Unit 3: Ecosystems
1. Concept and Structure of an Ecosystem
An Ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. The term was coined by A.G. Tansley in 1935.
Structure: Every ecosystem has two main components:
- Abiotic Components: Non-living factors like sunlight, temperature, water, soil, and minerals.
- Biotic Components: Living organisms like plants, animals, and microbes.
2. Producers, Consumers, and Decomposers
Biotic components are categorized based on how they obtain energy:
- Producers (Autotrophs): Green plants that prepare their own food via photosynthesis. They are the base of every ecosystem.
- Consumers (Heterotrophs): Organisms that depend on others for food.
- Primary Consumers: Herbivores (e.g., Deer, Rabbit).
- Secondary Consumers: Carnivores that eat herbivores (e.g., Fox).
- Tertiary Consumers: Top carnivores (e.g., Lion, Tiger).
- Decomposers (Saprotrophs): Fungi and bacteria that break down dead organic matter into nutrients, returning them to the soil.
3. Energy Flow in the Ecosystem
Energy flow in an ecosystem is unidirectional (one-way). It flows from the sun to producers and then to various consumers.
The 10% Law: Only about 10% of the energy available at one trophic level is transferred to the next level. The rest is lost as heat during respiration and metabolic processes.
4. Food Chains and Food Webs
Food Chain
A linear sequence of organisms where each is eaten by the next member in the chain.
Example: Grass → Grasshopper → Frog → Snake → Hawk
Food Web
In nature, food chains are rarely isolated. A Food Web is a network of interconnected food chains. It provides stability to the ecosystem because if one species disappears, predators have alternative food sources.
5. Ecological Pyramids
Ecological pyramids are graphical representations of the relationship between different trophic levels.
- Pyramid of Numbers: Shows the number of individuals at each level.
- Pyramid of Biomass: Shows the total dry weight of living matter. It can be inverted in aquatic ecosystems (e.g., Phytoplankton vs. Fish).
- Pyramid of Energy: Always upright because energy is lost as heat at each step.
6. Ecological Succession
The gradual and predictable change in the species composition of a given area over time is called Ecological Succession.
- Primary Succession: Starts in a barren area where no life existed before (e.g., bare rock, volcanic ash). It is a very slow process.
- Secondary Succession: Starts in an area that lost its previous community due to fire, flood, or abandoned farmland. It is much faster because soil is already present.
7. Exam Focus Enhancements
Exam Tips
- Upright vs Inverted: Remember that the Pyramid of Energy is never inverted. This is a very frequent 2-mark question.
- Trophic Levels: When drawing food chains, label the levels as T1 (Producers), T2 (Herbivores), etc.
- Succession Stages: The first species to inhabit an area is the Pioneer Species; the final stable community is the Climax Community.
Common Mistakes
- Flow of Energy: Saying energy is "recycled." Matter (nutrients) is recycled, but energy is lost as heat and never returns to the sun.
- Decomposers' Role: Forgetting to mention decomposers. Without them, nutrients would be locked in dead bodies, and the ecosystem would collapse.
- Food Chain Length: Thinking food chains can be infinitely long. Due to the 10% law, they usually don't exceed 4-5 levels.
Frequently Asked Questions
Q: Why is a food web better than a food chain?
A: A food web is more realistic and provides "insurance." If one path is blocked, the energy can flow through another path, making the ecosystem more resilient.
Q: What is a Trophic Level?
A: It is the specific functional role or "step" an organism occupies in a food chain based on its source of nutrition.