CIE iGCSE Biology-19.2 Food chains and food webs- Study Notes- New Syllabus
CIE iGCSE Biology-19.2 Food chains and food webs- Study Notes – New syllabus
CIE iGCSE Biology-19.2 Food chains and food webs- Study Notes -CIE iGCSE Biology – 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, tertiary and quaternary 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
- Draw, describe and interpret pyramids of numbers and pyramids of biomass
- Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain
- Describe a trophic level as the position of an organism in a food chain, food web or ecological pyramid
- Identify the following as the trophic levels in food webs, food chains and ecological pyramids: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers
Supplement
- Draw, describe and interpret pyramids of energyDiscuss the advantages of using a pyramid of energy rather than pyramids of numbers or biomass to represent a food chain
- 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
Food Chain – Transfer of Energy in Living Systems
Definition:
A food chain shows the transfer of energy from one organism to the next, starting with a producer.
🔍 Key Points:
- Producers (green plants or algae) make their own food using sunlight via photosynthesis.
- Primary consumers (herbivores) eat producers.
- Secondary consumers (carnivores/omnivores) eat herbivores.
- Tertiary consumers may eat other carnivores.
- Each level in the food chain is called a trophic level.
🧾 Example of a Simple Food Chain:
Grass → Grasshopper → Frog → Snake → Eagle
Grass = Producer
Grasshopper = Primary consumer
Frog = Secondary consumer
Snake = Tertiary consumer
Eagle = Top predator
Energy decreases at each trophic level due to loss as heat, movement, and waste.
Food chains are simplified models – most organisms are part of complex food webs.
🧠 Summary:
A food chain shows who eats whom and how energy flows from the Sun to producers, and then through various consumers in an ecosystem.
Constructing and Interpreting Simple Food Chains
What is a Food Chain?
A food chain shows the flow of energy in an ecosystem – beginning with a producer and moving through different levels of consumers.
🧱 How to Construct a Simple Food Chain:
- Start with a producer – a plant or algae that makes its own food using sunlight
- Add a primary consumer – herbivore that eats the producer
- Add a secondary consumer – eats the primary consumer (usually a carnivore/omnivore)
- Add a tertiary consumer (optional) – a top predator that eats other carnivores
🧾 Example 1: Grassland Food Chain
Grass → Grasshopper → Frog → Snake
Grass = Producer
Grasshopper = Primary consumer
Frog = Secondary consumer
Snake = Tertiary consumer
🧾 Example 2: Pond Food Chain
Algae → Small fish → Big fish → Kingfisher
Algae = Producer
Small fish = Primary consumer
Big fish = Secondary consumer
Kingfisher = Tertiary consumer / Top predator
🔍 How to Interpret a Food Chain:
- Arrow shows energy flow (from food to feeder)
- Each organism depends on the one before it for energy
- Some energy is lost as heat or waste at each stage
🧠 Key Terms:
| Term | Meaning |
|---|---|
| Producer | Makes food using sunlight (e.g. plants) |
| Consumer | Eats other organisms |
| Herbivore | Eats plants |
| Carnivore | Eats other animals |
| Omnivore | Eats both plants and animals |
| Top predator | Has no natural enemies |
Food Webs – A Network of Food Chains
Definition:
A food web is a network of interconnected food chains that shows how energy and nutrients flow through many different organisms in an ecosystem.
🔍 Why Use a Food Web Instead of a Food Chain?
- Food chains are simple – but real ecosystems are more complex.
- Animals often eat more than one type of food.
- Organisms belong to multiple food chains.
- A food web gives a more realistic picture of feeding relationships.
🧾 Example: Simplified Forest Food Web
Grass → eaten by Deer and Rabbits
Rabbits → eaten by Foxes and Hawks
Deer → eaten by Wolves
Insects → eaten by Birds, which are also eaten by Snakes
All these interactions connect to form a web – not just a straight line.
