Ecosystem

ECOSYSTEM – STRUCTURE AND FUNCTION

**Definition**: An ecosystem is a functional unit of nature comprising living organisms (biotic components) that interact with each other and with the surrounding physical environment (abiotic components).

**Key Structural Features**:

**Species composition**: Identification and enumeration of all plant and animal species present in the ecosystem

**Stratification**: Vertical distribution of different species at different levels

Example in forest: Trees occupy top layer (canopy), shrubs occupy middle layer, herbs and grasses occupy bottom layer (ground vegetation)

This creates distinct microhabitats with different light, temperature, and moisture conditions

**Classification of Ecosystems**:

**Terrestrial**: Forest, grassland, desert

**Aquatic**: Pond, lake, wetland, river, estuary

**Man-made**: Crop fields, aquarium

**Ecosystem Example – Pond as a Self-Sustainable System**:

A pond serves as an excellent model for understanding ecosystem function because it exhibits all four fundamental components:

**Abiotic component**: Water with dissolved nutrients, rich soil deposit at bottom, solar radiation, temperature cycles, day-length

**Biotic component**: Producers (phytoplankton, algae, floating/submerged/marginal plants), Consumers (zooplankton, free-swimming and bottom-dwelling animals), Decomposers (fungi, bacteria, flagellates abundant at bottom)

**Four Fundamental Functions of Any Ecosystem**:

1. **Productivity**: Conversion of inorganic into organic material using solar energy by autotrophs

2. **Consumption**: Heterotrophs feed on autotrophs and other organisms

3. **Decomposition**: Dead matter is broken down and minerals released for reuse by autotrophs

4. **Energy flow**: Unidirectional movement of energy toward higher trophic levels with dissipation as heat

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PRODUCTIVITY

**Definition**: Productivity is the rate of biomass or organic matter production per unit area over a time period by plants during photosynthesis. Expressed as gm⁻² yr⁻¹ or kcal m⁻² yr⁻¹.

**Primary Productivity**: Amount of biomass produced by plants (autotrophs) in photosynthesis.

**Two Types of Primary Productivity**:

**1. Gross Primary Productivity (GPP)**:

Total rate of photosynthetic production of organic matter by plants

Represents total energy captured from sunlight

All organic matter synthesized including that used in plant respiration

Formula: GPP = Total photosynthetically produced organic matter

**2. Net Primary Productivity (NPP)**:

Remaining biomass available for consumption by heterotrophs after accounting for plant respiration losses

Formula: **GPP – R = NPP** (where R = respiration losses of producers)

This is the actual energy available to herbivores and decomposers

NPP represents stored energy in plant tissues (leaves, stems, fruits, roots)

**Comparison**:

GPP is always greater than NPP because plants respire and use some energy for their own maintenance and growth

NPP is the net energy that fuels the entire food chain and decomposer community

**Secondary Productivity**:

Rate of formation of new organic matter by consumers (heterotrophs)

Energy assimilated from food consumed by animals at various trophic levels

Always less than NPP because energy is lost as heat during animal respiration and metabolic processes

**Factors Affecting Primary Productivity**:

Plant species composition in the ecosystem

Availability of essential nutrients (nitrogen, phosphorus, potassium)

Light intensity and duration

Temperature and moisture availability

CO₂ concentration in atmosphere

Photosynthetic capacity and efficiency of plants present

**Global Productivity Data**:

Annual net primary productivity of entire biosphere ≈ 170 billion tons (dry weight) of organic matter

Oceans cover ~70% of Earth's surface but contribute only 55 billion tons (low productivity due to nutrient limitation in surface waters and high light penetration requiring deep photosynthesis)

Terrestrial ecosystems contribute ~115 billion tons despite covering only 30% of surface (higher productivity due to nutrient availability and efficient vegetation)

**Exam Important Points**:

NPP, not GPP, represents energy available to food chains

Productivity varies significantly among ecosystems (tropical rainforests highest, deserts and deep oceans lowest)

Measurement units must include both mass and time for comparison

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DECOMPOSITION

**Definition**: Decomposition is the process by which decomposers break down complex organic matter into inorganic substances like CO₂, H₂O, and nutrients that return to the ecosystem for reuse.

