Ecosystem — NCERT Solutions

Conclusion: A food web is a more accurate and realistic depiction of energy flow in an ecosystem compared to a simple food chain.

In summary: All litter is detritus, but not all detritus is litter. Detritus is the broader category.

Definition: An ecosystem is a structural and functional unit of nature comprising living organisms (biotic components) interacting with each other and with their non-living (abiotic) environment.

Components of an Ecosystem

An ecosystem has two main components:

A. Abiotic Components (Non-living)

These are the physical and chemical factors of the environment:

1. Climatic factors: Temperature, light, humidity, rainfall, wind.
2. Edaphic factors: Soil — its texture, mineral content, pH, water-holding capacity.
3. Inorganic substances: $CO_2$, $O_2$, $N_2$, water, minerals (phosphorus, sulphur, etc.).
4. Organic substances: Proteins, carbohydrates, lipids, humus — these link biotic and abiotic components.

B. Biotic Components (Living)

These are all the living organisms in the ecosystem, classified based on their nutritional relationships:

1. Producers (Autotrophs)

• Green plants, algae, phytoplankton, and chemosynthetic bacteria.
• They fix solar energy through photosynthesis and produce organic matter from inorganic substances.
• They form the first trophic level and are the base of all food chains.
• Example: Grasses in a grassland, phytoplankton in a lake.

2. Consumers (Heterotrophs / Phagotrophs)

Organisms that obtain energy by consuming other organisms:

• Primary consumers (Herbivores): Feed directly on producers. Example: Grasshopper, deer, rabbit.
• Secondary consumers (Primary carnivores): Feed on herbivores. Example: Frog, small fish.
• Tertiary consumers (Secondary carnivores): Feed on primary carnivores. Example: Snake, large fish.
• Omnivores: Feed on both plants and animals. Example: Humans, cockroach.

3. Decomposers (Saprotrophs / Reducers)

• Bacteria and fungi that break down dead organic matter (detritus) into simple inorganic substances.
• They release nutrients back into the soil/water, making them available to producers again.
• They play a crucial role in nutrient cycling and are sometimes called reducers.
• Example: Bacillus, Aspergillus, mushrooms.

Interaction between Components

The abiotic and biotic components interact continuously:

• Producers absorb inorganic nutrients and solar energy → convert to organic matter.
• Consumers obtain energy by eating producers/other consumers.
• Decomposers break down dead matter → release inorganic nutrients back to the environment.

This interaction results in the four key processes of an ecosystem: productivity, decomposition, energy flow, and nutrient cycling.

Ecological Pyramids

Definition: An ecological pyramid is a graphical representation of the relationship between different trophic levels in an ecosystem in terms of number, biomass, or energy. The base of the pyramid represents the producers (first trophic level) and the successive tiers represent higher trophic levels.

Ecological pyramids were first described by Charles Elton (1927) and are also called Eltonian pyramids.

Types of Ecological Pyramids:

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

1. Pyramid of Numbers

Definition: It represents the number of individual organisms at each trophic level in an ecosystem.

(a) Upright Pyramid of Numbers — Grassland Ecosystem

In a grassland ecosystem:
$$\text{Grasses (millions)} \rightarrow \text{Grasshoppers (thousands)} \rightarrow \text{Frogs (hundreds)} \rightarrow \text{Snakes (tens)} \rightarrow \text{Hawks (few)}$$

• The number of producers (grasses) is maximum.
• The number decreases at each successive trophic level.
• The pyramid is upright (broad base, narrow apex).

(b) Inverted Pyramid of Numbers — Tree Ecosystem

In a forest ecosystem dominated by trees:
$$\text{Trees (1)} \rightarrow \text{Herbivorous insects (thousands)} \rightarrow \text{Birds (hundreds)} \rightarrow \text{Hawks (few)}$$

• A single large tree supports thousands of insects.
• The pyramid is inverted (narrow base, broad middle).

(c) Spindle-shaped Pyramid — Parasitic Food Chain

In a parasitic food chain:
$$\text{Tree (1)} \rightarrow \text{Birds (few)} \rightarrow \text{Lice/Mites (many)}$$

• The number of parasites exceeds the number of hosts.
• The pyramid has an irregular/spindle shape.

Limitation: The pyramid of numbers does not account for the size of organisms, so it can be misleading.

2. Pyramid of Biomass

Definition: It represents the total dry weight (biomass) of organisms at each trophic level at a given time. Biomass is expressed in $g\,m^{-2}$ or $kg\,ha^{-1}$.

