Ecosystem and Energy Flow: Aditi Ma'am.
Below is a one-shot HSC Board lecture for Chapter 14 – Ecosystems and Energy Flow, based on your uploaded textbook. It covers the major definitions, processes, diagrams, comparisons, and points that are most useful for board preparation. Ecosystem and energy flow tb
CHAPTER 14 — ECOSYSTEMS AND ENERGY FLOW
HSC BOARD ONE-SHOT LECTURE
1. Ecosystem
An ecosystem is a self-regulatory and self-sustaining structural and functional unit of nature containing both biotic and abiotic components.
The term ecosystem was coined by A. G. Tansley in 1935.
Think of an ecosystem as:
Living organisms + non-living environment + interaction between them
Examples:
- Pond
- Forest
- Grassland
- Desert
- River
- Ocean
The entire biosphere can also be considered one global ecosystem.
Types of ecosystems
Terrestrial ecosystems
- Forest
- Grassland
- Desert
Aquatic ecosystems
- Lakes
- Rivers
- Wetlands
- Estuaries
- Oceans
Another classification:
Natural ecosystem: self-sustainable and generally does not need regular human input.
Artificial ecosystem: requires human input of energy or materials.
Examples:
- Farmland
- Fish tank
- Fish-rearing pond
2. Structure of an Ecosystem
An ecosystem consists of two major components:
Abiotic components
Non-living components such as:
- Water
- Air
- Soil
- Temperature
- Light
- Minerals
- Nutrients
Biotic components
Living organisms:
- Producers
- Consumers
- Decomposers
The interaction of biotic and abiotic components produces the characteristic physical structure of an ecosystem.
Identification and enumeration of different plant and animal species gives the species composition of an ecosystem.
3. Spatial Patterns: Stratification and Zonation
This distinction is important for HSC.
Stratification
Vertical distribution of organisms into different layers is called stratification.
Example in a forest:
Emergent layer → Canopy → Understorey → Shrub layer → Herbs/grasses
Trees generally occupy upper strata, shrubs middle strata and herbs/grasses the lower strata.
In oceans also, different vertical zones occur.
Memory trick:
Stratification = Storeys = Vertical
Zonation
Horizontal distribution of plants and animals in an ecosystem is called zonation.
It can occur in terrestrial as well as aquatic habitats and is especially clearly visible near boundaries such as lake shores and beaches.
Examples:
- Intertidal zone
- Littoral zone
- Sublittoral zone
Memory trick:
Zonation = Zones beside one another = Horizontal
4. Four Functional Aspects of Ecosystem
Every self-sustaining ecosystem performs four major processes:
1. Productivity
2. Decomposition
3. Nutrient cycling
4. Energy flow
These four are very important for board exams. Ecosystem and energy flow tb
A simple flow can be remembered as:
Sun → Production of food → Consumption → Death/waste → Decomposition → Nutrients returned → Producers
5. Pond Ecosystem
A pond is a good example of a self-sustaining aquatic ecosystem.
Abiotic components
- Water
- Dissolved organic substances
- Dissolved inorganic substances
- Bottom soil
- Sunlight
- Temperature
Producers
- Phytoplankton
- Algae
- Aquatic plants
Consumers
- Zooplankton
- Aquatic insects
- Fish
Decomposers
- Bacteria
- Fungi
mostly found near the bottom.
6. Productivity
Productivity is the rate of generation of biomass in an ecosystem.
It is generally expressed as:
Mass / unit area / unit time
Example:
g m⁻² day⁻¹
Two major types are:
Gross Primary Productivity — GPP
It is the rate of production of organic matter by producers during photosynthesis.
But plants themselves use some of this food for respiration.
Net Primary Productivity — NPP
The biomass remaining after respiratory loss is called net primary productivity.
Most important formula
NPP = GPP − R
or
GPP − Respiration = NPP
NPP is the actual biomass available to:
- Herbivores
- Carnivores
- Decomposers
Primary productivity depends on:
- Plant species
- Environmental conditions
- Availability of nutrients
- Photosynthetic capacity
7. Secondary Productivity
Secondary productivity is the rate of formation of new organic matter by consumers.
It can also be described as the rate of assimilation of food energy by consumers.
It represents energy available at the consumer level for transfer to the next trophic level.
8. Decomposition
This is a major HSC question.
Decomposition is the process by which decomposers break down complex organic matter into simpler inorganic substances such as CO₂, water and nutrients.
Dead plant and animal matter and faecal matter collectively form:
Detritus
Detritus is the raw material for decomposition.
Major steps of decomposition:
F – L – C – H – M
Fragmentation
Leaching
Catabolism
Humification
Mineralisation
Memorise this order.
8.1 Fragmentation
Detritivores such as earthworms break large pieces of detritus into smaller particles.
