Edexcel iGCSE Biology-5.7 Food chains, Food webs & Pyramids- Study Notes- New Syllabus

Food Chains, Food Webs & Ecological Pyramids

🌱 Food Chains

Definition

• A food chain is a simple diagram that shows how food and energy pass from one organism to another in a single feeding pathway.
• The arrows in a food chain show the direction of energy flow, not the direction in which an organism moves.
• Energy always flows from the organism being eaten to the organism that eats it.

Parts of a Food Chain

A food chain usually includes:

• Producer
• Primary consumer
• Secondary consumer
• Tertiary consumer

Some food chains may have fewer or more trophic levels depending on the ecosystem.

Example of a Food Chain

Grass → Grasshopper → Frog → Snake → Eagle

Explanation:

• Grass makes its own food by photosynthesis, so it is the producer.
• Grasshopper eats grass, so it is the primary consumer.
• Frog eats grasshopper, so it is the secondary consumer.
• Snake eats frog, making it the tertiary consumer.
• Eagle is the top predator in this food chain.

Important Features of a Food Chain

• Shows only one feeding pathway.
• Starts with a producer.
• Arrows always point towards the organism receiving energy.
• Shows how energy moves from one trophic level to the next.
• Does not show all feeding relationships in an ecosystem.

🕸️ Food Webs

Definition

• A food web is a network of interconnected food chains that shows all the feeding relationships in an ecosystem.
• Most organisms eat more than one type of food, so food webs give a more realistic picture than a single food chain.

Why Food Webs are More Realistic

In nature:

• One animal may eat several different organisms.
• One organism may be eaten by many different predators.
• Organisms may belong to more than one food chain.

Food webs show these multiple feeding relationships.

Example of a Food Web

In a grassland ecosystem:

• Grass is eaten by rabbits, grasshoppers and mice.
• Rabbits may be eaten by foxes and hawks.
• Mice may be eaten by snakes and owls.
• Grasshoppers may be eaten by frogs and birds.
• Snakes may be eaten by hawks.

All these food chains connect to form a food web.

Importance of Food Webs

Food webs help us understand:

• How organisms depend on each other.
• How energy flows through an ecosystem.
• The effect of removing one species.

Example

If rabbit numbers decrease:

• Foxes may have less food.
• Hawks may eat more mice instead.
• Grass may increase because fewer rabbits are eating it.

One change can affect many other organisms.

📊 Pyramids of Numbers

Definition

• A pyramid of numbers shows the number of organisms present at each trophic level in a food chain.
• It does not show the size or mass of the organisms.

Features

• Producers usually have the largest number of organisms.
• The number of organisms usually decreases at higher trophic levels.
• Each level is drawn as a bar.

Example

The pyramid becomes narrower towards the top because fewer predators are supported.

Inverted Pyramid of Numbers

Sometimes the pyramid is not pyramid-shaped.

Example

One large oak tree → Hundreds of insects → Thousands of parasites

• One tree can support many insects.
• Many insects can support even more parasites.

This produces an inverted pyramid of numbers.

Advantages

• Simple to construct.
• Shows how the number of organisms changes between trophic levels.

Limitations

• Does not show the size of organisms.
• One large tree counts the same as one tiny plant.
• Does not show how much food or energy is available.

🌾 Pyramids of Biomass

Definition

• A pyramid of biomass shows the total dry mass of living organisms at each trophic level.
• Biomass is measured after removing water because water content varies greatly between organisms.

What is Biomass?

• Biomass is the total dry mass of living material in organisms.
• It is usually measured in:

grams per square metre (g/m²)

or

kilograms per square metre (kg/m²)

Features

• Producers usually have the greatest biomass.
• Biomass decreases at each higher trophic level.
• Gives a more accurate picture than a pyramid of numbers.

Example

Although there may be fewer trees than insects, trees usually have a much greater biomass.

Why Biomass Decreases

Not all food eaten becomes new body tissue.

Some food is:

• Used in respiration.
• Lost as heat.
• Lost in waste materials.
• Not completely digested.

As a result, less biomass is available for the next trophic level.

Advantages

• Takes organism size into account.
• Gives a better estimate of the amount of living material.
• Helps compare ecosystems more accurately.

⚡ Pyramids of Energy Transfer

Definition

• A pyramid of energy transfer shows the amount of energy available at each trophic level over a period of time.
• It shows how much energy is passed from one trophic level to the next.

Features

• Energy is measured in units such as kilojoules per square metre per year (kJ/m²/year).
• The producer always contains the greatest amount of energy.
• Each higher trophic level contains less energy.
• Energy pyramids are always upright.

Why Energy Decreases at Each Trophic Level

A large amount of energy is lost before it reaches the next trophic level.

Energy is lost because organisms:

• Carry out respiration.
• Release heat to the surroundings.
• Move from place to place.
• Maintain body temperature.
• Excrete waste materials such as urine and faeces.
• Leave some food uneaten or undigested.

Only a small amount of energy becomes new body tissue and can be passed to the next trophic level.

Example

The amount of energy becomes smaller at each trophic level.

Importance of Energy Pyramids

They help explain:

• Why food chains usually have only a few trophic levels.
• Why there are fewer top predators.
• Why large numbers of producers are needed to support consumers.

🌍 Comparison of Food Chains and Food Webs

📊 Comparison of Ecological Pyramids

📝 Quick Recap 
Food chain = single pathway, food web = network.
Pyramids:
  • Numbers → count of organisms.
  • Biomass → total dry mass.
  • Energy → energy transfer, always upright.
Energy flow = one way obeys 10% rule.
Food webs = more stable than chains.