(a) Upright Pyramid of Biomass — Terrestrial Ecosystem

In a grassland/forest ecosystem:

• Producers (grasses/trees) have the maximum biomass.
• Biomass decreases at each successive trophic level.
• The pyramid is upright.

$$\text{Producers} > \text{Herbivores} > \text{Carnivores}$$

Example values (approximate, $g\,m^{-2}$):

(b) Inverted Pyramid of Biomass — Aquatic Ecosystem

In an aquatic (ocean/lake) ecosystem:

• Phytoplankton (producers) have a very small biomass at any given time because they reproduce very rapidly and are consumed quickly.
• Zooplankton (primary consumers) have a larger biomass than phytoplankton at any given time.
• The pyramid is inverted.

$$\text{Phytoplankton (small biomass)} \rightarrow \text{Zooplankton (larger biomass)} \rightarrow \text{Fishes}$$

This is because phytoplankton have a very high turnover rate (rapid reproduction and consumption), so their standing crop biomass is low even though their productivity is high.

Limitations of Ecological Pyramids

1. They do not account for the same species belonging to two or more trophic levels.
2. They assume a simple food chain and cannot accommodate a food web.
3. Saprophytes (decomposers) are not given any place despite their vital role.
4. The pyramid of biomass does not consider the rate of production (productivity).

Primary Productivity

Definition: Primary productivity is defined as the rate of capture of solar energy (or the amount of organic matter/biomass produced per unit area per unit time) by the producers (autotrophs) through photosynthesis.

It is expressed in terms of weight ($g\,m^{-2}\,yr^{-1}$) or energy ($kcal\,m^{-2}\,yr^{-1}$).

Types of Primary Productivity

1. Gross Primary Productivity (GPP):

• The total rate of photosynthesis including the organic matter used up in respiration during the measurement period.
• It is the total amount of organic matter produced by producers.

$$GPP = \text{Total photosynthesis}$$

2. Net Primary Productivity (NPP):

• The organic matter that remains after the producers have used some for their own respiration ($R$).
• It is the biomass available for consumption by heterotrophs (herbivores and decomposers).

$$NPP = GPP - R$$
where $R$ = respiratory losses.

Note: The annual net primary productivity of the whole biosphere is approximately 170 × 10⁹ tonnes (dry weight) of organic matter.

Factors Affecting Primary Productivity

1. Light (Solar Radiation / PAR)

• Only about 50% of incident solar radiation is Photosynthetically Active Radiation (PAR).
• Of PAR, plants use only 2–10% for net primary production.
• Light intensity and duration (photoperiod) directly affect the rate of photosynthesis.
• In aquatic ecosystems, light penetration decreases with depth, limiting productivity to the photic zone.

2. Temperature

• Photosynthesis is an enzyme-mediated process; it increases with temperature up to an optimum (usually 25–35°C) and then decreases.
• Tropical regions with high temperatures and sunlight generally have higher productivity.

3. Water Availability

• Water is a raw material for photosynthesis.
• In terrestrial ecosystems, water availability (rainfall) is a major limiting factor.
• Deserts have very low productivity due to water scarcity.

4. Availability of Nutrients (Minerals)

• Macronutrients (N, P, K, S) and micronutrients are essential for plant growth.
• In aquatic ecosystems, nutrients (especially nitrogen and phosphorus) are often the limiting factors.
• Nutrient-rich (eutrophic) water bodies have higher productivity.

5. Carbon Dioxide Concentration

• $CO_2$ is the raw material for photosynthesis.
• Increased $CO_2$ concentration (up to a point) enhances the rate of photosynthesis and productivity.

6. Species Composition and Adaptations

• $C_4$ plants (e.g., sugarcane, maize) are more productive than $C_3$ plants under high light and temperature conditions.
• The type and diversity of plant species influence overall ecosystem productivity.

Conclusion: Primary productivity varies greatly across ecosystems — tropical rainforests and estuaries have the highest productivity, while deserts and deep oceans have the lowest. The interplay of light, temperature, water, and nutrients determines the productivity of any given ecosystem.

Decomposition

Definition: Decomposition is the process by which decomposers (bacteria and fungi) break down complex organic matter (detritus) into simpler inorganic substances such as carbon dioxide ($CO_2$), water ($H_2O$), and inorganic nutrients. It is essentially a catabolic process.

Substrate for decomposition: Dead plant remains (leaves, bark, flowers), dead animal remains, and fecal matter — collectively called detritus.

Steps/Processes of Decomposition

1. Fragmentation

• Detritivores (earthworms, millipedes, woodlice, certain insects) break down detritus into smaller particles, increasing the surface area available for microbial action.
• This is a physical process.