This process is called fragmentation.
8.2 Leaching
Water-soluble inorganic nutrients dissolve in water and move downward into deeper layers of soil.
Some may precipitate as unavailable salts.
8.3 Catabolism
Bacterial and fungal enzymes break complex organic matter into simpler substances.
This enzymatic degradation is called catabolism.
8.4 Humification
It results in the formation of:
Humus
Humus is:
- Dark coloured
- Amorphous
- Colloidal
- Partially decomposed organic material
- Highly resistant to microbial action
Humus decomposes very slowly.
Importance of humus
- Improves soil texture
- Increases water-holding capacity
- Acts as a reservoir of nutrients
8.5 Mineralisation
Microorganisms further break down humus and release inorganic nutrients.
This process is called mineralisation.
9. Factors Affecting Decomposition
Decomposition generally requires oxygen.
Two especially important environmental factors are:
Temperature + soil moisture
Faster decomposition
Warm + moist conditions
Slower decomposition
- Low temperature
- Anaerobic conditions
So remember:
Warm + Moist + Oxygen → Fast decomposition
10. Energy Flow in Ecosystem
The Sun is the major source of energy for almost all ecosystems, except some deep-sea ecosystems. Ecosystem and energy flow tb
Less than 50% of incident solar radiation is:
PAR — Photosynthetically Active Radiation
Plants capture only about 2–10% of PAR.
Still, this small amount supports almost the entire living world.
The important point is:
Energy flow is unidirectional.
Sun → Producers → Primary consumers → Secondary consumers → Tertiary consumers
It does not move backward.
Unlike nutrients, energy is not recycled.
Some energy is continuously lost as heat.
11. Producers and Consumers
Producers
Autotrophs manufacture their own food.
Terrestrial producers:
- Herbs
- Trees
- Other green plants
Aquatic producers:
- Phytoplankton
- Algae
Consumers
Consumers are heterotrophs.
Primary consumers
Eat producers.
They are usually herbivores.
Examples:
- Grasshopper
- Cow
- Sheep
- Goat
- Zooplankton
Secondary consumers
Eat primary consumers.
They are generally carnivores.
Tertiary consumers
Feed on secondary consumers and usually occupy higher trophic levels.
12. Trophic Levels
The specific position occupied by an organism in a food chain is called its trophic level.
First trophic level
Producers
Examples:
grass, phytoplankton, trees
Second trophic level
Primary consumers / herbivores
Examples:
cow, grasshopper, zooplankton
Third trophic level
Secondary consumers / carnivores
Examples:
fish, birds, wolf depending on the chain
Fourth trophic level
Tertiary/top consumers
Examples:
lion, humans in certain food chains
13. Food Chain
A food chain represents the linear transfer of food and energy from one organism to another.
Example:
Grass → Deer → Leopard
Food chains are usually limited to about 4–5 trophic levels because energy decreases at each successive level.
Three types are mentioned in the chapter:
1. Grazing food chain
2. Detritus food chain
3. Parasitic food chain
14. Grazing Food Chain — GFC
A grazing food chain begins with living green plants or producers.
Example:
Grass → Deer → Leopard
So:
Producer → Herbivore → Carnivore
15. Detritus Food Chain — DFC
A detritus food chain begins with:
Dead organic matter
It includes decomposers, especially:
- Fungi
- Bacteria
These organisms obtain nutrients and energy by degrading detritus.
They are also called saprotrophs.
Board distinction
Grazing chain begins with living producers.
Detritus chain begins with dead organic matter.
GFC and DFC may become connected at different levels.
16. Food Web
In nature, food chains normally do not exist independently.
Several interconnected food chains form a:
Food web
Example:
One animal may eat several types of food and may itself be eaten by several animals.
Food webs increase ecosystem stability.
Omnivores such as humans, crows and bears may occupy more than one trophic level.
17. 10% Law
One of the highest-yield board concepts.
According to the 10% law of energy transfer, only about:
10% of the energy
available at one trophic level is transferred to the next trophic level.
The law is associated in the textbook with R. Lindeman, 1942. Ecosystem and energy flow tb
Example:
Producer = 1000 J
Primary consumer receives ≈ 100 J
Secondary consumer receives ≈ 10 J
Tertiary consumer receives ≈ 1 J
Most remaining energy is lost through:
- Heat
- Respiration
- Metabolism
- Movement
- Waste
Therefore:
Higher trophic level = Lower available energy
This is why food chains usually cannot become indefinitely long.
18. Ecological Pyramids
An ecological pyramid is a graphic representation of the relationship between organisms at successive trophic levels with respect to number, biomass or energy.
The concept was developed by:
C. Elton — 1927
The base usually represents:
Producers
The apex represents:
Top consumers
Three types:
1. Pyramid of numbers
2. Pyramid of biomass
3. Pyramid of energy
19. Pyramid of Numbers
Shows the number of individuals at different trophic levels per unit area.
