CIE iGCSE Co-ordinated Sciences-B18.2 Food chains and food webs- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-B18.2 Food chains and food webs – Study Notes
CIE iGCSE Co-ordinated Sciences-B18.2 Food chains and food webs – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe a food chain as showing the transfer of energy from one organism to the next, beginning with a producer
- Construct and interpret simple food chains
- Describe a food web as a network of interconnected food chains and interpret food webs
- Describe a producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
- Describe a consumer as an organism that gets its energy by feeding on other organisms
- State that consumers may be classed as primary, secondary and tertiary according to their position in a food chain
- Describe a herbivore as an animal that gets its energy by eating plants
- Describe a carnivore as an animal that gets its energy by eating other animals
- Describe a decomposer as an organism that gets its energy from dead or waste organic material
- Use food chains and food webs to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat
Supplement
- Describe a trophic level as the position of an organism in a food chain and food web
- Identify the following as the trophic levels in food webs and food chains: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers
- Explain why the transfer of energy from one trophic level to another is often not efficient
- Explain, in terms of energy loss, why food chains usually have fewer than five trophic levels
- Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Food Chains
📌 Definition
A food chain shows the transfer of energy from one organism to the next, beginning with a producer.
🌞 Key Points
- Producers (plants/algae) → make their own food using sunlight (photosynthesis).
- Consumers (animals) → eat other organisms to get energy.
- Energy flows in one direction only.
- Each step in a food chain is called a trophic level.
🐾 Example Food Chain
Sun → Grass (producer) → Grasshopper (primary consumer) → Frog (secondary consumer) → Snake (tertiary consumer) → Eagle (top consumer)
📊 Roles in a Food Chain
| Organism type | Role in energy transfer | Example |
|---|---|---|
| Producer | Traps light energy, makes food | Grass |
| Primary consumer | Eats producers | Grasshopper |
| Secondary consumer | Eats primary consumer | Frog |
| Tertiary consumer | Eats secondary consumer | Snake |
| Top consumer | Highest predator | Eagle |
⚡ Quick Recap
Food chain = energy transfer diagram.
Starts with a producer → then consumers.
Energy decreases at each step.
👉 Memory tip: “Sun feeds plants, plants feed animals.”
Food Chains – Construction & Interpretation
📌 Definition
A food chain shows how energy flows from producers → consumers.
Always start with a producer (usually a green plant or algae).
Arrows (→) show the direction of energy transfer.
🐾 Example (a) Land Food Chain
Leaves (producer) → Caterpillars (primary consumer) → Blue tits (secondary consumer) → Kestrel (tertiary consumer)
- Leaves: Producers (photosynthesis).
- Caterpillars: Herbivores, feed on leaves.
- Blue tits: Small birds, feed on caterpillars.
- Kestrel: Predator bird, top consumer in this chain.
🐟 Example (b) Water Food Chain
Microscopic algae (producer) → Water fleas (primary consumer) → Minnows (secondary consumer) → Perch (tertiary consumer)
- Microscopic algae: Producers, trap sunlight energy.
- Water fleas: Small herbivores feeding on algae.
- Minnows: Small fish, feed on water fleas.
- Perch: Larger predatory fish, top consumer here.
📊 Producers vs Consumers
| Term | Definition | Example |
|---|---|---|
| Producer | Makes own food by photosynthesis | Leaves, algae |
| Primary consumer | Eats producers (herbivore) | Caterpillar, water flea |
| Secondary consumer | Eats primary consumers | Blue tit, minnow |
| Tertiary consumer | Eats secondary consumers | Kestrel, perch |
⚡ Quick Recap
Food chains start with producers.
Arrows = energy flow (who eats whom).
Consumers are levels above producers.
Example land chain: Leaf → Caterpillar → Blue tit → Kestrel.
Example water chain: Algae → Water flea → Minnow → Perch.
👉 Memory tip: “Plants feed herbivores, herbivores feed carnivores.”
Food Webs
📌 Definition
- A food web is a network of interconnected food chains in an ecosystem.
- It shows how different organisms are linked through feeding relationships.
- More realistic than a single food chain, since most animals eat more than one type of food.
🌱 Example Food Web (given organisms)
- Vegetation (producer) is eaten by: Rabbit, Beetle, Rat.
- Consumers:
- Rabbit → eaten by fox, owl, stoat.