2. Leaching

• Water-soluble inorganic nutrients from the detritus percolate (leach) down into the soil and are deposited as unavailable salts in the soil horizon.
• This is a physical/chemical process.

3. Catabolism

• Bacterial and fungal enzymes degrade detritus biochemically into simpler organic and inorganic substances.
• This is the core biochemical step of decomposition.

4. Humification

• Leads to the accumulation of a dark-coloured amorphous substance called humus.
• Humus is highly resistant to microbial action and decomposes very slowly.
• It serves as a reservoir of nutrients and improves soil water-holding capacity.
• Humus is further degraded by microbes.

5. Mineralisation

• The humus is further degraded by microbes to release inorganic nutrients (minerals) into the soil.
• This process of release of inorganic nutrients from organic matter is called mineralisation.
• Products: $CO_2$, $H_2O$, and inorganic salts (nitrates, phosphates, sulphates, etc.).

Products of Decomposition

Factors Affecting Decomposition

1. Temperature: Warm, moist conditions favour decomposition. Decomposition is slow in cold or dry conditions.
2. Moisture/Water: Adequate moisture is essential for microbial activity.
3. Oxygen: Aerobic decomposition is faster than anaerobic decomposition.
4. Chemical nature of detritus:

• Detritus rich in lignin and chitin decomposes slowly.
• Detritus rich in nitrogen and sugars decomposes quickly.

Conclusion: Decomposition is a vital ecosystem process that recycles nutrients, maintains soil fertility, and ensures the continuous availability of inorganic nutrients to producers, thereby sustaining the nutrient cycles of the ecosystem.

Energy Flow in an Ecosystem

Introduction

Energy flow is one of the most fundamental processes in an ecosystem. The sun is the ultimate source of energy for all ecosystems (except chemosynthetic ecosystems). Energy flows through the ecosystem in a unidirectional manner — from producers to consumers — and is non-cyclic (unlike nutrients).

Entry of Energy into the Ecosystem

• Of the total incident solar radiation, only about 50% is Photosynthetically Active Radiation (PAR).
• Of PAR, plants capture only 2–10% for net primary production through photosynthesis.
• This fixed energy enters the biotic component of the ecosystem.

$$\text{Solar Energy} \xrightarrow{\text{Photosynthesis}} \text{Chemical Energy (Organic Matter)}$$

Trophic Levels and Energy Transfer

Organisms are grouped into trophic levels based on their source of nutrition:

Energy is transferred from one trophic level to the next through feeding (predation).

The 10% Law (Lindeman's Law)

Proposed by R.L. Lindeman (1942), the 10% law states that:

> Only about 10% of the energy of any trophic level is transferred to the next trophic level.

The remaining 90% is lost as:

• Heat during metabolic activities (respiration)
• Incomplete digestion (fecal matter)
• Energy used for maintenance and movement

Example:
$$\text{Producers: } 1000\,kcal \rightarrow \text{Herbivores: } 100\,kcal \rightarrow \text{Carnivores: } 10\,kcal \rightarrow \text{Top carnivores: } 1\,kcal$$

This is why food chains are generally limited to 4–5 trophic levels — very little energy remains at higher levels.

Two Pathways of Energy Flow

1. Grazing Food Chain (GFC):
$$\text{Plants} \rightarrow \text{Herbivores} \rightarrow \text{Carnivores}$$

• Begins with living green plants.
• Dominant in most terrestrial ecosystems during the day.

2. Detritus Food Chain (DFC):
$$\text{Dead organic matter (Detritus)} \rightarrow \text{Detritivores} \rightarrow \text{Their predators}$$

• Begins with dead organic matter.
• Dominant in forest floor and aquatic ecosystems.
• In most ecosystems, more energy flows through the DFC than the GFC.

Both food chains are interconnected and together form the food web.

Key Features of Energy Flow

1. Unidirectional: Energy flows only from producers → consumers; it cannot flow back.
2. Non-cyclic: Unlike nutrients, energy is not recycled. Once used, it is lost as heat.
3. Decreasing availability: Energy decreases at each successive trophic level (10% law).
4. Two laws of thermodynamics apply:

• First Law: Energy is neither created nor destroyed; it is converted from one form to another (solar → chemical → heat).
• Second Law: At every energy transformation, some energy is lost as heat (entropy increases).

Conclusion

Energy flow in an ecosystem is a one-way process driven by solar energy. The progressive decrease in available energy at higher trophic levels (due to the 10% law) limits the length of food chains and determines the structure of ecosystems. Decomposers play a crucial role by releasing energy locked in dead organic matter and recycling nutrients back to the producers.