Usually upright:
Many producers → fewer herbivores → still fewer carnivores.
However, it may be inverted.
Example:
One large tree → many insects → many parasites
20. Pyramid of Biomass
Represents the amount of biomass present at different trophic levels per unit area.
It is generally upright in terrestrial ecosystems.
Example:
Grass → Mice → Snakes → Owls
But an important exception is:
Aquatic ecosystem
The biomass pyramid may be inverted because phytoplankton possess a small standing biomass but reproduce and turn over rapidly while supporting a relatively larger consumer biomass.
21. Pyramid of Energy
This is the easiest and most important pyramid fact:
Pyramid of energy is always upright.
Example:
Producers = 100%
Primary consumers = 10%
Secondary consumers = 1%
Tertiary consumers = 0.1%
Why can it never be inverted?
Because energy is lost as heat at every trophic level and cannot be recycled back.
Therefore:
Energy available always decreases upward.
22. Limitations of Ecological Pyramids
Very important theory question.
Ecological pyramids:
- Assume a simple food chain.
- Cannot properly represent a food web.
- Do not give adequate place to decomposers/saprophytes.
- Cannot easily represent organisms occupying more than one trophic level.
Example:
A sparrow may be:
- Primary consumer when eating seeds
- Secondary consumer when eating insects
23. Nutrient Cycling
Unlike energy, nutrients are recycled.
The movement of nutrients through biotic and abiotic components is called:
Nutrient cycling / Biogeochemical cycle
Nutrients move:
Abiotic environment → Organisms → Environment
Two main types:
Gaseous cycle
Main reservoir = atmosphere
Examples:
- Carbon cycle
- Nitrogen cycle
Sedimentary cycle
Main reservoir = Earth's crust
Example:
- Phosphorus cycle
24. Carbon Cycle
Carbon is extremely important because it forms the basic framework of organic molecules.
According to the chapter, approximately 71% of global carbon occurs dissolved in oceans, and the oceanic reservoir plays an important role in regulating atmospheric CO₂. Ecosystem and energy flow tb
The important carbon-cycle pathway is:
Atmospheric CO₂
↓ Photosynthesis
Plants
↓ Feeding
Animals
↓ Respiration/decomposition
Atmospheric CO₂
Some carbon enters:
- Oceans
- Sediments
- Rocks
- Fossil fuels
Long-term carbon storage sites are called:
Carbon sinks
Examples include:
- Oceans
- Fossil carbon deposits
- Sediments
25. Major Processes of Carbon Cycle
Remember these five:
P – R – D – S – C
Photosynthesis
Respiration
Decomposition
Sedimentation
Combustion
Photosynthesis
Atmospheric CO₂ → organic compounds in plants
Feeding
Plant carbon → animals
Respiration
Living organisms release CO₂
Decomposition
Decomposers release carbon from dead organic matter
Combustion
Burning fossil fuels and biomass releases CO₂
Other sources include:
- Forest fires
- Volcanic activity
- Fuel combustion
Human activities such as deforestation and fossil-fuel burning increase atmospheric CO₂.
26. Phosphorus Cycle
The cyclic movement of phosphorus through the lithosphere, hydrosphere and biosphere constitutes the phosphorus cycle.
Importance of phosphorus:
It is required for:
- Nucleic acids
- Biological membranes
- Cellular energy-transfer systems
- Bones
- Teeth
- Shells
Main reservoir:
Rocks
Rocks contain phosphorus mainly in the form of phosphates.
General flow:
Phosphate rocks
↓ Weathering
Phosphate in soil
↓ Plant absorption
Plants
↓ Feeding
Animals
↓ Death/waste
Decomposers
↓
Phosphate returned to soil
Some phosphorus enters aquatic systems by runoff and may eventually form sediments and new rocks.
27. Carbon Cycle vs Phosphorus Cycle
Very important comparison.
| Carbon cycle | Phosphorus cycle |
|---|---|
| Mainly gaseous cycle | Mainly sedimentary cycle |
| Atmospheric reservoir important | Rock is major reservoir |
| CO₂ moves through atmosphere | Practically no important gaseous phosphorus phase |
| Respiration releases CO₂ | No respiratory phosphorus release into atmosphere |
| Relatively rapid atmospheric exchange | Generally much slower cycle |
28. Phosphorus as Limiting Factor
Phosphorus is often available in relatively low quantity.
Therefore, it can become a:
Limiting factor for plant growth
Excess phosphorus reaching water bodies from:
- Agricultural runoff
- Industrial effluents
can cause:
Eutrophication
Eutrophication is excessive enrichment of water with nutrients leading to excessive algal growth.
