BIOLOGY CLASS NOTES FOR SENIOR SECONDARY SCHOOL 2 (SS 2)
SUBJECT: BIOLOGY
TOPIC: BALANCE IN NATURE (Ecological Balance, Food Chains, Food Webs, and Energy Flow)
LEVEL: SS 2
DURATION: Comprehensive Study Material
1. COMPREHENSIVE CORE CONCEPTS
A. The Concept of Ecological Balance
Nature exists in a state of dynamic equilibrium known as Ecological Balance. This is a state where the various components of an ecosystem—plants, animals, microorganisms, and their physical environment—interact in such a way that the ecosystem remains stable over time. In a balanced ecosystem, the birth rate and death rate of various species are roughly equal, and the resources (like water and nutrients) are recycled efficiently.
Ecological balance is maintained through several natural mechanisms. One major mechanism is population regulation. For instance, if the population of grasshoppers (prey) increases, the population of toads (predators) will eventually increase because there is more food available. As the toads eat more grasshoppers, the grasshopper population decreases, which eventually leads to a decrease in the toad population due to lack of food. This "see-saw" effect ensures that no single species dominates to the point of destroying the environment.
B. Food Chains: The Linear Path of Energy
A Food Chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. It represents "who eats whom" in an environment.
Every food chain begins with a Producer (usually a green plant or algae) that captures solar energy through photosynthesis.
- Producers (Autotrophs): These are the foundation. Example: Grass or Phytoplankton.
- Primary Consumers (Herbivores): Animals that eat producers. Example: A grasshopper or a small fish.
- Secondary Consumers (Carnivores): Animals that eat primary consumers. Example: A toad or a larger fish.
- Tertiary Consumers (Top Carnivores): Animals at the top of the chain. Example: A hawk or a shark.
- Decomposers: Organisms like fungi and bacteria that break down dead matter, returning nutrients to the soil.
Example of a Terrestrial Food Chain in Nigeria:
- Guinea Grass (Producer) → Grasshopper (Primary Consumer) → Toad (Secondary Consumer) → Snake (Tertiary Consumer) → Hawk (Quaternary Consumer).
C. Food Webs: The Complex Reality
In nature, a simple linear food chain is rare. Most animals eat more than one type of food. A Food Web is a network of interconnected food chains. It provides a more accurate picture of the feeding relationships in an ecosystem.
The complexity of a food web is vital for ecosystem stability. If one species in a food web disappears (e.g., due to disease), the predators of that species usually have alternative food sources. This prevents the entire system from collapsing. For example, if a hawk cannot find a snake, it might eat a lizard or a small bird instead. The more links a food web has, the more resilient the ecosystem is to changes.
D. Energy Flow and the 10% Rule
Energy enters an ecosystem from the sun. However, unlike nutrients (which cycle), energy flows in one direction and is eventually lost as heat.
The movement of energy is governed by the 10% Rule. This rule states that when energy is passed from one trophic level (feeding level) to the next, only about 10% of the energy is stored as biomass (body tissue) and passed on. The remaining 90% is lost through:
- Respiration: Energy used for movement, breathing, and keeping warm.
- Excretion/Egestion: Energy lost in waste products.
- Death: Energy in body parts that are not eaten (like bones or fur).
Because energy decreases at each level, food chains rarely have more than five steps. There simply isn't enough energy left at the top to support a large population of top predators.
2. REAL-WORLD EXAMPLES (SCENARIOS)
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Scenario 1: The Nigerian Farmland (Pest Control)
Imagine a maize farmer in Oyo State who decides to kill all the snakes on his farm because he is afraid of them. Soon, he notices a massive increase in the population of rats. The rats, no longer hunted by snakes, eat up his stored maize. This is a real-world example of how breaking a food chain upsets the ecological balance and affects human food security.
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Scenario 2: Overfishing in the Niger Delta
In coastal communities, if humans over-harvest large fish (tertiary consumers), the population of smaller fish and crustaceans may skyrocket temporarily. However, these small fish might over-consume the plankton (producers). Once the plankton is gone, the entire aquatic ecosystem collapses, leaving the fishermen with no fish at all.