**Raw Material for Decomposition – Detritus**:

Dead plant remains: leaves, bark, flowers, twigs, roots, woody debris

Dead animal remains: carcasses, bones, hair, feathers

Animal fecal matter and other wastes

All non-living organic matter in the ecosystem

**Five Important Steps in Decomposition Process** (occur simultaneously on detritus):

**1. Fragmentation**:

Physical breakdown of detritus into smaller particles

Carried out by detritivores (e.g., earthworms, millipedes, isopods, dung beetles, termites)

Increases surface area available for microbial action

No chemical change; purely mechanical breakdown

**2. Leaching**:

Water-soluble inorganic nutrients dissolve and move downward through soil layers

Water carries these soluble ions (K⁺, Ca²⁺, Mg²⁺, NO₃⁻, PO₄³⁻) into lower soil horizons

These nutrients precipitate as unavailable salts in lower soil layers

Also called elution when water moves nutrients out of the system entirely

**3. Catabolism**:

Bacterial and fungal enzymes chemically breakdown detritus into simpler inorganic substances

Complex organic compounds (cellulose, proteins, fats) → CO₂, H₂O, inorganic ions, simpler organic compounds

Primarily aerobic respiration by heterotrophic microorganisms

Major process releasing energy used by decomposer organisms

**4. Humification**:

Accumulation of dark-colored, amorphous substance called humus

Humus is highly resistant to further microbial decomposition

Decomposes extremely slowly (takes years to decades)

Colloidal in nature; serves as reservoir of nutrients

Improves soil water-holding capacity and soil structure

Supports plant growth and beneficial soil microorganisms

**5. Mineralisation**:

Further degradation of humus by specialized microbes

Release of inorganic nutrients (NH₄⁺, NO₃⁻, PO₄³⁻, K⁺, etc.) from humus back into soil

Makes nutrients available again for plant uptake

Completes the nutrient cycle

**Factors Controlling Decomposition Rate**:

**Chemical Composition of Detritus**:

**Rich in nitrogen and water-soluble substances** (sugars, proteins) → **Faster decomposition** (e.g., grass, legume leaves)

**Rich in lignin and chitin** (complex polymers) → **Slower decomposition** (e.g., woody materials, insect exoskeletons)

C:N ratio important; high C:N ratio slows decomposition

**Climatic Factors**:

**Temperature**: Warm environment (20-40°C) favors decomposition by increasing enzyme activity; low temperature inhibits decomposition (arctic/mountain ecosystems accumulate organic matter)

**Soil moisture**: Moist soil favors decomposition by supporting microbial growth; anaerobic conditions inhibit decomposition (waterlogged soils, peat bogs accumulate organic matter)

**Oxygen availability**: Decomposition is largely aerobic process; anaerobic conditions dramatically reduce decomposition rate

**Environmental Conditions That Inhibit Decomposition**:

Low temperature (polar regions, winters)

Anaerobiosis (waterlogged/flooded soils, peat bogs, swamps)

Low moisture (deserts, arid regions)

These conditions lead to accumulation of organic matter (coal, peat, fossil fuels formed under anaerobic conditions)

**Role of Decomposers**:

Bacteria (diverse metabolic capabilities)

Fungi (especially important in acidic soils; produce cellulase and lignin-degrading enzymes)

Actinomycetes (important in breaking down resistant compounds)

Protozoa and other microscopic organisms

Earthworms and arthropods (detritivores, not true decomposers but essential for fragmentation)

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ENERGY FLOW

**Fundamental Principle**: Energy flows through ecosystems in a **unidirectional manner** from sun → producers → consumers → decomposers. Energy cannot be recycled; it is dissipated as heat at each step.