🔍 How to Interpret a Food Web:
- Follow the arrows: Arrows show energy flow (from prey to predator)
- Identify producers: Usually plants or algae — they start the web
- Identify consumers:
- Primary consumers – eat producers
- Secondary consumers – eat herbivores
- Tertiary consumers – top predators
- Look for organisms in multiple chains: These connect different parts of the web
- Check effects of removing species: Removal of one species may unbalance the entire system
🧠 Key Point:
A food web shows how complex and interconnected life is in an ecosystem.
It helps us understand how energy is shared and balanced among organisms.
What Is a Producer?
Definition:
A producer is an organism that makes its own organic nutrients (like glucose) using energy from sunlight, usually through the process of photosynthesis.
🔍 Key Features of Producers:
- They do not rely on eating other organisms for food.
- They convert light energy into chemical energy (stored in food).
- They are the first organisms in all food chains.
– Green plants
– Algae
– Phytoplankton
🧠 Why Producers Are Important:
Producers form the base of every food chain, providing energy-rich food for herbivores and other consumers.
Without producers, life in ecosystems cannot exist, as they start the flow of energy.
What Is a Consumer?
Definition:
A consumer is an organism that gets its energy by feeding on other organisms.
🔍 Key Features of Consumers:
- Unlike producers, consumers cannot make their own food.
- They get chemical energy by eating plants, animals, or both.
- Consumers depend on producers directly or indirectly for their energy.
🧾 Types of Consumers:
| Type | What They Eat | Example |
|---|---|---|
| Herbivores | Only plants (producers) | Cows, rabbits, caterpillars |
| Carnivores | Only animals | Lions, eagles, sharks |
| Omnivores | Both plants and animals | Humans, bears, pigs |
| Scavengers | Dead animals | Vultures, hyenas |
🧠 Why Consumers Matter:
Consumers keep ecosystems balanced by controlling populations of other organisms and helping with the flow of energy through food chains.
Levels of Consumers in a Food Chain
Key Statement:
Consumers can be classified as primary, secondary, tertiary, or quaternary based on their position in a food chain.
🧾 Consumer Levels Explained:
| Consumer Level | What They Eat | Example |
|---|---|---|
| Primary consumer | Eat producers (plants) | Grasshopper, rabbit |
| Secondary consumer | Eat primary consumers | Frog, small bird, fox |
| Tertiary consumer | Eat secondary consumers | Snake, hawk, big cat |
| Quaternary consumer | Eat tertiary consumers | Eagle, killer whale |
🔍 How It Works:
- Producers start the chain (e.g., plants).
- Each consumer level depends on the one below it for energy.
- As you move up the chain, energy becomes less available.
🧠 Key Point:
The position of a consumer in a food chain shows how far it is from the original energy source – the Sun.
What Is a Herbivore?
Definition:
A herbivore is an animal that gets its energy by eating plants.
🔍 Key Features:
- Herbivores feed only on producers (such as green plants and algae).
- They are usually the primary consumers in a food chain.
- They have adaptations like flat teeth for grinding leaves or strong jaws for chewing tough plant parts.
🧾 Examples of Herbivores:
- Rabbit
- Cow
- Deer
- Giraffe
🧠 Why Herbivores Are Important:
Herbivores transfer energy from producers to higher levels in the food chain and help maintain balance in ecosystems.
What Is a Carnivore?
Definition:
A carnivore is an animal that gets its energy by eating other animals.
🔍 Key Features:
- Carnivores feed on other consumers (herbivores or other carnivores).
- They may be secondary, tertiary, or even quaternary consumers in a food chain.
- They often have sharp teeth and claws for catching and eating prey.
🧾 Examples of Carnivores:
- Snake
- Lion
- Hawk
- Wolf
- Shark
🧠 Why Carnivores Are Important:
Carnivores help control populations of other animals and keep the ecosystem balanced.
What Is a Decomposer?
Definition:
A decomposer is an organism that gets its energy from dead organisms or waste organic material.
🔍 Key Features:
- Decomposers break down dead plants, animals, and waste products (like fallen leaves or animal droppings).
- They release nutrients back into the soil, making them available for plants to use again.
