⚡ Quick Revision : Definition & Components
1. What is an Ecosystem?
An ecosystem is a self-contained unit where biotic (living) organisms interact with their abiotic (non-living) environment. The term was coined by A.G. Tansley.
- ✔ It is the basic functional unit of the biosphere.
- ✔ Can be Natural (Pond, Forest) or Artificial (Aquarium, Cropland).
2. Components of an Ecosystem
| Category | Components | Examples |
|---|---|---|
| Abiotic | Physical Factors | Sunlight, Temperature, Rainfall, Wind. |
| Chemical Factors | Soil nutrients, pH, Gases ($O_2, N_2, CO_2$). | |
| Biotic | Producers (Autotrophs) | Green plants, Phytoplankton. |
| Consumers (Heterotrophs) | Herbivores, Carnivores, Omnivores. | |
| Decomposers (Saprotrophs) | Bacteria, Fungi. |
Ecology: The study of interactions between organisms and their environment (Greek: Oikos - house; Logos - study).
Producers convert solar energy into chemical energy. Decomposers return minerals back to the soil. They are both essential for the cycle of life.
⚡ Quick Revision : Food Chains & Trophic Levels
1. The Food Chain
A Food Chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. It shows the "who eats whom" relationship in an ecosystem.
2. Trophic Levels
Each step or level in a food chain is called a Trophic Level. Energy decreases as we move to higher levels.
- 1 First Trophic Level: Producers (Green plants).
- 2 Second Trophic Level: Primary Consumers (Herbivores).
- 3 Third Trophic Level: Secondary Consumers (Small Carnivores).
- 4 Fourth Trophic Level: Tertiary Consumers (Top Carnivores).
3. Types of Food Chains
| Type | Starts With... | Example Habitat |
|---|---|---|
| Grazing Food Chain | Living green plants | Grasslands, Ponds |
| Detritus Food Chain | Dead organic matter | Forest floor, Mangrove swamps |
Apex Predator: An organism at the top of the food chain that has no natural predators (e.g., Lion, Tiger, Orca).
The direction of arrows in a food chain represents the flow of energy. They always point from the organism being eaten to the organism that eats it.
⚡ Quick Revision : Food Webs & Ecological Pyramids
1. The Food Web
In nature, simple food chains rarely exist in isolation. A Food Web is a network of interconnected food chains. It represents the actual, complex feeding relationships in an ecosystem.
- ✔ An organism may occupy more than one trophic level (e.g., an omnivore like a human).
- ✔ Food webs provide stability to an ecosystem; if one species population fails, others are available for the predator.
2. Ecological Pyramids
Introduced by Charles Elton, these are graphic representations of trophic structures. The base is always formed by producers.
-
A. Pyramid of Numbers: Shows the number of individual organisms at each level.
Note: Can be upright (Grassland) or inverted (Parasitic food chain). - B. Pyramid of Biomass: Shows the total living weight (dry mass) of organisms at each level.
- C. Pyramid of Energy: Shows the total amount of energy available at each level.
3. Why the Pyramid of Energy is ALWAYS Upright?
| Reason | Biological Explanation |
|---|---|
| Thermodynamics | Energy is lost as heat during metabolic activities at every step. |
| Uni-directional | Energy flows from lower to higher levels and never back to the sun. |
Biomass: The total mass of living organisms per unit area. Dry biomass is preferred because water weight fluctuates.
While the pyramid of numbers and biomass can sometimes be inverted (e.g., many insects feeding on one large tree), the pyramid of energy can NEVER be inverted.
⚡ Quick Revision : Energy Flow & The 10% Law
1. The 10% Law (Lindeman's Law)
This is one of the most important concepts in ecology. It states that only 10% of the energy available at a particular trophic level is transferred to the next higher trophic level.
Where does the remaining 90% go?
- Metabolic Heat: Lost during respiration and movement.
- Growth & Repair: Used for maintaining the organism's own body.
- Unconsumed Parts: Some parts (bones, fur, roots) are not eaten or digested.