**Source of Energy**:

**Sun** is the only energy source for all ecosystems except deep-sea hydrothermal vent ecosystems

Less than 50% of incident solar radiation is photosynthetically active radiation (PAR)

Plants capture only 2-10% of PAR available to them

This small amount of captured energy sustains entire biosphere

Energy input must be continuous to maintain ecosystem function (Second Law of Thermodynamics)

**Producers (First Trophic Level)**:

**Definition**: Autotrophic organisms that capture solar energy and convert it to chemical energy through photosynthesis

**Terrestrial**: Herbaceous plants, woody plants (trees, shrubs)

**Aquatic**: Phytoplankton, algae, higher aquatic plants

Producers form base of all food chains and pyramids

**Consumers (Higher Trophic Levels)**:

**Definition**: Heterotrophic organisms that obtain energy by consuming other organisms

Dependent on producers (directly or indirectly) for all energy and nutrient requirements

**Classification of Consumers**:

**1. Primary Consumers (Herbivores)** – Second Trophic Level:

Feed directly on producers (plants)

Examples: Insects, herbivorous birds, herbivorous mammals, molluscs (aquatic)

Examples: Grasshoppers, deer, rabbits, cows, zooplankton in aquatic ecosystems

Energy input: From plant biomass

**2. Secondary Consumers (Primary Carnivores)** – Third Trophic Level:

Feed on primary consumers (herbivores)

Are carnivores or insectivores

Examples: Sparrows eating seeds are primary consumers; same sparrows eating insects are secondary consumers

Show higher metabolic activity than herbivores

**3. Tertiary Consumers (Secondary Carnivores)** – Fourth Trophic Level:

Feed on secondary consumers

Top predators in grazing food chains

Examples: Hawks, eagles, large carnivores

Apex predators

**Food Chains and Food Webs**:

**Grazing Food Chain (GFC)**:

Begins with living producers (plants)

Energy flows: Producers → Herbivores → Primary Carnivores → Secondary Carnivores

Simple example: Grass → Goat → Man

Another example: Plants → Insects → Birds → Hawks

In aquatic ecosystems, GFC is major conduit for energy flow

Relatively simple and linear (though this is an oversimplification)

**Detritus Food Chain (DFC)**:

Begins with dead organic matter (detritus and waste products)

Energy flows: Detritus → Decomposers (Saprophytes) → Organisms feeding on decomposers

Decomposers: Bacteria and fungi primarily; also protozoa

**Saprotrophs** (sapro: to decompose): Heterotrophic organisms meeting energy needs by degrading dead organic matter

In terrestrial ecosystems, much larger fraction of energy flows through DFC than GFC

In aquatic ecosystems, relatively smaller proportion through DFC

**Interconnection of Food Chains**:

Some DFC organisms are prey to GFC animals (example: earthworms eaten by birds)

Omnivores consume both plants and animals (examples: cockroaches, crows, humans, bears, pigs)

These interconnections create **Food Web**: Complex network of interconnected food chains showing multiple feeding relationships

Food web is more realistic representation of energy flow than simple food chain

**Trophic Levels**:

**Definition**: Position of organism in food chain based on source of nutrition or feeding relationship

**Feeding relationship determines trophic level**, not species

**First trophic level**: Producers (plants, photosynthetic bacteria)

**Second trophic level**: Herbivores (primary consumers)

**Third trophic level**: Carnivores feeding on herbivores (secondary consumers)

**Fourth trophic level**: Carnivores feeding on carnivores (tertiary consumers)

**Important Concept – Organisms Occupying Multiple Trophic Levels**:

A single species can occupy more than one trophic level simultaneously

**Example – Sparrow**: Primary consumer when eating seeds/fruits/peas; Secondary consumer when eating insects/worms

**Example – Human**: Primary consumer when eating plants; Secondary/Tertiary consumer when eating meat

Trophic level represents **functional role**, not taxonomic classification

**Energy Loss at Each Trophic Level – 10% Law**:

Only **10% of energy is transferred** from one trophic level to next

90% is lost as heat through respiration, metabolic processes, movement, maintenance

Energy decreases at successive trophic levels in exponential manner

This explains why there are limited number of trophic levels in grazing food chains (typically 3-4)

**Why Limited Trophic Levels**:

Energy decreases from Producers (100%) → Herbivores (10%) → Primary Carnivores (1%) → Secondary Carnivores (0.1%)

When only 0.1% energy remains, insufficient to support viable population at next level

Detritus food chain may have more levels because decomposers utilize dead matter more efficiently