Consequences:
- Algal bloom
- Reduced light penetration
- Decomposition increases
- Dissolved oxygen decreases
- Aquatic organisms may die
29. Ecological Succession
A major long-answer topic.
Ecological succession is the gradual, predictable and sequential change in species composition of a given area over time.
Think:
Simple community → increasingly complex community → climax community
The first organisms to colonise an area are:
Pioneer species
The final relatively stable community is:
Climax community
30. Steps of Ecological Succession
Memorise this sequence:
N – I – E – A – C – R – S
Nudation
Invasion
Ecesis
Aggregation
Competition and co-action
Reaction
Stabilization
These steps can be asked directly.
31. Sere and Seral Community
The entire sequence of communities from pioneer stage to climax stage is called:
Sere
Each individual transitional community is called:
Seral community
As succession proceeds, generally:
- Species diversity increases
- Number of organisms increases
- Biomass increases
- Ecosystem complexity increases
32. Primary Succession
Primary succession starts in an area where no previous living community or developed soil is present.
Examples:
- Newly formed volcanic island
- Bare rocks
- Newly cooled lava
- Newly created pond
It is generally:
Very slow
because soil must first develop.
It may take hundreds or thousands of years.
33. Secondary Succession
Secondary succession occurs where a community previously existed but was destroyed.
Examples:
- Burnt forest
- Abandoned farmland
- Cut forest
- Flooded land
Since some soil already exists:
Secondary succession is faster than primary succession.
Board line:
Primary succession = starts almost from scratch.
Secondary succession = starts where soil or remnants of the previous community remain.
34. Hydrarch and Xerarch Succession
Based on starting habitat:
Hydrarch succession / Hydrosere
Occurs in wet or aquatic areas.
Progression:
Hydric → Mesic
Xerarch succession / Xerosere
Occurs in dry areas.
Progression:
Xeric → Mesic
Important conclusion:
Both ultimately move toward mesic conditions.
Mesic = neither excessively dry nor excessively wet.
35. Xerarch Succession on Bare Rock
Very important sequence:
Bare rock
→ Crustose lichens
→ Mosses/bryophytes
→ Herbs
→ Grasses
→ Shrubs
→ Trees
→ Climax forest
Lichens are important pioneer species because they release acids that help break rocks and initiate soil formation.
36. Hydrarch Succession
Textbook sequence:
Phytoplankton
→ Submerged plants
→ Rooted floating plants
→ Free-floating plants
→ Reed-swamp stage
→ Marsh-meadow stage
→ Scrub stage
→ Climax forest
Examples:
Submerged plant:
Hydrilla
Rooted floating:
Lotus
Free floating:
Pistia
Reed swamp:
Typha
Marsh meadow:
Cyperus
Scrub:
Alnus
Climax tree:
Quercus
Over time:
Water body → progressively shallower habitat → terrestrial community
37. Ecosystem Services
Ecosystem services are the benefits obtained by humans from ecosystems.
The Millennium Ecosystem Assessment classifies them into four categories:
1. Supporting services
Examples:
- Nutrient cycling
- Primary production
- Soil formation
- Habitat provision
- Pollination
2. Provisioning services
Things directly obtained:
- Food
- Seafood
- Timber
- Fuel wood
- Water
- Genetic resources
- Medicinal resources
- Ornamental resources
3. Regulating services
Examples:
- Carbon sequestration
- Regulation of prey populations
- Decomposition
- Detoxification
- Purification of water and air
- Pest control
4. Cultural services
Examples:
- Recreation
- Education
- Scientific value
- Spiritual value
- Historical importance
- Therapeutic benefits
38. Two Major Ecological Services to Remember
CO₂ fixation and O₂ release
Photoautotrophs:
- Remove CO₂ by photosynthesis
- Store carbon
- Release O₂
This helps maintain atmospheric balance.
Pollination
Pollination may occur through:
- Wind
- Water
- Animals
Without pollination, reproduction of many flowering plants and production of many crops and fruits would be severely affected.
MOST IMPORTANT BOARD DEFINITIONS
Learn these almost word-for-word:
Ecosystem: A self-regulatory and self-sustaining structural and functional unit of nature containing biotic and abiotic components.
Productivity: Rate of generation of biomass in an ecosystem.
Decomposition: Breakdown of complex organic matter into simpler inorganic substances by decomposers.
Detritus: Dead plant and animal matter including faecal matter that acts as raw material for decomposition.
Trophic level: Specific position occupied by an organism in a food chain according to its source of food.
Ecological pyramid: Graphic representation of trophic-level relationships with respect to number, biomass or energy.
Nutrient cycling: Movement of nutrient elements through biotic and abiotic components of an ecosystem.
Ecological succession: Gradual, predictable and sequential change in species composition of a given area over time.
Ecosystem services: Benefits obtained by humans from ecosystems.
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