- They play a key role in recycling matter in ecosystems.
🧾 Examples of Decomposers:
- Fungi
- Bacteria
- Some insects (e.g. dung beetles)
- Certain worms
Without decomposers, nutrients would not return to the soil, and dead matter would build up – ecosystems would stop functioning properly.
Human Impact on Food Chains and Food Webs
Key Idea:
Human activities like overharvesting and introducing foreign species can disrupt food chains and food webs, leading to serious consequences for ecosystems.
1. Overharvesting of Food Species
🔍 What It Means:
Overharvesting is when humans catch, hunt, or collect a species too much or too quickly, before it can naturally recover.
Small fish → Big fish → Shark
If humans overfish the small fish, the big fish and sharks will struggle to find food.
This causes their numbers to decrease and the food chain becomes unbalanced.
- Predators may starve or die out.
- Other prey species might overpopulate, causing damage (e.g. algae overgrowth).
- Ecosystem stability is lost.
2. Introducing Foreign (Invasive) Species
🔍 What It Means:
When humans bring a non-native species into a new habitat, the species may have no natural predators there and can outcompete or prey on local organisms.
In Australia, the cane toad was introduced to control pests.
It became invasive and started poisoning native predators that tried to eat it.
Native species declined, and food web connections were broken.
- Native species may become endangered or extinct.
- Predators may lose food sources.
- The foreign species can become dominant, pushing others out of their roles in the food web.
Overharvesting and introducing foreign species can disrupt energy flow, eliminate key organisms, and collapse food chains and food webs, making ecosystems less healthy and more unstable.
Pyramids of Numbers & Biomass
1. Pyramid of Numbers
Definition:
A pyramid of numbers shows the number of individual organisms at each level of a food chain.
✏️ Example Diagram:
Tree → Caterpillars → Birds → Hawk
- Each step represents a trophic level.
- Shows the number of organisms in each group.
- Does not consider size or mass.
🌿 2. Pyramid of Biomass
Definition:
A pyramid of biomass shows the total dry mass of organisms at each trophic level — a better indicator of energy available.
✏️ Example Diagram:
Grass → Grasshoppers → Frogs → Snake
- Reflects how much living material is present.
- More accurate than a pyramid of numbers.
- Better represents energy flow – energy decreases at each level.
🧠 Key Differences Summary:
| Feature | Pyramid of Numbers | Pyramid of Biomass |
|---|---|---|
| Shows | Number of organisms | Mass of organisms |
| May look distorted | Yes (e.g. with trees) | Rarely – usually pyramid-shaped |
| Energy flow accuracy | Low | High |
| Units used | Just numbers | Grams of dry mass per area (g/m²) |
Pyramid of Biomass vs Pyramid of Numbers
🔍 Pyramid of Numbers:
- Shows the number of individual organisms at each level of a food chain.
- Can be misleading because it doesn’t account for size or mass.
🌿 Pyramid of Biomass:
- Shows the total dry mass of living material at each trophic level.
- Gives a better idea of the actual amount of energy available.
🧠 Advantages of Using a Pyramid of Biomass:
| Advantage | Explanation |
|---|---|
| More accurate energy representation | Biomass reflects how much living material (and energy) is available at each level. |
| Avoids misleading results | Prevents odd-shaped pyramids caused by large but few organisms (e.g. trees). |
| Shows true structure of ecosystems | Maintains the typical pyramid shape – energy decreases as you go up the levels. |
🚫 Limitations of Pyramid of Numbers:
- Treats all organisms equally, regardless of size or energy value.
- Can be distorted when one organism supports many smaller ones.
A pyramid of biomass gives a more realistic and useful picture of how energy is distributed in a food chain compared to a pyramid of numbers, especially when organisms vary greatly in size.
What Is a Trophic Level?
✅ Definition:
A trophic level is the position an organism holds in a food chain, food web, or ecological pyramid based on how it gets its energy.