**Standing Crop**:

**Definition**: Total mass of living organisms (biomass) or total number of organisms present in ecosystem at particular time

Measured per unit area

**Biomass** expressed as fresh weight or dry weight (dry weight more accurate because water content variable)

Varies by trophic level: decreases in higher trophic levels

**Exam Important Points**:

Energy flow is unidirectional; cannot be recycled

Producers always have highest energy; energy decreases upward

10% law explains pyramid structure and limited trophic levels

Food web more realistic than food chain

One organism = multiple trophic levels possible

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ECOLOGICAL PYRAMIDS

**Definition**: Graphical representation of energy relationships, biomass, or numbers of organisms at different trophic levels in an ecosystem. Called pyramid because base (producers) is broad and progressively narrows toward apex (top consumers).

**Three Types of Ecological Pyramids**:

**1. Pyramid of Numbers**

**Definition**: Graphical representation showing number of individual organisms at each trophic level per unit area.

**Characteristics**:

X-axis: Number of organisms

Y-axis: Trophic levels

Each bar width represents number of individuals

Counted organisms at each trophic level must include ALL organisms of that level

**Usually Upright Shape**:

Producers > Herbivores > Primary Carnivores > Secondary Carnivores

Example: Grassland ecosystem

Grass: 6 million plants (producers)

Herbivores (grasshoppers, deer): 60,000 individuals

Primary Carnivores (birds, snakes): 600 individuals

Secondary Carnivores (hawks, eagles): 3-5 individuals

**Inverted Pyramid Example**:

Tree ecosystem (one large tree supporting thousands of insects)

Many insects on one tree

Few birds eating insects

Even fewer larger birds eating smaller birds

Creates inverted pyramid (exception to general rule)

**Limitation**: Does not account for size of organisms; a large elephant counts same as small ant

**2. Pyramid of Biomass**

**Definition**: Graphical representation of total dry biomass of all organisms at each trophic level per unit area.

**Characteristics**:

X-axis: Total biomass (gm⁻² or kg m⁻²)

Y-axis: Trophic levels

More accurate than numbers because considers total mass regardless of organism size

Dry weight measurement preferred over fresh weight (water content variable)

**Usually Upright Shape**:

Producers have largest biomass

Decreases at each higher trophic level

Example: Terrestrial ecosystem

Grass biomass: 10,000 kg/hectare

Herbivores: 1,000 kg/hectare

Primary carnivores: 100 kg/hectare

Secondary carnivores: 10 kg/hectare

**Inverted Pyramid in Aquatic Ecosystems** – Important Exception:

Sea ecosystem shows inverted biomass pyramid

Phytoplankton have small standing crop (fast reproduction, rapid turnover)

Fish biomass exceeds phytoplankton biomass (slower reproduction, longer lifespan)

Explanation: High productivity and rapid reproduction of phytoplankton allows them to support larger fish biomass despite lower standing crop at any given time

This is **paradox** explained by high turnover rate of phytoplankton

**3. Pyramid of Energy** – Always Upright

**Definition**: Graphical representation of energy available at each trophic level per unit area per unit time.

**Characteristics**:

X-axis: Energy (kcal m⁻² yr⁻¹)

Y-axis: Trophic levels

Each bar represents energy present at that level annually

Most accurate representation of ecosystem function

Primary producers: ~1% of incident solar radiation is converted to NPP

**Always Upright Shape** – Never Inverted:

Producers: Maximum energy (~1000 kcal m⁻² yr⁻¹)

Herbivores: 10% of producer energy (~100 kcal m⁻² yr⁻¹)

Primary carnivores: 10% of herbivore energy (~10 kcal m⁻² yr⁻¹)

Secondary carnivores: 10% of primary carnivore energy (~1 kcal m⁻² yr⁻¹)

**Why Never Inverted**:

Energy always decreases at successive trophic levels

Energy is lost as heat at each step (respiration, metabolic processes, movement)