- Beetle → eaten by rat.
- Rat → eaten by fox, owl, stoat.
- Stoat → eaten by fox, owl.
- Top predators: Fox and Owl (no natural predators shown).
📊 Key Points When Interpreting a Food Web
- Producers: Vegetation is the main energy source.
- Primary consumers: Rabbit, beetle, rat (herbivores/omnivores).
- Secondary consumers: Stoat (eats rabbits & rats).
- Tertiary consumers / Top predators: Owl, fox (eat rabbits, rats, stoats).
- Energy flow: Arrows show energy transfer, not “who eats who”.
- Stability of ecosystem: If one species is removed, the whole web is affected.
Example: If beetles die out → fewer rats → less food for foxes and owls.
⚡ Quick Recap
Food web = many food chains joined together.
Starts with producers → consumers → predators.
Shows energy flow and interdependence of species.
Example: Vegetation → rabbit → owl/fox.
👉 Memory tip: “Web = network of chains.”
Producers
📌 Definition
A producer is an organism that makes its own organic nutrients (like glucose).
Usually, this is done using energy from sunlight in the process of photosynthesis.
🧬 Key Points
- Producers form the first trophic level in food chains/webs.
- They convert light energy → chemical energy stored in food.
- Almost all life depends on producers as the primary energy source.
- Examples: Green plants, Algae, Some bacteria (chemosynthetic, rare compared to photosynthetic producers).
📊 Summary Table
| Feature | Producer |
| Role | Makes food for itself and other organisms |
| Energy source | Sunlight (photosynthesis) |
| Nutrients made | Glucose, other organic molecules |
| Examples | Grass, algae, phytoplankton |
⚡ Quick Recap
Producers = self-feeders (make their own food).
Use sunlight + CO₂ + H₂O → glucose + O₂.
Basis of all food chains.
👉 Memory trick: P for Producer = P for Photosynthesis.
Consumers
📌 Definition
A consumer is an organism that gets its energy by feeding on other organisms.
Unlike producers, consumers cannot make their own food.
🧬 Key Points
- Consumers depend directly or indirectly on producers for food.
- Different levels of consumers:
- Primary consumers → eat producers (herbivores).
- Secondary consumers → eat primary consumers (carnivores/omnivores).
- Tertiary consumers → eat secondary consumers (top predators).
- Examples: Rabbit (primary), Frog (secondary), Lion (tertiary).
📊 Summary Table
| Type of Consumer | Feeds On | Example |
| Primary consumer | Producers (plants, algae) | Cow, rabbit |
| Secondary consumer | Primary consumers | Snake, frog |
| Tertiary consumer | Secondary consumers | Lion, eagle |
⚡ Quick Recap
Consumers = eat to get energy.
Cannot photosynthesize.
Classified as primary, secondary, tertiary depending on what they eat.
👉 Memory trick: C for Consumer = C for “Chew” (they eat).
Consumers in a Food Chain
📌 Key Statement
Consumers may be classed as primary, secondary, or tertiary depending on their position in a food chain.
🧬 Types of Consumers
- Primary Consumers 🌱
Feed directly on producers (plants/algae).
Usually herbivores.
Example: Rabbit, cow, caterpillar. - Secondary Consumers 🐍
Feed on primary consumers.
Usually carnivores or omnivores.
Example: Snake, frog, blue tit. - Tertiary Consumers 🦅
Feed on secondary consumers.
Often top predators in a chain.
Example: Eagle, lion, kestrel.
📊 Summary Table
| Consumer Type | Feeds On | Example |
| Primary | Producers | Rabbit, cow |
| Secondary | Primary consumers | Snake, frog |
| Tertiary | Secondary consumers | Lion, eagle |
⚡ Quick Recap
Consumers = animals that eat others for energy.
Primary → eat plants.
Secondary → eat herbivores.
Tertiary → eat carnivores.
👉 Memory trick: P → S → T (Plants → Small herbivores → Top carnivores).
Herbivores
📌 Definition
A herbivore is an animal that gets its energy by eating plants (producers).
They are primary consumers in food chains.
🧬 Key Points
- Depend directly on producers for food.
- Cannot make their own food.
- Often have special teeth (flat molars) to grind plant material.
- Examples: Rabbit, cow, deer, caterpillar.