2. Numerical Example of Energy Transfer
| Trophic Level | Organism | Energy Available |
|---|---|---|
| $T_1$ (Producers) | Plants | 10,000 J |
| $T_2$ (Primary Consumer) | Deer | 1,000 J |
| $T_3$ (Secondary Consumer) | Wolf | 100 J |
| $T_4$ (Tertiary Consumer) | Lion | 10 J |
3. Characteristics of Energy Flow
- 1 Uni-directional: Energy always flows from Producers → Consumers. It can never flow back to the Sun.
- 2 Progressive Decrease: The amount of available energy decreases significantly at each subsequent trophic level.
- 3 Limited Levels: Due to energy loss, most food chains are limited to 3 or 4 trophic levels only.
Question: Why do top predators need to hunt large areas?
Answer: Because of the 10% Law, very little energy reaches the top level. They must eat a large volume of prey to meet their energy requirements.
Energy flows (enters and leaves), while minerals cycle (stay within the system). Energy is lost as heat, but minerals are recycled by decomposers.
⚡ Quick Revision : Biotic Interactions
Interspecific Interactions
In an ecosystem, different species interact with each other in various ways. These interactions can be beneficial, harmful, or neutral for the species involved.
| Type of Interaction | Species A | Species B | Key Definition & Example |
|---|---|---|---|
| Mutualism | (+) | (+) | Both benefit. Ex: Lichens (Algae & Fungi). |
| Commensalism | (+) | (0) | One benefits, other unaffected. Ex: Remora fish on Sharks. |
| Parasitism | (+) | (-) | Parasite benefits, host is harmed. Ex: Cuscuta, Tapeworm. |
| Predation | (+) | (-) | Predator eats prey. Ex: Lion and Deer. |
Special Focus: Parasitism
Parasites are organisms that live on or inside the body of another organism (host) for food and shelter.
- A. Ectoparasites: Live on the outside of the host's body (e.g., Lice, Ticks, Leech).
- B. Endoparasites: Live inside the host's body (e.g., Ascaris, Tapeworm, Plasmodium).
In ICSE, Symbiosis is often used interchangeably with Mutualism. It literally means "living together." The relationship is so close that the organisms often cannot survive without each other.
Predation vs Parasitism: A predator usually kills its prey immediately, whereas a parasite typically keeps its host alive as long as possible to ensure a steady food supply.
⚡ Quick Revision : Flora, Fauna & The Nitrogen Cycle
1. Flora and Fauna
These terms describe the collective biological diversity of a specific region or time period.
- Flora: The plant life occurring in a particular region (e.g., Cactus in a desert ecosystem).
- Fauna: The animal life occurring in a particular region (e.g., Camels and Scorpions in a desert).
2. The Nitrogen Cycle
Nitrogen is essential for making proteins and nucleic acids. Since plants cannot use atmospheric $N_2$ directly, it must be converted into nitrates.
Key Processes:
- Nitrogen Fixation: Conversion of atmospheric $N_2$ into ammonia/nitrates by lightning or bacteria (Rhizobium).
- Nitrification: Conversion of ammonia into nitrites and then nitrates by bacteria like Nitrosomonas and Nitrobacter.
- Ammonification: Conversion of dead organic matter into ammonia by putrefying bacteria.
- Denitrification: Conversion of nitrates back into $N_2$ gas by bacteria like Pseudomonas.
3. Specialized Bacteria Roles
| Bacteria Name | Process | Role in Ecosystem |
|---|---|---|
| Rhizobium | Fixation | Lives in root nodules of legumes; fixes $N_2$. |
| Nitrosomonas | Nitrification | Converts Ammonia to Nitrites. |
| Pseudomonas | Denitrification | Reduces soil fertility by releasing $N_2$ gas. |
Question: Why do farmers grow leguminous crops (like pulses) between two main crops?
Answer: Leguminous plants host Rhizobium bacteria in their root nodules, which naturally enrich the soil with nitrates, reducing the need for chemical fertilizers.