1. Producers, Consumers, and Decomposers

Introduction

Every living organism needs energy to survive. Energy is required for growth, movement, reproduction, repair, and all other life processes. However, not all organisms obtain energy in the same way. Some organisms produce their own food, while others must eat plants, animals, or both to obtain energy.

In every ecosystem, energy enters through sunlight and is transferred from one organism to another through feeding relationships. Ecologists group organisms into three important categories based on how they obtain energy: producers, consumers, and decomposers. Together, these groups ensure that energy flows through ecosystems while nutrients are continually recycled.

How Energy Enters an Ecosystem

Almost all ecosystems on Earth depend on the Sun as their primary source of energy.

Plants and other photosynthetic organisms capture sunlight and convert it into chemical energy through photosynthesis.

The chemical energy stored in plants is then transferred to other organisms when they are eaten.

The flow of energy can be summarised as:

Sun → Producers → Consumers → Decomposers

Unlike nutrients, energy flows in one direction and is not recycled.

 

 

 

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Figure 1. Energy enters ecosystems from the Sun and flows through producers, consumers, and decomposers.

Producers

Producers are organisms that make their own food.

They are also called autotrophs.

Most producers use photosynthesis, combining sunlight, carbon dioxide, and water to produce glucose (food).

Examples include:

• Trees
• Grass
• Flowers
• Mosses
• Algae
• Phytoplankton

Some bacteria living near deep-sea hydrothermal vents produce food using chemosynthesis, obtaining energy from chemicals instead of sunlight.

Why Producers Are Important

Producers:

• Capture energy from the Sun.
• Form the base of almost every food chain.
• Produce oxygen during photosynthesis.
• Provide food for consumers.

Without producers, most ecosystems could not exist.

 

 

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Figure 2. Producers use photosynthesis to convert sunlight into chemical energy stored as food.

Consumers

Consumers cannot make their own food.

They obtain energy by eating other organisms.

Consumers are also called heterotrophs.

There are several types of consumers.

Herbivores (Primary Consumers)

Herbivores eat plants or algae.

Examples include:

• Rabbits
• Deer
• Cows
• Caterpillars
• Grasshoppers
• Zebras

Primary consumers obtain energy directly from producers.

Carnivores (Secondary and Tertiary Consumers)

Carnivores eat other animals.

Examples include:

• Lions
• Hawks
• Wolves
• Sharks
• Snakes

Some carnivores eat herbivores, while others eat other carnivores.

Omnivores

Omnivores eat both plants and animals.

Examples include:

• Humans
• Bears
• Foxes
• Crows
• Raccoons

Their flexible diet allows them to survive in many different environments.

Scavengers

Scavengers feed on dead animals without hunting them.

Examples include:

• Vultures
• Hyenas
• Crabs
• Some beetles

Scavengers help remove dead organisms from ecosystems.

 

 

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Figure 3. Consumers obtain energy by feeding on plants, animals, or both.

Decomposers

Decomposers break down dead plants, animals, and waste materials.

Examples include:

• Fungi
• Bacteria

Unlike scavengers, decomposers digest dead material externally using enzymes before absorbing the nutrients.

Why Decomposers Are Important

Decomposers:

• Break down dead organisms.
• Recycle nutrients into the soil.
• Help plants obtain essential minerals.
• Prevent dead material from accumulating.

Although energy is lost as heat during decomposition, valuable nutrients are returned to the ecosystem.

Without decomposers, ecosystems would eventually run out of nutrients.

 

 

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Figure 4. Decomposers recycle nutrients from dead organisms, making them available for producers once again.

Classifying Organisms by Feeding Role

Scientists classify organisms according to how they obtain energy.

How Energy Moves Through Ecosystems

Energy moves through ecosystems whenever one organism eats another.

For example:

• Grass captures sunlight.
• A rabbit eats the grass.
• A fox eats the rabbit.
• Fungi and bacteria decompose the remains.

The energy originally captured from the Sun has moved through several organisms.

At each step:

• Some energy is used for movement, growth, and reproduction.
• Much of the energy is released as heat.
• Only a small amount is passed to the next feeding level.

This is why food chains usually contain only a few levels.

 

 

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Figure 5. Energy flows in one direction through a food chain, while nutrients are eventually recycled by decomposers.

