Ecosystem

Ecosystem – Structure & Function

Ecosystem – Core Concept

• Definition
  • An ecosystem is a structural and functional unit of nature in which living organisms interact with each other and with the physical environment.
• Key Features
  • Self-regulating and self-sustaining system
  • Interaction between biotic and abiotic components
  • Exchange of materials between living and non-living components
  • Open system – receives solar energy input
  • Energy flow is unidirectional
• Term Coined By
  • The term ecosystem was introduced by A. G. Tansley in 1935.
  • The entire biosphere can be considered a global ecosystem.

Types of Ecosystems

1. Terrestrial Ecosystems
   • Forest
   • Grassland
   • Desert
2. Aquatic Ecosystems
   • Pond
   • Lake
   • Wetland
   • River
   • Estuary
   • Sea
3. Man-Made Ecosystems
   • Crop fields
   • Aquarium

Ecosystems vary in size from a small pond to a vast ocean.

Structure of an Ecosystem

Every ecosystem has two major components:

A. Abiotic Components (Non-Living)

• Include physical and chemical factors such as:
  • Light
  • Temperature
  • Water
  • Soil
  • Wind
  • Minerals
• These determine the physical structure and environmental conditions of the ecosystem.

B. Biotic Components (Living)

Organized into three functional groups:

1. Producers (Autotrophs)

• Role:
  • Convert solar energy into chemical energy through photosynthesis.
• Examples:
  • Algae, bryophytes, vascular plants
• Function:
  • Fix inorganic substances into complex organic compounds.

2. Consumers (Heterotrophs)

• Role:
  • Depend on producers or other consumers for food.
• Types:
  1. Herbivores
  2. Carnivores
  3. Omnivores
• Function:
  • Transfer energy across trophic levels.

3. Decomposers (Reducers / Microconsumers)

• Role:
  • Break down dead organic matter and waste products.
• Examples:
  • Bacteria, fungi, moulds, mushrooms
• Mechanism:
  • Secrete extracellular enzymes → decompose organic matter → release nutrients
• Importance:
  • Recycle nutrients back to the abiotic environment.

Physical Structure of Ecosystem

Species Composition

• Refers to the types and number of plant and animal species present in an ecosystem.
  • High diversity → Tropical rainforests, coral reefs
  • Low diversity → Deserts, Arctic regions

Stratification

• Vertical distribution of species in distinct layers.
• Example: Forest Ecosystem
  • Emergent layer
  • Canopy
  • Understorey
  • Shrub layer
  • Herb/grass layer
• Stratification improves efficient utilization of light and other resources.

Functions of an Ecosystem

Four major functional aspects:

1. Productivity

• Definition
  • Rate of production of organic matter by producers.
• Mechanism
  • Photosynthesis converts radiant solar energy into chemical energy stored in organic compounds.

2. Decomposition

• Definition
  • Breakdown of dead organic matter into simpler inorganic substances.
• Stages
  • Fragmentation
  • Leaching
  • Catabolism
  • Humification
  • Mineralisation
• Result
  • Release of nutrients into soil and water.

3. Energy Flow

• Principle
  • Energy enters the ecosystem as sunlight
  • Transferred from producers → consumers → decomposers
  • Flow is unidirectional
  • Energy is lost as heat at each trophic level
• Energy cannot be recycled.

4. Nutrient Cycling

• Definition
  • Movement of nutrients between biotic and abiotic components.
  • Matter is continuously recycled within ecosystems.
• Examples
  • Carbon cycle
  • Nitrogen cycle
  • Phosphorus cycle

Example of an Aquatic Ecosystem: Pond

Abiotic Components

• Water with dissolved nutrients
• Bottom soil deposits
• Solar radiation
• Temperature
• Day length

Biotic Components

• Producers
  • Phytoplankton, algae, floating and submerged plants
• Consumers
  • Zooplankton, small fish, large fish, bottom-dwelling organisms
• Decomposers
  • Bacteria, fungi, flagellates

Functional Processes

• Conversion
  • Autotrophs convert inorganic matter into organic matter using sunlight.
• Consumption
  • Heterotrophs feed on producers or other consumers.
• Decomposition
  • Dead organisms are broken down → minerals released.
• Energy Flow
  • Energy moves in one direction and is dissipated as heat.

