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Ecosystem Class 12 Biology Notes

Ecosystem-Structure and Function

An ecosystem is a functional unit of nature where living organisms interact with each other and with their physical environment. Ecosystems can be as small as a pond or as vast as a forest or an ocean. The entire Earth's biosphere can be considered a global ecosystem, which is a combination of all local ecosystems.

To study ecosystems more easily, they are divided into two main categories:

Terrestrial ecosystems: Examples include forests, grasslands, and deserts.
Aquatic ecosystems: Examples include ponds, lakes, wetlands, rivers, and estuaries.
Man-made ecosystems: Crop fields and aquariums are also considered ecosystems created by humans.

The structure of an ecosystem is determined by the interaction between its living (biotic) and non-living (abiotic) components. Key structural features include:

Species Composition: The identification and counting of all plant and animal species within an ecosystem.
Stratification: The vertical distribution of different species occupying different levels. For instance, in a forest, trees form the top layer, shrubs the middle layer, and herbs/grasses the bottom layer.

An ecosystem's function is understood by studying four key aspects:

Productivity: The rate of biomass or energy creation.
Decomposition: The breakdown of dead organic matter.
Energy Flow: The movement of energy through different organisms.
Nutrient Cycling: The circulation of nutrients within the ecosystem.

Example

A Pond as an Aquatic Ecosystem

A small pond is a great example of a self-sustaining ecosystem that showcases all its fundamental components and functions.

Abiotic Components: These are the non-living parts, including the water with its dissolved inorganic and organic substances, the soil at the bottom, and climatic factors like solar energy, temperature, and day-length.
Biotic Components:
- Producers (Autotrophs): These organisms create their own food. In a pond, they include phytoplankton (microscopic algae), larger algae, and various plants (floating, submerged, and those on the edges).
- Consumers (Heterotrophs): These organisms feed on others. They are represented by zooplankton (microscopic animals), free-swimming animals, and bottom-dwelling creatures.
- Decomposers: These are mainly fungi, bacteria, and flagellates found in abundance at the bottom of the pond. They break down dead organic material.

Functions performed in the pond:

Energy Conversion: Autotrophs convert inorganic materials into organic matter using sunlight.
Consumption: Heterotrophs consume the autotrophs.
Decomposition: Decomposers break down dead organisms, releasing inorganic nutrients back into the pond for the autotrophs to use again. This cycle repeats continuously, driven by a unidirectional movement of energy from the sun to producers and then to consumers, with energy being lost as heat at each step.

Productivity

For any ecosystem to function, it needs a constant supply of energy, which primarily comes from the sun.

Primary production is the total amount of biomass or organic matter produced by plants through photosynthesis in a specific area over a period. It is measured in terms of weight ( $\text{g m}^{-2}$ ) or energy ( $\text{kcal m}^{-2}$ ).

Productivity is the rate at which biomass is produced. It is expressed in units like $\text{g m}^{-2} \text{yr}^{-1}$ or ( $\text{kcal m}^{-2}$ ) $\text{yr}^{-1}$ , which allows for the comparison of different ecosystems.

Productivity is divided into two types:

Gross Primary Productivity (GPP): This is the total rate at which plants produce organic matter during photosynthesis.
Net Primary Productivity (NPP): Plants use a significant amount of the GPP for their own respiration (R). The remaining biomass is the NPP. It is the biomass that is available for consumption by herbivores and decomposers. The relationship is given by the formula: $GPP - R = NPP$

Secondary productivity is the rate at which consumers (heterotrophs) form new organic matter by consuming plants or other organisms.

Primary productivity is influenced by several factors, including the types of plant species, nutrient availability, and environmental conditions. The annual NPP of the entire biosphere is about 170 billion tons (dry weight) of organic matter. Interestingly, oceans cover 70% of the Earth's surface but only account for 55 billion tons of this productivity, with the rest occurring on land.

Decomposition

Decomposers, like the earthworm, are known as the "farmer's friend" because they break down complex organic matter and help loosen the soil.

Decomposition is the process where decomposers break down complex organic matter into simple inorganic substances like carbon dioxide, water, and nutrients. The raw material for this process is detritus, which includes dead plant parts (leaves, bark, flowers), dead animal remains, and fecal matter.

The key steps in decomposition are:

Fragmentation: Detritivores (like earthworms) chew or break down detritus into smaller particles.
Leaching: Water-soluble inorganic nutrients seep down into the soil, where they may become locked away as unavailable salts.
Catabolism: Bacterial and fungal enzymes digest the detritus, converting it into simpler inorganic substances.

These steps occur simultaneously. During decomposition in the soil, two other important processes happen:

Humification: This leads to the formation of humus, a dark-coloured, amorphous substance. Humus is very resistant to microbial action and decomposes very slowly. It is colloidal and acts as a reservoir of nutrients.
Mineralisation: Some microbes further degrade the humus, releasing inorganic nutrients back into the soil. This makes the nutrients available for plants to use again.