Energy Flow vs Nutrient Cycling

It is important to distinguish between energy and nutrients.

This difference is one of the most important ideas in ecology.

Worked Example

Question

Classify each organism as a producer, consumer, or decomposer.

*Although earthworms feed on dead organic matter, they are detritivores, not true decomposers. They physically break down material but do not chemically decompose it like fungi and bacteria.

Real-World Connection

Composting is an excellent example of decomposers at work. In compost bins, bacteria, fungi, worms, and other organisms break down food scraps and garden waste into nutrient-rich compost. This compost can then be added to soil, improving plant growth without the need for as much chemical fertilizer.

Did You Know?

Tiny marine producers called phytoplankton are responsible for producing around half of the Earth's oxygen through photosynthesis. Although microscopic, they form the foundation of most ocean food webs and play a vital role in regulating Earth's climate.

Key Terms

Autotroph – An organism that produces its own food using sunlight or chemical energy.

Chemosynthesis – The process of producing food using energy from chemical reactions instead of sunlight.

Consumer – An organism that obtains energy by eating other organisms.

Decomposer – An organism that breaks down dead organic matter and recycles nutrients.

Detritivore – An organism that feeds on dead organic matter by ingesting it.

Energy flow – The movement of energy through an ecosystem from one organism to another.

Heterotroph – An organism that cannot produce its own food.

Photosynthesis – The process by which producers convert sunlight into chemical energy.

Producer – An organism that makes its own food and forms the base of most food chains.

Scavenger – An animal that feeds on dead organisms but does not hunt them.

Key Takeaways

• Almost all ecosystems obtain their energy from the Sun.
• Producers capture solar energy through photosynthesis.
• Consumers obtain energy by eating other organisms.
• Decomposers recycle nutrients from dead organisms and waste.
• Energy flows in one direction through ecosystems, while nutrients are continually recycled.

2. Food Chains

Introduction

Every organism depends on other organisms, directly or indirectly, for food and energy. Plants capture energy from the Sun, herbivores feed on plants, carnivores feed on other animals, and decomposers recycle nutrients when organisms die. These feeding relationships connect organisms and allow energy to move through ecosystems.

One of the simplest ways to represent these relationships is with a food chain. A food chain shows the path that energy follows as it moves from one organism to another. Although real ecosystems are much more complex, food chains help us understand the basic flow of energy through living systems.

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What Is a Food Chain?

A food chain is a diagram that shows who eats whom in an ecosystem.

It illustrates the movement of energy from one organism to the next.

Every food chain begins with a producer because producers capture energy directly from the Sun.

A simple food chain looks like this:

Sun → Grass → Rabbit → Fox

In this example:

• The grass captures sunlight.
• The rabbit eats the grass.
• The fox eats the rabbit.

Each arrow shows the direction that energy moves.

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Figure 1. A food chain shows the movement of energy from the Sun through producers and consumers.

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The Parts of a Food Chain

The Sun

The Sun is the original source of energy for almost every ecosystem.

Without sunlight, most producers could not carry out photosynthesis.

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Producers

Producers make their own food using photosynthesis.

Examples include:

• Grass
• Trees
• Algae
• Phytoplankton

They form the first trophic level of every food chain.

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Primary Consumers

Primary consumers are herbivores that eat producers.

Examples include:

• Rabbits
• Caterpillars
• Deer
• Grasshoppers
• Zebras

They obtain energy directly from plants.

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Secondary Consumers

Secondary consumers eat primary consumers.

Examples include:

• Frogs
• Snakes
• Foxes
• Small fish

Some are carnivores, while others are omnivores.

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Tertiary Consumers

Tertiary consumers feed on secondary consumers.

Examples include:

• Eagles
• Sharks
• Lions
• Wolves

Many are apex predators, meaning they have few or no natural predators.

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Figure 2. Food chains are organised into trophic levels based on how organisms obtain their energy.

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Energy Flows Through a Food Chain

Food chains represent the movement of energy, not just food.

When an organism eats another organism:

• Chemical energy stored in food is transferred.
• Some energy is used for movement.
• Some is used for growth and reproduction.
• Much is released as heat.
• Only a small amount is passed to the next trophic level.

As a result, less energy is available at each successive feeding level.

This is why ecosystems support many producers but relatively few top predators.

