Ecosystem and energy flow
Ecosystem
PYQ from 2024
14.1 Introduction
1. Definition of Ecosystem
Ecosystem: A self-regulating and self-sustaining structural and functional unit of nature (biosphere).
It consists of both:
Biotic components: Living organisms (plants, animals and microorganisms).
Abiotic components: Non-living factors (air, water, soil, sunlight and temperature).
Biotic components interact with one another and with their surrounding environment.
A. G. Tansley (1935) coined the term ecosystem.
2. Size of Ecosystems
Ecosystems vary greatly in size:
Small: A pond or a small farmland.
Large: Oceans or villages.
The entire biosphere can be considered one global ecosystem, consisting of many local ecosystems.
3. Types of Ecosystems
A. Based on Environment
1. Terrestrial Ecosystems (Land)
Forest, Grassland, Desert
2. Aquatic Ecosystems (Water)
Lakes, Wetlands, Rivers, Seas, Oceans
Estuaries (areas where freshwater mixes with seawater).
B. Based on Origin
1. Natural Ecosystems Develop naturally.
Do not require regular human input of energy or materials. They are self-sustainable.
Examples: Forests, rivers and natural lakes.
2. Artificial Ecosystems
Created or maintained by humans.
Require regular input of energy or materials.
Examples: Farmland, fish tanks and fish-rearing ponds.
4. What Do We Study in an Ecosystem?
We study the structure and functions of ecosystems to understand:
1. Input (Net Productivity): Production of organic matter by organisms.
2. Energy Transfer: Movement of energy through food chains and food webs.
3. Nutrient Cycling: Movement and reuse of nutrients in the ecosystem.
4. Output: Decomposition (breakdown of organic matter) and energy loss.
14.1 Ecosystem: Structure and Function
Species Composition
Species composition: Identification and counting (enumeration) of different plant and animal species present in an ecosystem.
It helps us understand the variety of organisms present in that ecosystem.
3. Spatial Patterns
Biotic and abiotic components vary according to location and time. Variation due to space (location) produces two types of spatial patterns:
A. Stratification (Vertical Distribution)
Stratification: Vertical distribution of different plant and animal species at different levels or layers of an ecosystem.
Example 1: Forest
Top layer: Trees
Middle layer: Shrubs
Bottom layer: Herbs and grasses
Example 2: Open Sea
Epipelagic zone
Mesopelagic zone
Bathypelagic zone
Benthic zone
B. Zonation (Horizontal Distribution)
Zonation: Horizontal distribution of plants and animals across different areas of land or water.
It occurs in both terrestrial and aquatic ecosystems.
It is especially noticeable at the junction of land and water.
Example: Edges of a large lake or beach
Intertidal zone
Littoral zone
Sublittoral zone
Functional Aspects of an Ecosystem
The biotic (living) and abiotic (non-living) components of an ecosystem are linked to function as ' ecosystem unit' through four basic functional processes:
1. Productivity
2. Decomposition
3. Nutrient Cycling
4. Energy Flow
All four processes are essential for an ecosystem to remain self-sustaining (able to function continuously on its own).
1. Productivity
Conversion of inorganic substances into organic matter by autotrophs (producers), using solar energy.
Heterotrophs (consumers) obtain food by consuming autotrophs or other organisms.
2. Decomposition
● Breakdown of dead organic matter.
● Includes mineralisation (release of inorganic nutrients/minerals from dead organic matter).
3. Nutrient Cycling
● Storage and transportation of nutrients within the ecosystem.
● Minerals released during decomposition are reused by autotrophs.
4. Energy Flow
● Energy flows in one direction (unidirectional): Producers → Consumers → Heat loss
● Energy is finally dissipated (released) as heat.
Example: Pond Ecosystem
A pond is a relatively small, shallow aquatic ecosystem. It is a self-sustaining unit that demonstrates all four functional processes.
● Water containing dissolved inorganic and organic substances.
● Rich soil deposits at the bottom of the pond.
● Solar energy, temperature variations, day length and other climatic conditions regulate the functioning of the pond.
Biotic Components - A Producers- Phytoplankton, Algae, Other aquatic plants
B. Consumers - Zooplankton, Aquatic insects, Fish
C. Decomposers - Bacteria, Fungi
Mainly located at the bottom of the pond.
a. Productivity
● Productivity is the rate of generation of biomass (organic matter) in an ecosystem.
● A constant input of solar energy is essential for an ecosystem to function and sustain itself.
Unit of Productivity
Expressed as mass per unit area (or volume) per unit time.
Example: g/m²/day (grams per square metre per day).
Types of Productivity
1. Primary Productivity
Rate of production of organic matter by autotrophs (green plants), through photosynthesis.