Conceptual Integration

• Ecosystem links structure and function:
  • Structure → Species composition and stratification
  • Function → Productivity, decomposition, energy flow, nutrient cycling
• Energy flows through the system.
• Matter cycles within the system.
• Together, these processes maintain ecosystem stability and sustainability.

Productivity

Concept of Productivity

• Definition
  • Productivity is the rate of biomass (organic matter) production per unit area per unit time in an ecosystem.
  • It represents the rate at which solar energy is converted into chemical energy by producers.
• Units of Measurement
  • g m⁻² year⁻¹ (weight basis)
  • kcal m⁻² year⁻¹ (energy basis)

Types of Productivity

1. Primary Productivity

• Definition
  • Amount of biomass produced by plants during photosynthesis per unit area per unit time.
  • Occurs at the producer level.
  • Primary productivity is of two types:

A. Gross Primary Productivity (GPP)

• Total rate of organic matter production during photosynthesis.
• Represents total photosynthetic output.

B. Net Primary Productivity (NPP)

• Biomass remaining after plants use some energy for respiration.
• Formula:
  • NPP = GPP − R
  • Where: R = Respiration losses by plants
• Significance
  • Represents energy available to herbivores and decomposers
  • Determines energy transfer to higher trophic levels

2. Secondary Productivity

• Definition
  • Rate of formation of new organic matter by consumers.
  • Represents the rate at which heterotrophs assimilate food energy.
  • Occurs at the consumer level.

Factors Affecting Productivity

• Biotic Factors
  • Plant species present
  • Photosynthetic efficiency
  • Leaf area and canopy structure
• Abiotic Factors
  • Light intensity
  • Temperature
  • Water availability
  • Nutrient availability

Primary productivity varies greatly among ecosystems due to these factors.

Global Productivity

• Annual Net Primary Productivity of Biosphere
  • ≈ 170 billion tons (dry weight) of organic matter per year.
• Distribution
  • Oceans (≈ 70% of Earth’s surface) → ≈ 55 billion tons
  • Land ecosystems → ≈ 115 billion tons

Important Concept

• Despite covering a larger surface area, oceans have lower productivity due to:
  • Nutrient limitation in open oceans
  • Limited light penetration depth
  • Low phytoplankton concentration in many regions

Decomposition

Concept of Decomposition

• Definition
  • Decomposition is the physical and chemical breakdown of complex organic matter into simpler inorganic substances such as:
    • Carbon dioxide
    • Water
    • Mineral nutrients
  • It is essential for nutrient recycling in ecosystems.

Detritus – The Raw Material

• Definition
  • Detritus consists of:
    • Dead plant parts (leaves, bark, flowers)
    • Dead animals
    • Animal excreta
• Types
  • Above-ground detritus (leaf litter, droppings, carcasses)
  • Below-ground detritus (dead roots, buried organisms)

Steps in Decomposition

1. Fragmentation

• Process
  • Detritivores (e.g., earthworms, termites) break detritus into smaller particles.
• Significance
  • Increases surface area
  • Enhances microbial action

2. Leaching

• Process
  • Water-soluble nutrients dissolve and percolate into deeper soil layers.
• Outcome
  • Nutrients temporarily become unavailable to plants.