🔍 How Trophic Levels Work:
Each trophic level represents a step in the flow of energy:
| Trophic Level | Who It Includes | Examples |
|---|---|---|
| 1st – Producer | Organisms that make their own food | Plants, algae |
| 2nd – Primary consumer | Herbivores that eat producers | Grasshopper, rabbit |
| 3rd – Secondary consumer | Carnivores/omnivores that eat herbivores | Frog, small birds |
| 4th – Tertiary consumer | Eat secondary consumers | Snake, fox |
| 5th – Quaternary consumer | Top predators | Hawk, killer whale |
| Decomposers (varied) | Break down dead organisms; act across levels | Bacteria, fungi |
The higher the trophic level, the less energy is available, because some is lost as heat at each step.
Trophic Levels in Food Chains, Webs & Pyramids
Trophic Level Breakdown:
| Trophic Level | Group Name | What They Do | Examples |
|---|---|---|---|
| 1st Trophic Level | Producers | Make their own food using sunlight | Plants, algae, phytoplankton |
| 2nd Trophic Level | Primary Consumers | Eat producers (usually herbivores) | Rabbit, cow, caterpillar |
| 3rd Trophic Level | Secondary Consumers | Eat primary consumers (usually carnivores) | Frog, fox, small bird |
| 4th Trophic Level | Tertiary Consumers | Eat secondary consumers | Snake, eagle |
| 5th Trophic Level | Quaternary Consumers | Eat tertiary consumers (top predators) | Killer whale, lion, hawk |
🧠 Key Notes:
- Producers are always at the base – they create energy-rich food from sunlight.
- Each level above feeds on the level below.
- Energy decreases with each step up the trophic levels.
📌 Found In:
- Food Chains → Show a simple, straight-line energy flow.
- Food Webs → Show interconnected feeding relationships.
- Ecological Pyramids → Visually represent trophic levels by number, biomass, or energy.
Pyramid of Energy – Description, Drawing & Interpretation
What Is a Pyramid of Energy?
A pyramid of energy shows how energy is transferred from one trophic level to the next in a food chain, measured in joules (J) over a certain area and time.
📊 Diagram Explanation:
| Trophic Level | Organism | Energy Available |
|---|---|---|
| Tertiary consumer | Hawk | 10 J |
| Secondary consumer | Mouse | 100 J |
| Primary consumer | Grasshopper | 1,000 J |
| Primary producer | Flowering plants | 10,000 J |
| Sunlight input | (Solar energy source) | 1,000,000 J |
🖊️ How to Read This Pyramid:
- The base of the pyramid (producers) has the most energy (10,000 J).
- As you move up, energy decreases rapidly – only about 10% is transferred to the next level.
- Most energy is lost through:
- Heat (from respiration)
- Movement
- Undigested materials
🧠 Why Is This Pyramid Useful?
- Shows the actual energy flow through an ecosystem.
- More accurate than pyramids of number or biomass.
- Explains why food chains rarely have more than 4–5 trophic levels.
📌 Key Interpretations:
- Plants absorb sunlight and convert it to chemical energy via photosynthesis.
- Only about 1% of sunlight is turned into plant biomass.
- Each consumer gets less energy than the one below it.
- Top predators (like the hawk) receive the least energy.
🧾 Conclusion:
The pyramid of energy is always pyramid-shaped because energy decreases at each level.
It is the most accurate tool to represent energy flow in ecosystems.
Why Use a Pyramid of Energy?
Key Statement:
A pyramid of energy gives a more accurate and reliable picture of energy flow in a food chain than pyramids of numbers or biomass.
🧠 What It Shows:
- Displays the actual amount of energy transferred from one trophic level to the next.
- Measured in joules (J) or kilojoules (kJ).
- Includes area and time: kJ/m²/year.
🔍 Advantages Over Other Pyramids:
| Advantage | Why It Matters |
|---|---|
| Most accurate | Shows real energy transfer, not just organism counts or biomass. |
| Always pyramid-shaped | Energy always decreases — never distorted or upside down. |
| Includes time | Reflects energy transfer rate — not just a static moment. |
| Shows energy loss | Clearly illustrates heat loss, movement, and waste at each level. |
| Explains food chain limits | Helps explain why food chains are short — limited energy for top predators. |
⚠️ Limitations of Other Pyramids:
- Pyramid of Numbers: Can be misleading if one large organism (like a tree) supports many small ones.