A. Gross Primary Productivity (GPP)
● Total rate of production of organic matter during photosynthesis.
● Plants use a considerable part of GPP for their own respiration.
B. Net Primary Productivity (NPP)
Organic matter remaining after subtracting respiratory losses (R) from GPP.
This is the biomass available for consumption by heterotrophs, such as herbivores, carnivores and decomposers.
Formula:
NPP = GPP - R
Where:
GPP = Gross Primary Productivity
R = Respiratory losses by plants
NPP = Net Primary Productivity
Important facts:
● Annual NPP of the entire biosphere ≈ 170 billion tonnes (dry weight) of organic matter.
● Contribution of oceans ≈ 55 billion tonnes.
● The remaining productivity comes from land-based ecosystems.
PYQ from 2023
Define the term:
a) Gross primary productivity
b) Net primary productivity
Factors affecting Primary Productivity
●Plant species present in a particular area.
●Environmental factors.
●Availability of nutrients.
●Photosynthetic capacity of plants.
Therefore, primary productivity varies in different ecosystems.
2. Secondary Productivity
● of formation of new organic matter by consumers.
● It is also the rate of assimilation (conversion into usable form) of food energy at the consumer level.
● Represents the energy available to consumers for transfer to the next trophic level.
b. Decomposition
Decomposition is the process by which decomposers break down complex organic matter into simpler inorganic substances such as carbon dioxide, water and nutrients.
Dead remains of plants and animals, including faecal matter, form detritus (raw material for decomposition).
Steps of Decomposition
There are five important steps:
1. Fragmentation
2. Leaching
3. Catabolism
4. Humification
5. Mineralisation
1. Fragmentation
Detritivores (organisms that feed on dead organic matter), such as earthworms, break detritus into smaller fragments or particles.
2. Leaching
Water-soluble inorganic nutrients move down into deeper soil layers through percolating water.
These nutrients may get precipitated (deposited) as unavailable salts.
3. Catabolism
Bacterial and fungal enzymes break down detritus into simpler inorganic substances.
Note: Fragmentation, leaching and catabolism operate simultaneously on detritus.
4. Humification
Leads to the accumulation of a partially decomposed, dark-coloured, amorphous (without a definite shape), colloidal organic substance called humus.
Humus: Is resistant to microbial action.
Decomposes at an extremely slow rate.
Improves soil texture.
Increases the water-holding capacity of soil.
Acts as a reservoir of nutrients due to its colloidal nature.
5. Mineralisation
Some microbes further degrade humus.
This releases inorganic nutrients into the soil.
This process is called mineralisation.
Factors Affecting Decomposition
Decomposition requires oxygen.
The two most important factors regulating decomposition are:
1. Temperature
2. Soil moisture
●Favourable conditions:
Warm and moist environment favours decomposition because it supports soil microbial activity.
● Unfavourable conditions:
Low temperature.
Anaerobic conditions (absence of oxygen).
These conditions inhibit decomposition.
PYQ FROM 2025
What is decomposition?
Give diagrammatic representation of 'decomposition cycle.'?
Energy Flow
1. Source of Energy
The Sun is the main source of energy for all ecosystems on Earth, except deep-sea ecosystems.
Less than 50% of the total incoming solar radiation is Photosynthetically Active Radiation (PAR).
PAR is the portion of sunlight used by plants and photosynthetic bacteria for photosynthesis.
Plants capture only 2–10% of PAR.
Although this is a small amount, it sustains the entire living world.
2. Flow of Energy in an Ecosystem
Plants and photosynthetic bacteria (autotrophs) capture solar energy and convert it into chemical energy (food).
All organisms depend directly or indirectly on producers for food.
Energy flows in one direction only:
Sun → Producers → Consumers
Energy flow cannot be reversed.
Energy can be used only once in an ecosystem; it is not recycled like nutrients.
3. Producers (Autotrophs)
Organisms that prepare their own food using solar energy and simple inorganic substances.
They require a continuous supply of energy to synthesise essential molecules.
Examples:
Terrestrial ecosystems: Herbaceous plants (soft-stemmed plants) and woody plants (trees and shrubs).
Aquatic ecosystems: Phytoplankton and algae.
4. Consumers (Heterotrophs)
Organisms that depend directly or indirectly on plants for food.
They cannot prepare their own food.
Consumers are classified according to their feeding levels:
A. Primary Consumers
Feed directly on producers.
Also called herbivores.
Examples:
Terrestrial: Grasshopper, aphids, parrot, sheep, cattle, goat and donkey.
Aquatic: Molluscs.
B. Secondary Consumers
Feed on primary consumers (herbivores).
Generally called primary carnivores.
C. Tertiary Consumers
Feed on secondary consumers.