3. Catabolism

• Process
  • Bacteria and fungi secrete extracellular enzymes that break complex organic matter into simpler substances.
• Outcome
  • Formation of simpler organic and inorganic compounds
  • Temporary immobilisation of nutrients in microbial biomass

4. Humification

• Process
  • Formation of humus from partially decomposed organic matter.
• Characteristics of Humus
  • Dark coloured
  • Amorphous
  • Colloidal
  • Resistant to microbial action
  • Decomposes very slowly
• Functions
  • Acts as a nutrient reservoir
  • Improves soil aeration
  • Enhances water-holding capacity

5. Mineralisation

• Process
  • Further microbial breakdown of humus.
• Outcome
  • Release of inorganic nutrients back into soil, making them available to producers.

Factors Affecting Decomposition

A. Chemical Composition of Detritus

• Slow Decomposition
  • High lignin content
  • High chitin content
• Fast Decomposition
  • Rich in nitrogen
  • Rich in water-soluble compounds (e.g., sugars)

B. Climatic Factors

• Favourable Conditions
  • Warm temperature
  • Moist environment
  • Adequate oxygen (aerobic conditions)
• Unfavourable Conditions
  • Low temperature
  • Dry conditions
  • Anaerobic conditions (oxygen deficiency)

Anaerobiosis slows decomposition and may cause accumulation of organic matter.

Ecological Significance

• Recycles nutrients back to producers
• Maintains soil fertility
• Prevents accumulation of organic waste
• Supports ecosystem sustainability

Energy Flow in Ecosystems

Concept of Energy in Ecosystem

• Primary Source
  • Sun is the ultimate source of energy for all ecosystems
  • (exception: deep-sea hydrothermal ecosystems).
• Photosynthetically Active Radiation (PAR)
  • Less than 50% of incident solar radiation is PAR
  • Only 2–10% of PAR is captured by autotrophs
  • This small fraction sustains the entire biosphere

Primary Producers

• Definition
  • Autotrophs that convert solar energy into chemical energy through photosynthesis.
• Terrestrial Producers
  • Herbaceous plants
  • Woody plants
• Aquatic Producers
  • Phytoplankton
  • Algae
  • Higher aquatic plants
• Role
  • Fix solar energy
  • Form the base of all food chains
  • Support all heterotrophs directly or indirectly

Laws Governing Energy Flow

• First Law of Thermodynamics
  • Energy cannot be created or destroyed; it can only be transformed from one form to another.
  • Energy flows:
    • Sun → Producers → Consumers → Decomposers
• Second Law of Thermodynamics
  • Energy transfer is inefficient.
  • At each trophic level, some energy is lost as heat.

Key Implications

• Continuous energy input from the Sun is essential
• Ecosystem functions as an open system
• Energy flow is unidirectional and cannot be recycled

Food Chain

• Definition
  • A linear sequence of organisms through which energy and nutrients pass via feeding relationships.
• General Characteristics
  • Usually 3–6 trophic levels
  • Progressive reduction in energy and biomass
  • Large energy loss at each transfer
  • Sustained by producers and decomposers

Trophic Levels

• Trophic Level
  • Position in a food chain based on mode of nutrition.

1. First Trophic Level – Producers
   • Autotrophic organisms
   • Examples: Grass, trees, phytoplankton
2. Second Trophic Level – Primary Consumers
   • Herbivores
   • Examples: Grasshopper, cow, zooplankton
3. Third Trophic Level – Secondary Consumers
   • Primary carnivores
   • Examples: Frog, fish, birds
4. Fourth Trophic Level – Tertiary Consumers
   • Top carnivores
   • Examples: Eagle, lion

• Important Note
  • Parasites are not fixed to a single trophic level
  • They feed across multiple levels

Standing Crop

• Definition
  • Total living biomass present at each trophic level per unit area.
• Measured as:
  • Fresh weight (less accurate)
  • Dry weight (more accurate; avoids moisture variation)

Types of Food Chains

1. Grazing Food Chain (GFC)

• Starts from living green plants.
• Example: Grass → Goat → Man
• Most common type
• Predation occurs at each step