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Scenario 3: The Garden Ecosystem
In your backyard, aphids (tiny insects) might be sucking the sap from your hibiscus plants. Ladybird beetles (Ladybugs) often arrive to eat the aphids. If you spray harsh chemicals to kill the aphids, you might also kill the ladybirds. If the aphids return (which they often do faster than the ladybirds), they will multiply without any natural "checks," destroying your garden faster than before.
3. PRACTICAL APPLICATIONS (STEP-BY-STEP GUIDES)
How to Identify and Map a Local Food Web
- Step 1: Observation. Go to a safe, bushy area or a school garden. Spend 15 minutes silently observing the organisms.
- Step 2: Documentation. List every living thing you see: weeds, insects, birds, lizards, spiders.
- Step 3: Research Feeding Habits. For each organism, determine what it eats. (e.g., "I saw a lizard; I know lizards eat insects").
- Step 4: Drawing the Web. Write the names of the organisms on a large sheet of paper. Draw arrows pointing in the direction of energy flow (from the one being eaten to the one doing the eating).
- Step 5: Analysis. Identify the producers, consumers, and potential decomposers.
4. SUGGESTED HOME PROJECTS (PROJECT-BASED LEARNING)
Project: The "Ecosystem in a Bottle" (A Mini-Biosphere)
Objective: To observe how plants and decomposers maintain a mini-ecological balance.
Materials Needed:
- A large, clear plastic soda bottle (2 liters).
- Moist (not soaking) garden soil.
- Small weeds or grass with roots attached.
- A few small stones or pebbles.
- A few "decomposers" (earthworms or millipedes found in damp soil).
- Clear adhesive tape.
Procedure:
- Cut the top third of the bottle off.
- Place a layer of pebbles at the bottom for drainage.
- Add 3-4 inches of garden soil.
- Plant your small weeds/grass in the soil.
- Gently place the earthworms or millipedes on the soil.
- Lightly mist the inside with water (do not overwater).
- Replace the top of the bottle and seal the seam tightly with clear tape so it is airtight.
- Place the bottle in a spot with indirect sunlight (not direct sun, or it will cook!).
Observation Task: Over the next two weeks, observe how the water cycles (condenses on the sides and falls back) and how the plants stay green. The worms will turn the soil, and the plants will provide oxygen. This represents a closed, balanced system.
5. LIFE SKILLS INTEGRATION
- Critical Thinking: Understanding food webs helps you realize that every action has a reaction. In business or personal life, this "systems thinking" allows you to predict how a change in one area will affect another.
- Environmental Stewardship: You learn that humans are part of the food web, not outside of it. Our survival depends on the health of producers (plants) and the stability of the climate.
- Career Connections:
- Ecologist: Studying how organisms interact.
- Agricultural Scientist: Using natural predators to control pests (Biological Control) instead of chemicals.
- Wildlife Conservationist: Protecting endangered species to maintain nature’s balance.
- Environmental Lawyer: Creating laws to prevent pollution that destroys food chains.
6. ASSESSMENT THROUGH APPLICATION
- The "Impact Prediction" Task: If a mysterious disease wiped out all the toads in a Nigerian savanna, explain three specific consequences this would have on the other members of that ecosystem.
- Energy Calculation: If a field of maize produces 50,000 Kilojoules (kJ) of energy, according to the 10% rule, how much energy is available to:
- The cows that eat the maize?
- The humans who eat the cows?
- Design a Solution: Your community is facing a mosquito outbreak. Instead of using chemical sprays that kill all insects, propose a "biological" way to restore balance and reduce the mosquito population using your knowledge of food chains.
7. STUDENT REFLECTION QUESTIONS
- If you were to remove yourself from your local food web for one month, what would change? Would any species increase or decrease?
- Why is a food web more beneficial to the survival of a species than a simple food chain?
- "The sun is the ultimate source of all energy." Trace the energy in the "Suya" (meat) you ate yesterday back to the sun in four steps.
- Humans often consider decomposers (like maggots or fungi) "gross." How would the world look after one year if all decomposers suddenly went extinct?
GOLDEN STARS MONTESSORI ACADEMY