ICSE 9 Biology Ecosystem Basic

1.0 The Ecosystem: Nature's Unit

An Ecosystem is a self-contained area where living organisms (Biotic) interact with each other and with their non-living (Abiotic) environment. It is the basic functional unit of the biosphere.

1.1 Components of an Ecosystem

Component	Description	Examples
Abiotic	The non-living physical and chemical factors.	Sunlight, Temperature, Soil, Water, Air.
Biotic	All living organisms inhabiting the area.	Plants, Animals, Fungi, Bacteria.

1.2 Functional Classification of Biotic Components

Living organisms are classified based on how they obtain their nutrition within the ecosystem:

Producers (Autotrophs): Green plants that manufacture food through Photosynthesis.

Consumers (Heterotrophs): Organisms that depend on producers for food (Herbivores, Carnivores, Omnivores).

Decomposers (Saprotrophs): Microorganisms like bacteria and fungi that break down dead organic matter, recycling nutrients.

⚠️ Exam Tip: Defining Species vs. Population

In "Define the term" questions: A Population is a group of individuals of the same species living in an area. A Community consists of different populations interacting in the same area.

The Sun: The Primary Source

Sunlight is the ultimate source of energy for almost all ecosystems. Only about 1% of the solar energy reaching Earth is actually captured by green plants for photosynthesis.

2.0 Energy Flow in Ecosystems

Energy flow is the most vital process in an ecosystem. Unlike nutrients, which are recycled, energy flow is always unidirectional (one-way)—from the sun to producers, and then to consumers.

2.1 Food Chains and Food Webs

A Food Chain is a linear sequence of organisms where each is eaten by the next member in the chain. A Food Web is a complex network of interconnected food chains.

Feature	Food Chain	Food Web
Structure	Simple, linear link.	Complex, branching network.
Stability	Unstable (if one link fails, the chain breaks).	More stable (organisms have alternative food sources).

2.2 Trophic Levels and the 10% Law

Each step or level of a food chain is called a Trophic Level. Energy decreases as we move to higher levels.

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The 10% Law:

Proposed by Raymond Lindeman. It states that only 10% of the energy available at one trophic level is transferred to the next level. The remaining 90% is lost as heat during respiration and other metabolic processes.

⚠️ Exam Tip: Arrows in a Food Chain

In the exam, always ensure the arrows point toward the eater, not the one being eaten. The arrow represents the direction of energy flow. Example: Grass → Deer → Lion.

Why are Food Chains short?

Because of the 10% Law, by the time we reach the 4th or 5th trophic level, the amount of available energy is too small to support another group of organisms. This is why most food chains have only 3 to 4 levels.

3.0 Pyramids and Relationships

Ecologists use pyramids to represent the structure of an ecosystem graphically. Additionally, organisms in a community rarely live in isolation; they form various types of associations.

3.1 Ecological Pyramids

An ecological pyramid is a graphical representation of the relationship between different organisms in an ecosystem.

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Type of Pyramid	Description
Pyramid of Number	Shows the total number of individual organisms at each trophic level.
Pyramid of Biomass	Shows the total living weight (dry weight) of organisms at each level.
Pyramid of Energy	Shows the total amount of energy at each level. It is always upright.

3.2 Biotic Interactions: Living Together

Interactions between different species can be beneficial, harmful, or neutral. Here are the types you must know:

Mutualism (Symbiosis):

Both organisms benefit from each other.
Example: Lichens (Algae + Fungus).

Parasitism:

One benefits (parasite) and the other is harmed (host).
Example: Cuscuta on a host plant.

Commensalism:

One benefits, and the other is neither helped nor harmed.
Example: Epiphytes on tree branches.

⚠️ Exam Tip: Lichens

In "Give Reasons," you might be asked why Lichens are an example of symbiosis. Reason: The Alga provides food (photosynthesis), and the Fungus provides water, minerals, and protection.

Predation vs. Parasitism

While both involve one organism feeding on another, a Predator usually kills its prey immediately for food, whereas a Parasite usually lives on or inside the host for a long time without killing it immediately.

4.0 Biogeochemical Cycles

Unlike energy, which flows one way and is lost, matter (nutrients) is recycled. The circulation of chemical elements between the living (biotic) and non-living (abiotic) world is called a Biogeochemical Cycle.

4.1 The Carbon Cycle

Carbon is the building block of life. It moves through the ecosystem via two main processes:

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Carbon Addition: CO₂ is added to the atmosphere by Respiration, Combustion (burning fuels), and Decay.

Carbon Removal: CO₂ is removed primarily by Photosynthesis in green plants.

4.2 The Nitrogen Cycle

Nitrogen is essential for proteins, but plants cannot use atmospheric N₂ directly. It must be "fixed" into nitrates.

Process	Bacteria Involved	Description
Nitrogen Fixation	Rhizobium / Azotobacter	Converting atmospheric N2 into ammonia/nitrates.
Nitrification	Nitrosomonas / Nitrobacter	Converting ammonia into nitrites and then nitrates.
Denitrification	Pseudomonas	Converting soil nitrates back into N2 gas.

⚠️ Exam Tip: Root Nodules

In "Name the following," remember that Rhizobium bacteria live in the root nodules of Leguminous plants (like peas and beans). This is an example of Symbiosis.

✅ Unit Complete: Ecosystem

• Can you explain why the energy pyramid is always upright?
• Do you know the difference between Nitrifying and Denitrifying bacteria?
• How does Carbon return to the atmosphere?
• What is the role of Lightning in nitrogen fixation?