
Begins with green plants (producers).	Begins with dead organic matter (detritus).
Involves herbivores (primary consumers) and carnivores (secondary consumers).	Involves decomposers (bacteria, fungi) and detritivores (organisms that feed on detritus).
Energy is obtained from living plants.	Energy is derived from decaying organic matter.
Transfers energy directly from plants to herbivores and then to carnivores.	Decomposes dead organic matter into simpler substances, recycling nutrients.
Example: Grass → Grasshopper → Frog → Snake.	Example: Fallen leaves → Decomposer bacteria → Earthworms → Birds.
Common in terrestrial ecosystems, such as grasslands and forests.	Common in various ecosystems, including soil and aquatic environments.

(b) The difference between Production and Decomposition:

Production	Decomposition
The process by which autotrophs (producers) use energy from the sun to convert inorganic substances into organic matter.	The process by which decomposers break down organic matter into simpler substances, releasing nutrients.
Adds new organic matter to the ecosystem.	Recycles nutrients and breaks down organic matter, returning it to the ecosystem.
Autotrophs (e.g., plants, algae) are primarily responsible for production.	Decomposers (e.g., bacteria, fungi) and detritivores are primarily responsible for decomposition.
Moves from the lower trophic levels (producers) to the higher trophic levels (consumers).	Facilitates the recycling of nutrients within the ecosystem.
Takes place in the cells of autotrophic organisms, typically in plant leaves and chloroplasts.	Occurs in various parts of ecosystems where organic matter accumulates, such as soil and dead organic material.

Upright Pyramid	Inverted Pyramid
Has a traditional pyramid shape, with a broad base (producers) and a narrow top (top consumers).	Has an inverted shape, with a narrow base (producers) and a broad top (top consumers).
Typically represents the pyramid of numbers or biomass in an ecosystem.	Represents the pyramid of numbers or biomass, but inverted, often seen in certain ecosystems.
Each level represents the different trophic levels in the ecosystem.	Each level represents trophic levels but may not necessarily reflect actual population or biomass numbers.
Common in terrestrial ecosystems and ecosystems where there's a clear energy flow from producers to consumers.	More commonly observed in ecosystems with parasitic relationships, where one or a few top consumers support a larger base of host organisms.
Example: Grass (producers) → Herbivores → Carnivores.	Example: Parasitoid wasps (top consumers) → Host insects

(d) The difference between a Food Chain and a Food Web:

Food Chain	Food Web
Linear representation of the transfer of energy and nutrients from one organism to another, typically in a single, simplified pathway.	Complex, interconnected network of multiple food chains within an ecosystem, showing the interactions between various organisms.
Consists of a single, linear sequence of organisms where each species feeds on the one below it and is eaten by the one above it.	Involves multiple interconnected food chains, creating a more accurate representation of the complex feeding relationships in an ecosystem.
Simple and straightforward, providing a clear, direct pathway of energy transfer.	More complex and realistic, capturing the intricate and often overlapping interactions among various species in an ecosystem.
Typically represents a specific and limited portion of the ecosystem, focusing on a single path of energy flow.	Represents a broader view of the ecosystem, illustrating the myriad relationships and interactions among different species.
Example: Grass → Grasshopper → Frog → Snake.	Example: In a forest ecosystem - Plants → Herbivores (rabbits, deer) → Carnivores (foxes, hawks) → Decomposers (bacteria, fungi).

(e) The difference between Litter and Detritus:

Litter	Detritus
Freshly fallen or partially decomposed organic matter on the soil surface, such as leaves, twigs, and other plant debris.	Fully decomposed organic matter found deeper in the soil or sediment, often with a lower energy content.
Relatively intact and not significantly broken down, retaining recognizable plant materials.	Broken down into simpler substances, with little or no recognizable plant material remaining.
Located on the surface of the soil, covering the ground in forests and other ecosystems.	Typically found deeper in the soil, sediments, or at the bottom of aquatic ecosystems.
Contains relatively higher energy content due to its freshness and the presence of less decomposed organic matter.	Contains lower energy content because it has undergone substantial decomposition, and complex organic compounds have been broken down.
Serves as a source of organic matter for decomposers and detritivores, contributing to the nutrient cycling process.	Represents the final stage of organic matter decomposition and plays a significant role in nutrient recycling.
Fallen leaves, twigs, branches, and other surface plant debris in a forest.	Compost in a compost bin, humus in soil, and partially decomposed matter in sediments.

(f) The difference between Primary and Secondary Productivity:

Primary Productivity	Secondary Productivity
The rate at which autotrophic organisms (producers) convert solar energy into organic matter through photosynthesis.	The rate at which heterotrophic organisms (consumers) at higher trophic levels convert the organic matter from primary producers into their biomass.
Primary productivity is primarily driven by autotrophic organisms such as plants, algae, and phytoplankton.	Secondary productivity involves heterotrophic organisms such as herbivores, carnivores, and omnivores, as they assimilate energy from the consumption of other organisms.
Measured in terms of the rate of biomass or energy production by primary producers (e.g., grams of plant material produced per square meter per year).	Measured in terms of the rate at which energy or biomass is incorporated into the bodies of consumers at higher trophic levels (e.g., grams of herbivore biomass produced per square meter per year).
Represents the energy input into the ecosystem, serving as the base of the food chain.	Represents the flow of energy from primary producers to herbivores, then to carnivores or omnivores, further up the trophic levels.
At the bottom of the food chain, responsible for creating organic matter from inorganic substances.	Occurs at higher trophic levels, showing the efficiency of energy transfer and nutrient cycling in an ecosystem.
Example: Plants converting sunlight into biomass through photosynthesis in a forest.	Example: Herbivores like rabbits converting plant biomass into their own biomass, and then carnivores like foxes converting herbivore biomass into their own biomass.