This follows Second Law of Thermodynamics

Cannot accumulate energy; continuous loss as entropy increases

**Comparison of Three Pyramids**:

| Feature | Numbers | Biomass | Energy |

|---------|---------|---------|--------|

| Usually Shape | Upright (with exceptions) | Upright (inverted in aquatic) | Always Upright |

| Reflects | Individual count | Total mass | Energy flow |

| Most Accurate | No | Better | Yes |

| Can Be Inverted | Yes | Yes | No |

| Ecological Significance | Shows population structure | Shows trophic relationships | Shows energy availability |

**Important Exam Concepts**:

All pyramids must include ALL organisms at each trophic level

Organism may occupy multiple trophic levels (functional, not taxonomic)

Pyramid of energy is most reliable; cannot be inverted

Pyramid of biomass can be inverted in aquatic systems due to high turnover

Pyramid of numbers can be inverted (one plant, many insects)

**Limitations of Ecological Pyramids**:

1. **Do not show organism functioning at multiple trophic levels**: Single organism at two or more levels simultaneously not properly represented

2. **Assume simple linear food chain**: Reality is complex food web; many alternative feeding pathways not shown

3. **Saprophytes/Decomposers not shown**: Despite crucial ecological role, decomposers often excluded (though they are part of DFC)

4. **Do not show energy stored in detritus**: Dead organic matter pool not separately shown

5. **Oversimplification**: Complex ecological relationships reduced to simple geometric shape

**Exam Important Points**:

Energy pyramid most reliable indicator of ecosystem function

10% rule applies: only 10% energy transferred between levels

Biomass and number pyramids can have exceptions; energy pyramid cannot

Pyramids represent functional (trophic) relationships, not taxonomic classifications

Each pyramid must include complete census of trophic level organisms

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KEY EXAM FORMULAS AND RELATIONSHIPS

**Energy Relationships**:

**GPP – R = NPP** (Gross Primary Productivity minus Respiration = Net Primary Productivity)

**Energy Transfer = 10% Law** (Only 10% of energy passed to next trophic level)

**Biomass/Energy Pyramid**: Producer 100% → Herbivore 10% → Carnivore 1% → Top Carnivore 0.1%

**Decomposition Stages** (Sequential but Simultaneous):

1. Fragmentation (physical breakdown by detritivores)

2. Leaching (water-soluble nutrient movement)

3. Catabolism (enzymatic breakdown by microbes)

4. Humification (humus formation)

5. Mineralisation (nutrient release from humus)

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SUMMARY OF ECOSYSTEM CONCEPTS

**Structural Features**:

**Species composition**: Types and numbers of species

**Stratification**: Vertical layering of organisms

**Abiotic components**: Water, air, soil, sunlight, temperature

**Biotic components**: Producers, consumers, decomposers

**Functional Components**:

1. **Productivity**: Energy capture and biomass production (GPP, NPP, Secondary productivity)

2. **Decomposition**: Complex organic matter → inorganic substances (5-step process)

3. **Energy Flow**: Unidirectional, follows 10% law, represented by food chains/webs

4. **Nutrient Cycling**: Circular movement of elements (gaseous and sedimentary cycles)

**Trophic Relationships**:

Producers (1st) → Herbivores (2nd) → Carnivores (3rd-4th) → Decomposers

Grazing food chain (energy from living plants)

Detritus food chain (energy from dead organic matter)

Food web (interconnected chains)

**Ecological Pyramids**:

Pyramid of numbers (individual count)

Pyramid of biomass (total mass)

Pyramid of energy (energy availability) – always upright

**Ecosystem Services**:

Air and water purification

Nutrient cycling

Soil formation

Climate regulation

Pollination

Waste decomposition

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COMMON EXAM QUESTION PATTERNS

**Definition Questions**: Define productivity, decomposition, food chain, trophic level, NPP, GPP

**Comparison Questions**: Distinguish between GFC and DFC; GPP and NPP; biomass and energy pyramids

**Calculation Questions**: Calculate energy at each trophic level using 10% rule; determine organism trophic level

**Conceptual Questions**: Why is pyramid of energy always upright? Explain inverted biomass pyramid in aquatic ecosystems. Why are there fewer top predators than herbivores?

**Diagram Questions**: Label food chains, pyramids, decomposition stages, ecosystem components

**Case Study Questions**: Analyze specific ecosystem (pond, forest) identifying all components and energy flow