📊 Summary Table
| Feature | Herbivores |
| Food source | Plants (producers) |
| Food chain role | Primary consumers |
| Adaptations | Grinding teeth, long digestive system |
| Examples | Rabbit, cow, caterpillar |
⚡ Quick Recap
Herbivores = plant-eaters.
Always primary consumers.
Examples: rabbit, cow, deer.
👉 Memory trick: “Herbi = Herb (plants).”
Carnivores
📌 Definition
A carnivore is an animal that gets its energy by eating other animals.
They may be secondary or tertiary consumers in food chains.
🧬 Key Points
- Depend on herbivores or other carnivores for food.
- Usually have sharp teeth and claws for catching and tearing prey.
- Many are predators, while some are scavengers.
- Examples: Lion, tiger, eagle, snake, frog.
📊 Summary Table
| Feature | Carnivores |
|---|---|
| Food source | Other animals |
| Food chain role | Secondary or tertiary consumers |
| Adaptations | Sharp teeth, claws, strong digestive enzymes |
| Examples | Lion, eagle, snake |
⚡ Quick Recap
Carnivores = meat-eaters.
Feed on herbivores or other carnivores.
Examples: lion, snake, eagle.
👉 Memory trick: “Carni = meat.”
Decomposers
📌 Definition
A decomposer is an organism that gets its energy from dead or waste organic material.
They break down dead plants, animals, and waste products into simpler substances.
🧬 Key Points
- Decomposers recycle nutrients back into the soil, making them available for producers.
- Important for nutrient cycling and ecosystem health.
- Examples:
- Fungi (mushrooms, moulds)
- Bacteria
- Some invertebrates (earthworms feed on decaying matter)
📊 Summary Table
| Feature | Decomposers |
|---|---|
| Food source | Dead or waste organic material |
| Role in ecosystem | Nutrient recycling |
| Energy source | Chemical energy from organic matter |
| Examples | Fungi, bacteria, earthworms |
⚡ Quick Recap
Decomposers = nature’s recyclers.
Feed on dead/waste material.
Return nutrients to soil for plants.
👉 Memory trick: “Decompose = break down → feed self + feed soil.”
Human Impact on Food Chains and Food Webs
📌 Introduction
Humans can alter food chains and food webs, which can disrupt ecosystems. Two major ways are:
Overharvesting of food species
Introducing foreign (invasive) species
1️⃣ Overharvesting of Food Species
Definition: Removing too many organisms from an ecosystem for food or other resources.
Effects on food chains/webs:
- Reduces prey availability → predators struggle to survive.
- Can cause population crashes of both prey and predators.
- Example: Overfishing of cod → fewer cod → seabird populations decline → other fish populations increase unexpectedly.
2️⃣ Introducing Foreign Species
Definition: Humans bring species into a habitat where they did not naturally exist.
Effects on food chains/webs:
- New species may compete with native species for food.
- May prey on native species, reducing their numbers.
- Can disrupt energy flow in the food web.
- Example: Rabbits introduced in Australia → overgrazed vegetation → native herbivores starved → predators affected.
📊 Summary Table
| Human Action | Impact on Food Chain/Web | Example |
|---|---|---|
| Overharvesting | Predator starvation, prey population collapse | Overfishing cod |
| Introduced species | Competition, predation, ecosystem disruption | Rabbits in Australia |
⚡ Quick Recap
Humans directly (overharvesting) or indirectly (introducing species) change energy flow.
Can cause population decline, extinction, or ecosystem imbalance.
Memory tip: “Too much take or new arrivals → chain breaks.”
Trophic Levels
📌 Definition
A trophic level is the position of an organism in a food chain or food web.
It shows how energy flows from one level to the next.
🌱 Key Points
- Producers → first trophic level (make their own food).
- Primary consumers → second trophic level (herbivores eating producers).
- Secondary consumers → third trophic level (eat primary consumers).
- Tertiary consumers → fourth trophic level (eat secondary consumers).
- Decomposers → recycle nutrients, can interact at all trophic levels.
📊 Example (Land Food Chain)
| Trophic Level | Organism | Role |
|---|---|---|
| 1 | Grass | Producer |
| 2 | Rabbit | Primary consumer |
| 3 | Fox | Secondary consumer |
| 4 | Eagle | Tertiary consumer |
⚡ Quick Recap
Trophic level = “energy step” in a food chain/web.