7. Describe the components of an ecosystem.

Answer : An ecosystem is a structural and functional unit of nature and it comprises abiotic and biotic components.

The components of an ecosystem are :

Abiotic Components:

Air: Comprising atmospheric gases like oxygen, carbon dioxide, and nitrogen, it influences respiration and photosynthesis in living organisms.

Water: Aquatic ecosystems depend on water bodies, while terrestrial ecosystems require soil moisture and rainfall.

Soil: Soil composition and nutrient content are vital for plant growth and provide habitat for microorganisms.

Biotic Components:

Producers (Autotrophs) : These include green plants, algae, and certain bacteria that perform photosynthesis, converting sunlight into energy and organic matter.

Consumers (Heterotrophs) : Organisms in various trophic levels consume others for energy. Herbivores feed on producers, carnivores prey on other consumers, and omnivores consume both plants and animals.

Decomposers (microconsumers) : Bacteria, fungi, and detritivores break down dead organic matter, recycling nutrients back into the ecosystem.

Structural Features:

Species Composition: The types and diversity of organisms present in an ecosystem contribute to its species composition. This affects the ecosystem's complexity and ecological interactions.

Stratification: Ecosystems often exhibit a vertical layering, or stratification, of different organisms based on their habitat preferences and resource utilization.

Nutritional Niches:

Every organism within the ecosystem occupies a specific ecological niche based on its source of nutrition. This niche determines an organism's role and place within the ecosystem, helping regulate population dynamics and maintain the ecosystem's balance.

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

Answer : Ecological pyramids are graphical representations of the trophic (feeding) structure and relationships within an ecosystem. These pyramids help illustrate the distribution of energy, biomass, or the number of organisms across different trophic levels. There are three main types of ecological pyramids: pyramids of number, pyramids of biomass, and pyramids of energy.

Pyramid of Number :

A pyramid of number depicts the number of organisms at each trophic level in an ecosystem. It typically starts with a broad base representing a large number of primary producers and narrows as you move up the trophic levels.

This pyramid can take various shapes. In a healthy ecosystem, it is usually upright, with producers outnumbering herbivores and herbivores outnumbering carnivores.

Example: In a forest ecosystem, the pyramid of numbers may have a broad base of grasses and shrubs, a smaller number of herbivores (e.g., rabbits), and an even smaller number of carnivores (e.g., foxes).

Pyramid of Biomass:

A pyramid of biomass represents the total dry weight of all the organisms in each trophic level, measured in units like grams per square meter.

It typically shows a reduction in biomass as you move up the trophic levels. Producers have the most biomass, followed by herbivores, and then carnivores.

Example: In a grassland ecosystem, the pyramid of biomass may have a wide base of grasses, with more biomass, followed by a smaller amount of biomass in herbivores (e.g., deer), and even less biomass in carnivores (e.g., lions).

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

Answer : Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight () or energy ( ). The rate of biomass production is called productivity.

Factors affecting primary productivity, which is the capture of solar energy or biomass production by producers, include :

Gross Primary Productivity (GPP): GPP represents the total rate of solar energy capture and organic matter production by producers. It is influenced by factors such as available sunlight, temperature, and water supply, as well as the photosynthetic capacity of plants.

Net Primary Productivity (NPP): NPP is the biomass or energy left after producers utilize a portion of the energy captured through GPP. NPP depends on the efficiency of energy conversion by producers, nutrient availability, and the extent of energy used for plant maintenance and growth.

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

Answer : Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients, and this process is called decomposition. Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition .

The decomposition process involves several steps and leads to various products are :

Fragmentation: Detritivores like earthworms break down detritus into smaller particles. This step is called fragmentation. It physically reduces the size of detritus, making it more accessible to decomposers.

Leaching: During leaching, water-soluble inorganic nutrients from the detritus are dissolved and transported downward into the soil horizon. These nutrients may later precipitate as less available salts.

Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This biochemical process is known as catabolism. It transforms complex organic matter into more basic compounds.

Humification: As decomposition proceeds, a dark-colored, amorphous substance called humus accumulates. Humus is highly resistant to microbial action and decomposes at an extremely slow rate. It is colloidal in nature and serves as a reservoir of nutrients.

Mineralisation: Some microbes further degrade the humus, releasing inorganic nutrients. This process is called mineralisation. It makes the nutrients available for uptake by plants.

All of these steps in decomposition occur simultaneously on detritus, contributing to the breakdown of organic matter and the release of nutrients. The rate of decomposition is influenced by the chemical composition of the detritus and climatic factors. In warmer and moister environments, decomposition is favored, while low temperatures and anaerobic conditions can inhibit decomposition, leading to the accumulation of organic materials. The composition and availability of detritus, as well as climatic conditions, play a crucial role in regulating the overall decomposition process in ecosystems.

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

Answer : Energy flows unidirectionally through an ecosystem, starting with solar energy captured by primary producers. Plants convert sunlight into chemical energy through photosynthesis. This energy is then transferred to consumers as they feed on producers or other organisms. Decomposers break down organic matter, releasing energy for recycling. Energy loss as heat occurs at each trophic level, following the 10 percent law. This one-way flow of energy sustains life within the ecosystem, underlining the fundamental role of producers and trophic relationships.