D. Secondary Carnivores
Animals that depend on primary carnivores (secondary consumers) for food.
1. Food Chain
A food chain is a sequence of organisms in which one organism feeds on another and is itself eaten by another organism.
● Food chains are usually straight (linear) and generally contain 4–5 trophic levels.
● There are three types of food chains:
1. Grazing Food Chain (GFC)
2. Detritus Food Chain (DFC)
3. Parasitic Food Chain
Energy trapped by producers is either:
Transferred to consumers, or
Remains stored in the producer until it dies.
● The death of an organism marks the beginning of the detritus food chain.
2. Detritus Food Chain (DFC)
Begins with dead organic matter (detritus).
Mainly consists of decomposers such as:
Fungi, Bacteria
● These are heterotrophic organisms that obtain energy and nutrients by breaking down detritus.
They are called saprotrophs (saprobes).
● Saprotrophs secrete enzymes that break down dead organic matter into simple inorganic substances, which are then absorbed by them.
DFC may be connected with the grazing food chain at different levels.
3. Food Web
● In natural ecosystems, food chains are interconnected and form a food web.
● Some animals are omnivores (organisms that eat both plants and animals).
Examples: Cockroaches, crows, bears and humans.
● Omnivores connect different food chains because they feed on both producers and consumers.
● These natural interconnections help maintain ecosystem stability.
4. Trophic Levels
Trophic level: The specific position occupied by an organism in a food chain according to its food source and method of obtaining food.
Trophic levels:
First trophic level: Producers (plants).
Second trophic level: Herbivores (primary consumers).
Third trophic level: Carnivores (secondary consumers).
Fourth trophic level: Secondary carnivores (tertiary consumers).
5. 10% Law of Energy Transfer
Energy available decreases at each successive trophic level.
According to the 10% Law, only 10% of the energy from one trophic level is transferred to the next trophic level as net energy.
Proposed by R. Lindemann (1942).
Example:
If producers have 10,000 units of energy:
Producers: 10,000 units
Primary consumers: 1,000 units
Secondary consumers: 100 units
Tertiary consumers: 10 units
Only 10% is transferred; the remaining energy is not passed to the next trophic level.
● A tertiary carnivore feeding exclusively on secondary carnivores is generally not supported due to insufficient energy.
● Secondary carnivores may sometimes feed directly on herbivores.
PYQs from 2025
Q. (ix) In the given food chain, if plants absorb 1000 Joules of light energy, then how much energy is available for Lion?
Food Chain:
Plants → Deer → Lion
(Producer) → (Herbivore) → (Carnivore)
Ecological Pyramids
C. Elton (1927): Developed the concept of ecological pyramids
An ecological pyramid is a graphical representation of the relationship between organisms at successive trophic levels in terms of:
1. Number
2. Biomass
3. Energy
The concept was developed by C. Elton (1927).
A typical pyramid has a broad base and gradually narrows towards the top.
2. Structure of an Ecological Pyramid
Base: Producers (first trophic level).
Middle levels: Primary and secondary consumers.
Apex: Tertiary or top-level consumers.
There are three main types:
A. Pyramid of Numbers
Represents the relative number of individuals per unit area at each trophic level.
B. Pyramid of Biomass
Represents the biomass (total mass of living organisms) per unit area at each trophic level.
C. Pyramid of Energy
Represents the amount of accumulated energy per unit area at each trophic level.
4. Upright Ecological Pyramids
In most well-balanced ecosystems, pyramids of number, biomass and energy are upright.
Producers generally have greater number and biomass than herbivores.
Herbivores generally have greater number and biomass than carnivores.
5. Inverted Ecological Pyramids
A. Inverted Pyramid of Biomass
● Found in oceanic ecosystems.
● The biomass of producers (mainly phytoplankton) is lower than the biomass of consumers at a particular time.
B. Inverted pyramid of number.
● Can occur in certain ecosystems.
● Example: A single tree supports many insects, which are eaten by a smaller number of birds. Those birds may further support numerous parasites.
This produces an inverted pyramid of numbers.
The pyramid of energy is always upright.
It can never be inverted because:
Only 10% of energy is transferred to the next trophic level.
A large amount of energy is lost as heat at every trophic level.
Describe pyramid of energy with the help of diagram.
7. Limitations of Ecological Pyramids
1. Assumes a simple food chain
2. Does not represent food webs: It cannot accommodate complex food-web relationships.
3. Saprophytes are not included: Decomposers play an important role in ecosystems but are not given a place in ecological pyramids.
4. One species may occupy multiple trophic levels: A species can feed at different trophic levels in the same ecosystem.
Example – Sparrow:
When a sparrow eats seeds, fruits and peas → Primary consumer.
When it eats insects and worms → Secondary consumer.
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