Energy decreases at each higher level.
Memory tip: “Producer → Herbivore → Carnivore → Top predator.”
Trophic Levels in Food Chains and Food Webs
📌 Definition
A trophic level is the position of an organism in a food chain or web.
Organisms are classified based on what they eat and who eats them.
🌱 Trophic Levels
| Trophic Level | Description | Examples |
|---|---|---|
| Producers | Make their own food using sunlight (photosynthesis) | Grass, algae, phytoplankton |
| Primary consumers | Eat producers (herbivores) | Rabbit, cow, water flea |
| Secondary consumers | Eat primary consumers (carnivores/omnivores) | Frog, minnow, blue tit |
| Tertiary consumers | Eat secondary consumers (top predators) | Snake, perch, kestrel |
| Quaternary consumers | Eat tertiary consumers (apex predators, rare in food chains) | Eagle, lion, large sharks |
🔑 Key Points
- Energy flows from producers → consumers.
- Higher trophic levels have less energy available.
- Most food chains do not go beyond 4–5 trophic levels due to energy loss.
⚡ Quick Recap
Producers → Primary → Secondary → Tertiary → Quaternary
Energy decreases at each level.
Memory tip: “Plants → Herbivores → Small carnivores → Big carnivores → Apex predators.”
Energy Transfer Between Trophic Levels
📌 Key Point
Energy transfer from one trophic level to the next is often not efficient.
Only a small fraction of energy (~10%) is passed on to the next level.
🌱 Reasons for Energy Loss
- Respiration
Energy is used by organisms for movement, growth, and maintaining body temperature.
Lost as heat to the environment. - Incomplete consumption
Not all parts of an organism are eaten (bones, roots, shells). - Indigestible materials
Some food cannot be digested and is excreted as waste (faeces, urine).
📊 Summary Table
| Reason | How energy is lost |
|---|---|
| Respiration | Energy released as heat |
| Incomplete consumption | Some parts not eaten |
| Indigestible materials | Energy passes out as waste |
⚡ Quick Recap
Only ~10% of energy moves to the next trophic level.
Most energy is lost as heat, waste, or undigested parts.
Memory tip: “Energy diminishes as you go up the chain.”
Why Food Chains Usually Have Fewer Than Five Trophic Levels
📌 Key Point
Food chains rarely have more than 4–5 trophic levels because energy is lost at each step, making higher levels unsustainable.
🌱 Explanation
- Energy loss at each trophic level
Only about 10% of energy is transferred to the next level.
~90% is lost as:- Heat (from respiration)
- Waste (faeces, urine)
- Uneaten parts (bones, roots)
- Insufficient energy for top predators
By the 5th or 6th level, there is too little energy to support another consumer.
Populations at high trophic levels are smaller or may not exist.
📊 Summary Table
| Trophic Level | Energy Available | Notes |
|---|---|---|
| Producer | 100% | Base of the chain |
| Primary consumer | ~10% | Herbivore |
| Secondary consumer | ~1% | Small carnivore |
| Tertiary consumer | ~0.1% | Top predator |
| Quaternary consumer | ~0.01% | Rare due to low energy |
⚡ Quick Recap
Energy decreases at each level → limits chain length.
Usually, 4–5 levels max in a food chain.
Memory tip: “Energy runs out before you reach the top.”
Energy Efficiency in Human Diet
📌 Key Point
It is more energy efficient for humans to eat crop plants directly than to eat livestock fed on crops.
🌱 Explanation
- Energy loss in food chains
Only ~10% of energy from one trophic level is passed to the next. - Crops → eaten by humans: energy moves directly from producer → human.
- Crops → livestock → humans: energy passes through two levels, so ~90% is lost to:
- Respiration (movement, heat)
- Waste (faeces, urine)
- Uneaten parts
- Less energy available at higher trophic levels
Feeding livestock on crops and then eating meat requires more crops to produce the same amount of human food energy.
📊 Summary Table
| Pathway | Energy Efficiency |
|---|---|
| Crop → Human | High (direct transfer) |
| Crop → Livestock → Human | Low (~10% of energy transferred to next level) |
⚡ Quick Recap
Eat plants directly → more energy for humans.
Energy lost at each trophic level reduces efficiency.
Memory tip: “Skip the cow → get more power!”