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Reading the Arrows

Many students mistakenly believe the arrows point toward the organism being eaten.

In fact, the arrows show the direction of energy flow.

For example:

Grass → Rabbit → Fox

This means:

• Energy moves from the grass to the rabbit.
• Energy then moves from the rabbit to the fox.

A useful way to remember this is:

The arrow points to the organism receiving the energy.

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Figure 3. The arrows in a food chain show the direction that energy is transferred between organisms.

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Constructing Food Chains

When constructing a food chain:

1. Begin with a producer.
2. Add a primary consumer.
3. Continue with higher-level consumers.
4. Draw arrows showing the direction of energy flow.

Example 1 – Grassland

Grass → Grasshopper → Frog → Snake → Hawk

Example 2 – Ocean

Phytoplankton → Zooplankton → Small Fish → Tuna → Shark

Example 3 – Forest

Oak Tree → Caterpillar → Robin → Hawk

Each food chain represents one possible pathway for energy through an ecosystem.

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What Happens When a Food Chain Changes?

Food chains are interconnected.

A change to one organism often affects all the others.

Example 1: Fewer Rabbits

If rabbit numbers decrease:

• Foxes have less food.
• Fox populations may decrease.
• Grass populations may increase because fewer rabbits are eating it.

Example 2: More Predators

If fox numbers increase:

• Rabbit populations may decline.
• Grass populations may increase.

Example 3: Drought

A drought may reduce plant growth.

As a result:

• Herbivore populations decline.
• Carnivore populations also decline.

Small changes can produce large effects throughout an ecosystem.

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Figure 4. Changes to one population can affect every organism connected in a food chain.

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Food Chains Are Simplified Models

Real ecosystems are much more complex than a single food chain.

For example:

• Foxes eat rabbits, mice, insects, and berries.
• Hawks eat snakes, rabbits, and birds.
• Rabbits eat many different plants.

Because organisms usually have several food sources, scientists often use food webs, which combine many food chains into one network.

Food chains, however, are an excellent starting point for understanding energy flow.

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Worked Example

Question

Construct a food chain using the following organisms:

• Grass
• Hawk
• Snake
• Mouse

Solution

Grass → Mouse → Snake → Hawk

Explanation:

• Grass is the producer.
• The mouse is the primary consumer.
• The snake is the secondary consumer.
• The hawk is the tertiary consumer.

The arrows show the direction of energy flow.

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Real-World Connection

Food chains help scientists understand how pollutants move through ecosystems. Chemicals such as pesticides or mercury can enter producers and become more concentrated as they move up the food chain, a process known as biomagnification. This is one reason why top predators, such as eagles, sharks, and polar bears, are often the most affected by environmental pollution.

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Did You Know?

One of the longest food chains ever discovered occurs in the deep ocean, where tiny phytoplankton support a complex network of organisms, eventually leading to large predators such as sharks and orcas. Even these enormous animals ultimately depend on the microscopic producers at the base of the chain.

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Key Terms

Apex predator – A predator at the top of a food chain with few or no natural predators.

Consumer – An organism that obtains energy by eating other organisms.

Energy flow – The movement of energy through organisms in an ecosystem.

Food chain – A diagram showing the pathway of energy transfer from one organism to another.

Primary consumer – A consumer that feeds directly on producers.

Producer – An organism that makes its own food through photosynthesis or chemosynthesis.

Secondary consumer – A consumer that feeds on primary consumers.

Tertiary consumer – A consumer that feeds on secondary consumers.

Trophic level – The feeding position an organism occupies in a food chain.

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Key Takeaways

• A food chain shows how energy moves through an ecosystem.
• Every food chain begins with a producer that captures energy from the Sun.
• Consumers occupy different trophic levels depending on what they eat.
• The arrows show the direction of energy flow, from food to feeder.
• Changes to one organism can affect every other organism in the food chain.
• Food chains are simplified models that help explain energy transfer in ecosystems before studying more complex food webs.

 

 

 

3. Food Webs

Introduction

A food chain is a useful way to show how energy moves through an ecosystem, but it tells only part of the story. In reality, most organisms eat a variety of foods and are themselves eaten by several different predators. For example, a fox may eat rabbits, mice, insects, berries, and even birds, while a rabbit may feed on many different kinds of plants.