2. Detritus Food Chain (DFC)

• Starts from dead organic matter (detritus).
• Flow: Detritus → Detritivores → Carnivores
• Example: Detritus → Earthworm → Frog → Snake
• Key Points:
  • Dominant in terrestrial ecosystems
  • Major pathway of energy flow on land

3. Parasitic Food Chain (PFC)

• Begins with host and ends with parasite.
  • Energy flows from large host to smaller parasite
  • Also called auxiliary food chain

Food Web

• Definition
  • Network of interconnected food chains.
• Significance
  • Provides alternate feeding pathways
  • Enhances ecosystem stability
  • Reduces risk of population collapse
• Example:
  • Grass may be eaten by rabbit or mouse.
  • Mouse may be eaten by snake or hawk.

These interconnected pathways form a food web.

Energy Transfer Efficiency

• Lindemann’s 10% Law (1942)
  • Only about 10% of energy at one trophic level is transferred to the next.
• Implications
  • About 90% energy lost as heat
  • Energy decreases progressively
  • Higher trophic levels support fewer organisms
  • Food chains are short (usually 3–5 levels)

Shorter food chains are more energy efficient.

Direction of Energy Flow

• Pattern
  • Sun → Producers → Herbivores → Carnivores → Decomposers
• Key Characteristics
  • Unidirectional
  • Not recycled
  • Lost as heat at each trophic level

• Matter cycles in ecosystem.
• Energy does not cycle.

Ecological Significance

• Explains trophic structure
• Explains limited trophic levels
• Explains pyramid of energy
• Determines ecosystem productivity
• Maintains ecological balance

Ecological Pyramids

Concept and Definition

• An ecological pyramid is a graphical representation of trophic structure in terms of:
  • Number
  • Biomass
  • Energy
• It shows quantitative relationships between trophic levels in a food chain.

Historical Note

• Proposed by Charles Elton (1927).
• Also called Eltonian Pyramids.

Basic Structure

• Shape
  • Broad base → Narrow apex
• Base
  • Producers (First trophic level)
• Apex
  • Tertiary or top consumers

Important Principles

• All organisms at each trophic level must be included for accurate calculation
• Trophic level represents function, not species
• Example:
  • Sparrow → Primary consumer when eating seeds
  • Sparrow → Secondary consumer when eating insects

Types of Ecological Pyramids

1. Pyramid of Number
2. Pyramid of Biomass
3. Pyramid of Energy

1. Pyramid of Numbers

• Represents
  • Number of individuals per unit area at each trophic level.
• General Pattern
  • Usually upright:
  • Producers > Herbivores > Carnivores
• Example (Grassland):
  • Millions of plants → Thousands of herbivores → Few carnivores

Exceptions

• Inverted Pyramid of Numbers (Tree Ecosystem):
  • One large tree
  • Many insects feeding on it
  • Several birds feeding on insects
  • Parasites and hyperparasites increase further
• Result → Number increases upward → Inverted shape

Forest ecosystem may appear spindle-shaped.

2. Pyramid of Biomass

• Represents
  • Total dry weight of organisms at each trophic level per unit area.
  • (Dry weight preferred because moisture content varies seasonally.)

• Terrestrial Ecosystem
  • Usually upright:
  • Producer biomass > Herbivore biomass > Carnivore biomass
  • Examples: Grassland, forest

• Aquatic Ecosystem
  • Often inverted.
  • Reason:
    • Phytoplankton have low standing biomass
    • Rapid turnover rate
    • Zooplankton biomass may exceed phytoplankton biomass
  • Thus → Biomass increases upward → Inverted pyramid

3. Pyramid of Energy

• Represents
  • Rate of energy flow or productivity per unit area per unit time.
  • Units: kcal/m²/year or g/m²/year

• Always Upright
  • Energy decreases at each trophic level.
• Reason:
  • Energy lost as heat
  • Respiration losses
  • Follows Second Law of Thermodynamics
  • Supported by Lindemann’s 10% Law
• Only about 10% of energy transfers to the next trophic level.
• Therefore, energy pyramid can never be inverted.