- Pyramid of Biomass: More accurate than numbers but doesn’t reflect energy use or transfer speed.
📌 Final Summary:
The pyramid of energy is the best tool for understanding energy flow through ecosystems.
It explains:
- Why energy is lost
- Why top predators are few
- Why ecosystems rely on a strong producer base
Why Is Energy Transfer Between Trophic Levels Inefficient?
Key Idea:
As energy moves from one trophic level to the next, a large amount is lost – making the transfer inefficient.
Only about 10% of energy is passed on to the next level.
🔍 Where the Energy Goes:
| Reason for Energy Loss | Explanation |
|---|---|
| Heat loss (respiration) | Organisms use energy for respiration, which releases heat into the environment. |
| Movement | Energy is used for physical activity and locomotion rather than being stored as biomass. |
| Waste (excretion and egestion) | Not all parts of food are digested or absorbed, some is lost in faeces and urine. |
| Undigested body parts | Parts like bones, fur, and shells are often left uneaten or unabsorbed. |
| Metabolism | Energy is used for internal body processes like growth, repair, and homeostasis. |
🧠 Why This Matters:
- Less energy is available at each higher trophic level.
- This limits food chains to usually 4-5 levels.
- Top predators are fewer – not enough energy to support large numbers.
📌 Summary:
Energy transfer is inefficient because most energy is lost as heat, waste, or used for life processes.
Only a small portion becomes available biomass for the next level in the food chain.
Why Do Food Chains Have Fewer Than Five Trophic Levels?
Key Explanation:
Food chains are usually short, with fewer than five trophic levels, because too much energy is lost at each step.
🔥 How Energy Loss Affects the Chain:
Each time energy moves from one trophic level to the next, about 90% is lost, and only ~10% is passed on.
🔍 Main Reasons for Energy Loss:
| Energy Loss | Why It Happens |
|---|---|
| Heat (respiration) | Organisms use energy to stay alive and maintain body temperature. |
| Movement | Energy is spent on activities like hunting, walking, flying, or swimming. |
| Waste | Some food remains undigested or unabsorbed and is lost as faeces. |
| Body processes | Energy is used for growth, repair, metabolism, and maintaining bodily functions. |
📉 Result:
Very little energy remains by the time you reach the 4th or 5th trophic level.
There isn’t enough energy to support large populations of top predators.
That’s why most food chains end after the tertiary or quaternary consumer.
🧠 Final Summary:
Because energy is lost at every trophic level, less energy is available for organisms higher up the food chain.
This limits the number of levels and explains why food chains are usually short.
Why Is It More Energy Efficient for Humans to Eat Crops Than Livestock?
Key Idea:
It is more energy-efficient for humans to eat crop plants directly than to eat animals (like cows or chickens) that were fed on those crops.
🔍 Reason: Energy Loss Between Trophic Levels
- When humans eat crops, they act as primary consumers.
- When humans eat animals fed on crops, they become secondary consumers.
- But at each trophic level, about 90% of the energy is lost!
🔥 How Energy Is Lost in Livestock:
| Where the Energy Goes | Explanation |
|---|---|
| Respiration | Animals use crops’ energy for their own body processes like staying alive and warm. |
| Movement | Energy used by animals for walking, breathing, and daily activities. |
| Waste | Not all food is digested – energy is lost in faeces and urine. |
| Uneaten Parts | Bones, fat, and other inedible parts are not consumed by humans. |
📊 Example:
1000 kJ of crop energy →
Human eats crops directly: ~100 kJ of usable energy
Human eats meat from animals fed on crops: only ~10 kJ remains!
🧠 Final Summary:
Eating crops directly saves energy by skipping a trophic level.
Less energy is wasted, making it better for food security and more sustainable for feeding large human populations.