Because ecosystems contain many interconnected feeding relationships, scientists use food webs to represent the flow of energy more accurately. Food webs reveal the complexity of ecosystems and help us understand how changes to one species can affect many others.

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What Is a Food Web?

A food web is a network of interconnected food chains.

It shows the many feeding relationships that exist within an ecosystem.

Unlike a food chain, which follows one pathway of energy flow, a food web shows that organisms often have:

• Multiple food sources.
• More than one predator.
• Connections with many other species.

Food webs provide a much more realistic picture of how ecosystems function.

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Figure 1. A food web consists of many interconnected food chains, showing multiple pathways for energy flow.

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Food Chains vs Food Webs

Although both diagrams show how energy moves through ecosystems, they are different.

Food Chain	Food Web
One pathway of energy flow	Many interconnected pathways
Simple	More complex
Easier to study	More realistic
Shows one feeding relationship.	Shows many feeding relationships
Limited information	Better represents real ecosystems

Food chains are useful for introducing energy flow, while food webs provide a more complete understanding of ecosystems.

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Reading a Food Web

Food webs use arrows to show the direction of energy transfer.

Remember:

The arrow points toward the organism receiving the energy.

For example:

Grass → Rabbit

means that:

• The rabbit eats the grass.
• Energy moves from the grass to the rabbit.

When many arrows connect different organisms, they form a food web.

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Figure 2. The arrows in a food web show the direction of energy transfer from one organism to another.

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Interconnected Feeding Relationships

Many organisms eat a variety of foods.

For example, in a forest ecosystem:

A fox may eat:

• Rabbits
• Mice
• Birds
• Insects
• Fruit

A hawk may eat:

• Snakes
• Rabbits
• Mice
• Small birds

A rabbit may eat:

• Grass
• Leaves
• Wildflowers

Because organisms have several food sources, the loss of one food source does not always mean they will die. They may switch to another available food source.

This flexibility helps ecosystems remain stable.

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Example Food Web

Imagine a grassland ecosystem.

Producer

• Grass

Primary Consumers

• Rabbit
• Mouse
• Grasshopper

Secondary Consumers

• Frog
• Snake
• Fox

Top Predators

• Hawk
• Owl

Some feeding relationships include:

• Grass → Rabbit
• Grass → Mouse
• Grass → Grasshopper
• Grasshopper → Frog
• Mouse → Snake
• Rabbit → Fox
• Rabbit → Hawk
• Snake → Hawk
• Mouse → Owl

Notice that several predators share the same prey.

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Figure 3. Real ecosystems contain many interconnected feeding relationships rather than a single food chain.

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What Happens If a Species Is Removed?

Removing one species from a food web can affect many other organisms.

Example: Rabbits Disappear

Possible effects include:

• Foxes have less food.
• Hawks have less food.
• Grass populations increase because fewer rabbits are eating it.
• Foxes may begin eating more mice.
• Increased predation may reduce mouse populations.

This shows that changes to one population often spread throughout the ecosystem.

This is sometimes called a ripple effect or cascade effect because one change causes many others.

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Keystone Species

Some species have a much greater influence on an ecosystem than their population size might suggest.

These are called keystone species.

If a keystone species disappears:

• Food webs may become unstable.
• Some populations increase dramatically.
• Others may decline or disappear.
• Biodiversity may decrease.

Examples of keystone species include:

• Sea otters
• Wolves
• Beavers
• Coral species in reef ecosystems

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Figure 4. Removing a keystone species can trigger widespread changes throughout an entire food web.

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Why Food Webs Are More Realistic

Real ecosystems are extremely complex.

Most organisms:

• Eat several different foods.
• Have multiple predators.
• Change diets during different life stages.
• Adapt to seasonal changes in food availability.

Food webs capture these complex relationships far better than simple food chains.

Scientists use food webs to:

• Study ecosystems.
• Predict environmental changes.
• Understand biodiversity.
• Plan conservation efforts.

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Worked Example

Question

Study the following food web relationships:

• Grass → Rabbit
• Grass → Mouse
• Rabbit → Fox
• Rabbit → Hawk
• Mouse → Snake
• Snake → Hawk

Which organisms would be directly affected if rabbit numbers declined?

Solution

Directly affected organisms include:

• Fox (less food)
• Hawk (less food)
• Grass (less grazing, so grass increases)

Indirect effects might include:

• Foxes hunting more mice.
• Snake populations competing more with foxes for mice.
• Changes in predator populations over time.