Ecological Efficiency

• Definition
  • Percentage of energy transferred from one trophic level to the next.
• Formula
  • Ecological Efficiency (%) = (Energy at higher trophic level ÷ Energy at lower trophic level) × 100

Related Efficiencies

• Assimilation efficiency – Percentage of ingested energy assimilated
• Photosynthetic efficiency – Percentage of solar energy converted to chemical energy
• Net production efficiency = (NPP ÷ GPP) × 100

Common Patterns Summary

• Numbers → Usually upright; may be inverted (tree ecosystem)
• Biomass → Upright (terrestrial); inverted (aquatic)
• Energy → Always upright
• Most reliable pyramid → Energy

Limitations of Ecological Pyramids

• Trophic overlap not properly represented
• Assumes simple food chains (ignores food webs)
• Decomposers not included
• Does not show food web interconnections

Ecological Significance

• Explains energy distribution
• Explains trophic structure
• Shows why food chains are short
• Demonstrates energy loss at each level
• Helps understand ecosystem stability

Ecological Succession

• Definition
  • Ecological succession is the gradual, orderly, and predictable change in species composition of a given area over time.
• Core Idea
  • Communities are dynamic, not static
  • They respond to environmental changes
  • Changes proceed in a definite sequence
  • Ultimately lead to a stable climax community

Basic Terminology

• Sere
  • The complete sequence of communities that develop successively in an area.
• Seral Stages (Seral Communities)
  • Transitional communities appearing during succession.
• Pioneer Species (Primary Colonisers)
  • First organisms to invade a bare or disturbed area
  • Hardy and stress-tolerant
• Climax Community
  • Final stable community
  • Self-sustaining
  • In equilibrium with physical environment
  • Characterized by maximum diversity and niche specialization

General Trends During Succession

• Across successive seral stages:
  • Species diversity increases
  • Number of individuals increases
  • Total biomass increases
  • Food webs become more complex
  • Stability increases

Succession and evolution have operated parallelly over geological time.

Types of Ecological Succession

• Based on the initial condition of the area:
  1. Primary succession
  2. Secondary succession

1. Primary Succession

• Definition
  • Occurs in an area that was previously unoccupied and lifeless.
• Examples
  • Bare rock
  • Newly cooled lava
  • Newly formed ponds or reservoirs

• Initial Conditions
  • No soil
  • No humus
  • No previous biological community
  • Highly hostile environment

• Key Characteristics
  • Pioneer species come from outside
  • Soil formation is essential before complex life establishes
  • Extremely slow process
  • May take hundreds to thousands of years
• Reason for Slowness
  • Soil formation from rock through weathering and organic matter accumulation is time-consuming.

2. Secondary Succession

• Definition
  • Occurs in areas where a previous community existed but was destroyed.
• Examples
  • Burned forest
  • Abandoned farmland
  • Flooded land
  • Deforested region

• Initial Conditions
  • Soil already present
  • Humus present
  • Seeds, spores, or vegetative propagules may remain

• Key Characteristics
  • Faster than primary succession
  • Pioneer community arises from surviving organisms and migrants
  • Climax reached more quickly

Effect of Disturbances

• Disturbances may be:
  • Natural (fire, flood, storm)
  • Human-induced (deforestation, agriculture)
• Effect
  • May revert succession to an earlier seral stage
  • Create new environmental conditions
  • Favor some species while suppressing others

Ecological succession explains long-term changes in community structure and ecosystem development.

Succession of Plants

Classification Based on Moisture Conditions

• Plant succession is classified according to habitat moisture into:
  1. Hydrarch succession (Hydrosere)
  2. Xerarch succession (Xerosere)

1. Hydrarch Succession (Hydrosere)

• Starts In
  • Wet habitat such as pond or lake.
• Pioneer Stage
  • Phytoplankton stage.