Factors Affecting Decomposition:

Oxygen: Decomposition is largely an oxygen-requiring process.
Chemical Composition: Decomposition is slower for detritus rich in lignin and chitin. It is faster for detritus rich in nitrogen and water-soluble substances like sugars.
Climatic Factors: Warm and moist environments speed up decomposition by promoting the activity of soil microbes. Low temperatures and lack of oxygen (anaerobiosis) slow it down, causing organic materials to build up.

Energy Flow

The sun is the ultimate source of energy for almost all ecosystems on Earth. However, not all sunlight is used.

Photosynthetically Active Radiation (PAR) is the portion of solar radiation that plants can use for photosynthesis, which is less than 50% of the total incident radiation.
Plants capture only 2-10% of the PAR. This small fraction of energy is what sustains the entire living world.

Energy flow in an ecosystem is unidirectional. It flows from the sun to producers, and then from producers to consumers. This movement of energy follows the laws of thermodynamics.

First Law of Thermodynamics: Energy is conserved as it is transferred.
Second Law of Thermodynamics: During energy transfer, some energy is always lost as heat, leading to increased disorder (entropy). Ecosystems need a constant supply of energy to counteract this tendency.

Food Chains and Trophic Levels

Organisms in an ecosystem are linked through feeding relationships, forming food chains and food webs.

Producers: These are autotrophs that produce their own food. In terrestrial ecosystems, they are mainly herbaceous and woody plants. In aquatic ecosystems, they are phytoplankton, algae, and other aquatic plants.
Consumers: These are heterotrophs that depend on producers for food.
- Primary Consumers: Herbivores that feed directly on plants.
- Secondary Consumers: Carnivores that eat primary consumers.
- Tertiary Consumers: Carnivores that eat secondary consumers.

There are two main types of food chains:

Grazing Food Chain (GFC): This chain starts with producers (e.g., grass) being eaten by primary consumers (e.g., goat), which are then eaten by secondary consumers (e.g., man).
Detritus Food Chain (DFC): This chain begins with dead organic matter (detritus). It is made up of decomposers like fungi and bacteria, also known as saprotrophs (from sapro, meaning "to decompose"). They break down dead material for energy.

Note

In aquatic ecosystems, the Grazing Food Chain (GFC) is the primary pathway for energy flow. In contrast, in terrestrial ecosystems, a much larger portion of energy flows through the Detritus Food Chain (DFC).

These individual food chains are interconnected, forming a complex food web. For example, some animals from the DFC (like earthworms) can be prey for animals in the GFC (like birds). Omnivores like crows or humans can also link different food chains.

Trophic Levels and The 10% Law

A trophic level refers to the specific position an organism occupies in a food chain based on its food source.

First Trophic Level: Producers (plants)
Second Trophic Level: Primary consumers (herbivores)
Third Trophic Level: Secondary consumers (carnivores)
Fourth Trophic Level: Tertiary consumers (top carnivores)

A key principle of energy flow is that the amount of energy decreases at each successive trophic level.

Standing Crop: This is the total mass of living material (biomass) or the number of organisms in a unit area at a particular trophic level. Biomass is expressed as fresh or dry weight, with dry weight being a more accurate measure.
10 Per Cent Law: Only about 10% of the energy from one trophic level is transferred to the next. The rest is lost, primarily as heat during metabolic processes. This law restricts the number of trophic levels in a food chain, as there is not enough energy to support many levels.

Ecological Pyramids

An ecological pyramid is a graphical representation of the relationship between organisms at different trophic levels. The base of the pyramid always represents the producers (first trophic level), and the apex represents the top-level consumers. The relationship can be shown in terms of number, biomass, or energy.

There are three main types of ecological pyramids:

1. Pyramid of Number

This pyramid shows the total number of individual organisms at each trophic level.

Upright Pyramid: In most ecosystems, like a grassland, the number of producers (grasses) is far greater than the number of herbivores, which in turn are more numerous than the carnivores. This results in an upright pyramid.
Inverted/Spindle-Shaped Pyramid: There are exceptions. For example, a single large tree (producer) can support thousands of insects (primary consumers). This would result in an inverted or spindle-shaped pyramid of numbers.

2. Pyramid of Biomass

This pyramid represents the total mass of living organisms (biomass) at each trophic level.

Upright Pyramid: In most terrestrial ecosystems, the biomass of producers is greater than the biomass of herbivores, and so on. This creates an upright pyramid.
Inverted Pyramid: The pyramid of biomass in the sea is often inverted. This is because the producers are microscopic phytoplankton, which have a small standing crop (biomass at a given time). They are consumed rapidly by zooplankton, which have a larger biomass. The phytoplankton reproduce very quickly, so they can support a larger biomass of zooplankton.

3. Pyramid of Energy

This pyramid represents the amount of energy present at each trophic level.

Always Upright: The pyramid of energy can never be inverted. This is because when energy flows from one trophic level to the next, a significant amount (around 90%) is lost as heat at each step, according to the second law of thermodynamics. Therefore, the energy available at each successive level is always less than the level below it.