• Sequence of Stages
  1. Phytoplankton stage
  2. Rooted submerged plants
  3. Rooted floating plants
  4. Free-floating plants
  5. Reed swamp stage
  6. Marsh-meadow stage
  7. Woodland stage
  8. Climax forest

• Final Outcome
  • Water body is gradually converted into land.
  • Results in a mesic climax community (moderate moisture conditions).

2. Xerarch Succession (Xerosere)

• Starts In
  • Dry habitat such as bare rock or desert.
• Pioneer Stage
  • Lichens.

• Role of Lichens
  • Secrete acids
  • Break down rocks
  • Contribute to soil formation

• Sequence of Stages
  1. Crustose lichen stage
  2. Foliose lichen stage
  3. Moss stage
  4. Herb stage
  5. Shrub stage
  6. Forest stage (climax)

• Final Outcome
  • Dry habitat gradually develops into mesic conditions.
  • Ends in a mesic climax forest.

Common Endpoint

• Both hydrarch and xerarch successions ultimately lead to a mesic climax community.
• Mesic = Moderate moisture conditions.

Impact on Animals

• As vegetation changes during succession:
  • Food sources change
  • Shelter availability changes
  • Animal species composition changes
  • Decomposer community also changes
• Thus, plant succession drives animal succession.

Time Scale

• Primary succession → Very slow (may take thousands of years)
• Secondary succession → Comparatively rapid

Quick Comparison

• Primary Succession
  • No soil
  • No previous life
  • Very slow
  • Pioneer species migrate from outside
  • Example: Bare rock, lava
• Secondary Succession
  • Soil present
  • Previous life existed
  • Faster
  • Seeds and propagules remain
  • Example: Burned forest

Exam-Focused Key Points

• Succession is orderly and directional
• Sere = Complete sequence of communities
• Seral stage = Transitional community
• Climax = Stable, self-perpetuating community
• Species diversity and biomass increase during succession
• All successions tend toward a mesic climax community

Nutrient Cycling

Importance of Nutrients

• Organisms require nutrients to:
  • Grow
  • Reproduce
  • Regulate physiological functions

Standing State

• Definition
  • Amount of nutrients (C, N, P, Ca, etc.) present in soil at a given time
• Important Points
  • Varies seasonally
  • Differs across ecosystems
  • Represents immediately available nutrients

What is Nutrient Cycling?

• Definition
  • Movement of nutrient elements through the biotic and abiotic components of an ecosystem.
• Also Called
  • Biogeochemical cycles
    • Bio → Living organisms
    • Geo → Rocks, air, water
• Key Concept
  • Nutrients are not permanently lost.
  • They are continuously recycled within ecosystems.

Pools of Nutrients

1. Reservoir Pool

• Large storage of nutrients
• Transfer to cycling pool is slow
• Acts as long-term reserve
• Examples
  • Atmospheric nitrogen
  • Rock phosphates
• Function
  • Maintains balance when input and output rates differ.

2. Cycling Pool

• Nutrients actively exchanged
• Rapid movement between organisms and environment

Types of Nutrient Cycles

1. Gaseous Cycles
   • Main reservoir → Atmosphere
   • Examples
     • Carbon cycle
     • Nitrogen cycle
2. Sedimentary Cycles
   • Main reservoir → Earth’s crust
   • Examples
     • Phosphorus cycle
     • Sulphur cycle

Factors Affecting Nutrient Release

• Rate of nutrient cycling depends on:
  • Soil characteristics
  • Moisture
  • pH
  • Temperature
• These factors regulate movement of nutrients from reservoir pool to active cycling pool.

Nutrient cycling maintains ecosystem productivity and long-term sustainability.

Carbon Cycle

• Importance of Carbon
  • Constitutes approximately 49% of the dry weight of organisms
  • Second most abundant element in living organisms after water

Carbon Reservoirs

• Major Reservoirs
  • Oceans → ~71% (mainly as dissolved carbon)
  • Atmosphere → ~1% (as CO₂)
  • Fossil fuels → Significant long-term storage
  • Sedimentary rocks

• Role of Oceans
  • Oceans act as a major regulator of atmospheric CO₂ levels by absorbing and releasing carbon.

Carbon Movement

• Carbon circulates continuously through:
  • Atmosphere
  • Oceans
  • Living organisms
  • Dead organic matter

• Annual Carbon Fixation
  • Approximately 4 × 10¹³ kg of carbon is fixed annually through photosynthesis.

Carbon Inputs and Outputs

• Carbon Fixation
  • Photosynthesis converts atmospheric CO₂ into organic compounds.
• Carbon Return to Atmosphere
  • Respiration by producers and consumers
  • Decomposition of organic matter
  • Combustion of fossil fuels and biomass
  • Volcanic activity
• Sedimentation
  • A portion of carbon becomes locked in sediments and fossil deposits, removing it from active circulation for long periods.

Human Impact on Carbon Cycle

• Major Human Activities
  • Deforestation
  • Burning of fossil fuels
  • Forest fires
• Effects
  • Increased atmospheric CO₂ concentration
  • Enhanced greenhouse effect
  • Contribution to global climate change

The carbon cycle maintains balance between atmospheric CO₂ and biological systems, but human activities are altering this equilibrium.

Phosphorus Cycle

Importance of Phosphorus

• Phosphorus is a key component of:
  • Nucleic acids (DNA, RNA)
  • Biological membranes (phospholipids)
  • ATP (energy transfer molecule)
  • Bones and teeth

Phosphorus Reservoir

• Main Reservoir
  • Rocks (as phosphate minerals).
• Process
  • Weathering of rocks → Phosphate released into soil → Absorbed by plants.

Movement Through Food Chain

• Plants → Herbivores → Carnivores
• After death and excretion:
• Decomposers release phosphorus back into soil.

In Aquatic Systems

• Phosphate absorbed by seaweeds and phytoplankton
• Passed to fish and seabirds
• Guano (bird droppings) may return phosphorus to land

Special Characteristics of Phosphorus Cycle

• Key Differences from Carbon Cycle
  • No significant gaseous phase
  • No respiratory release into atmosphere
  • Atmospheric input is negligible
  • Faster loss to sediments than return to land

• Additional Note
  • Phosphate may combine with metals such as Al, Ca, and Fe → Form insoluble salts → Become less available to plants.

Quick Comparison

• Carbon Cycle
  • Gaseous cycle
  • Major atmospheric exchange
  • Large ocean reservoir
  • Strongly influenced by human activities
• Phosphorus Cycle
  • Sedimentary cycle
  • No atmospheric phase
  • Rock reservoir
  • Slower recycling

Ecosystem Services

• Definition
  • Benefits provided to humans by functioning ecosystems.
  • They are outcomes of ecosystem processes.

Major Ecosystem Services

1. Environmental Services
   • Air purification
   • Water purification
   • Flood and drought control
   • Climate regulation
   • Carbon storage
2. Biological Services
   • Nutrient cycling
   • Soil formation
   • Pollination
   • Biodiversity maintenance
   • Wildlife habitat
3. Cultural Services
   • Recreation
   • Aesthetic value
   • Spiritual significance

Economic Value of Ecosystem Services

• Estimated global value ≈ 33 trillion US dollars per year.
• Estimated by Robert Costanza and colleagues.
• This value is nearly double the global GNP (~18 trillion US dollars).

Approximate Cost Distribution

• Soil formation → ~50%
• Recreation → <10%
• Nutrient cycling → <10%
• Climate regulation → ~6%
• Wildlife habitat → ~6%

Key Points

• Nutrients cycle between reservoir and cycling pools
• Standing state = nutrients present at a given time
• Carbon cycle is gaseous; phosphorus cycle is sedimentary
• Phosphorus lacks significant atmospheric exchange
• Ecosystem services are free but economically invaluable
• Soil formation contributes the highest economic value

Chapter Summary

ECOSYSTEM – BASIC CONCEPT

• An ecosystem is a self-sustaining and functional unit of nature in which living organisms interact with one another and with their physical environment.

Components of an Ecosystem

1. Abiotic Components
   • Inorganic substances – air, water, soil, minerals
   • Physical factors – temperature, light, wind
2. Biotic Components
   • Producers (autotrophs)
   • Consumers (herbivores, carnivores, omnivores)
   • Decomposers (bacteria, fungi)

• The interaction between abiotic and biotic components determines the structure and functioning of the ecosystem.

STRUCTURE OF AN ECOSYSTEM

• Each ecosystem has a characteristic physical structure formed due to continuous interactions.
• Main Structural Features
  • Species Composition – types and number of organisms present
  • Stratification – vertical layering of organisms (e.g., trees, shrubs, herbs)
• Every organism occupies a specific trophic position based on its source of nutrition.

FUNCTIONS OF AN ECOSYSTEM

• Four major functional aspects:
  1. Productivity
  2. Decomposition
  3. Energy Flow
  4. Nutrient Cycling

PRODUCTIVITY

• Primary Productivity
  • Rate at which producers capture solar energy and convert it into biomass.
• Types:
  1. Gross Primary Productivity (GPP)
     • Total organic matter produced during photosynthesis.
  2. Net Primary Productivity (NPP)
     • Biomass remaining after subtracting respiration losses.
     • NPP = GPP − Respiration

• Secondary Productivity
  • Rate at which consumers assimilate food energy into new biomass.

DECOMPOSITION

• Decomposers break down complex organic matter into simple inorganic substances such as:
  • Carbon dioxide
  • Water
  • Mineral nutrients
• Major Steps of Decomposition
  • Fragmentation (by detritivores)
  • Leaching (soluble nutrients washed downward)
  • Catabolism (enzymatic breakdown by microbes)
  • Humification and mineralisation
• Decomposition ensures nutrient recycling and maintains soil fertility.

ENERGY FLOW

• Energy flow is unidirectional:
• Sun → Producers → Consumers → Decomposers

• Key Principles
  • Energy decreases at each trophic level
  • Large portion is lost as heat
  • Food chains represent feeding relationships
  • Food webs show interconnected food chains
• Energy cannot be recycled; it must be continuously supplied.

NUTRIENT CYCLING

• Nutrients move repeatedly between biotic and abiotic components.
• Two Major Types
  1. Gaseous Cycles
     • Reservoir → Atmosphere or hydrosphere
     • Example: Carbon cycle
  2. Sedimentary Cycles
     • Reservoir → Earth’s crust
     • Example: Phosphorus cycle
• Reservoirs regulate nutrient balance in ecosystems.

ECOSYSTEM SERVICES

• Ecosystem services are benefits obtained from ecosystem processes.
• Examples:
  • Air and water purification
  • Nutrient cycling
  • Soil formation
  • Climate regulation
  • Biodiversity maintenance
• These services support ecological stability and human survival.

ECOLOGICAL SUCCESSION

Biotic communities are not static; they change gradually over time.

• Definition
  • Orderly and predictable changes in species composition of an area over time.
• Process
  • Begins with pioneer species in a lifeless or disturbed area
  • Series of intermediate stages (seral stages)
  • Ends in a stable climax community

Climax Community

• Stable and self-perpetuating
• Remains unchanged unless disturbed

• Primary succession is slow (soil formation required).
• Secondary succession is faster (soil already present).

FINAL POINTS

• Ecosystem = biotic + abiotic interaction system
• Structure → species composition + stratification
• Functions → productivity, decomposition, energy flow, nutrient cycling
• Energy flow is unidirectional
• Nutrients are recycled
• Succession leads to climax community
• Ecosystem services sustain human life