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Real-World Connection

Food webs help conservationists understand how ecosystems respond to change. For example, when wolves were reintroduced to Yellowstone National Park, they reduced elk populations, allowing young trees and shrubs to recover. This created new habitats for birds, beavers, insects, and fish. A single change in the food web improved biodiversity throughout the ecosystem.

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Did You Know?

The sea otter is considered a keystone species along the Pacific coast of North America. Sea otters eat sea urchins, which feed on kelp. Without otters, sea urchin populations can grow rapidly and destroy entire kelp forests, affecting hundreds of other marine species that depend on kelp for food and shelter.

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Key Terms

Biodiversity – The variety of living organisms within an ecosystem.

Food chain – A simple diagram showing one pathway of energy flow.

Food web – A network of interconnected food chains showing multiple feeding relationships.

Interconnected – Linked together in many ways.

Keystone species – A species whose influence on an ecosystem is much greater than its abundance would suggest.

Predator – An organism that hunts and eats other organisms.

Prey – An organism that is hunted and eaten by a predator.

Ripple effect (Cascade effect) – A series of changes that spread through an ecosystem after one species changes in abundance or is removed.

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Key Takeaways

• A food web is a network of interconnected food chains.
• Food webs show multiple feeding relationships and pathways of energy flow.
• Most organisms have more than one food source and more than one predator.
• Removing one species can affect many other populations throughout the ecosystem.
• Food webs provide a more realistic representation of ecosystems than food chains because they reflect the complexity of natural feeding relationships.

 

 

 

4. Trophic Level

Introduction

Every organism in an ecosystem occupies a particular position in the flow of energy. Plants capture energy from the Sun, herbivores feed on plants, carnivores feed on herbivores, and decomposers recycle nutrients after organisms die. These feeding positions are known as trophic levels.

Understanding trophic levels helps scientists explain how energy moves through ecosystems and why there are usually many more plants than large predators. As energy is transferred from one trophic level to the next, much of it is used by organisms or lost as heat, leaving less energy available for higher levels.

What Is a Trophic Level?

A trophic level is the feeding position an organism occupies in a food chain or food web.

Organisms are grouped according to how they obtain their energy.

Each step in a food chain represents a different trophic level.

For example:

Grass → Rabbit → Snake → Hawk

 

 

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Figure 1. Each organism occupies a different trophic level based on how it obtains energy.

The Main Trophic Levels

First Trophic Level – Producers

Producers form the base of every food chain.

They capture energy from the Sun through photosynthesis.

Examples include:

• Grass
• Trees
• Algae
• Phytoplankton

Producers contain the greatest amount of available energy because they receive energy directly from sunlight.

Second Trophic Level – Primary Consumers

Primary consumers are herbivores that feed on producers.

Examples include:

• Rabbits
• Deer
• Caterpillars
• Grasshoppers
• Zooplankton

They obtain energy by eating plants or algae.

Third Trophic Level – Secondary Consumers

Secondary consumers feed on primary consumers.

Examples include:

• Frogs
• Snakes
• Small fish
• Foxes

Some are carnivores, while others are omnivores.

Fourth Trophic Level – Tertiary Consumers

Tertiary consumers feed on secondary consumers.

Examples include:

• Hawks
• Eagles
• Sharks
• Lions

Many are apex predators, meaning they have few or no natural predators.

Decomposers

Although decomposers are not usually assigned a single trophic level, they play a vital role by breaking down dead organisms from all trophic levels.

Examples include:

• Fungi
• Bacteria

They recycle nutrients back into the ecosystem, allowing producers to grow again.

 

 

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Figure 2. An energy pyramid illustrates the major trophic levels in an ecosystem.

Why Does Energy Decrease at Higher Trophic Levels?

When one organism eats another, not all of the energy is passed on.

Energy is used for:

• Movement
• Growth
• Reproduction
• Breathing (respiration)
• Maintaining body temperature

Much of this energy is eventually released as heat.

As a result, only a small proportion of the energy stored in one trophic level becomes available to the next.

This explains why ecosystems have:

• Many producers
• Fewer herbivores
• Even fewer carnivores
• Very few top predators

The 10% Rule

A useful guideline in ecology is the 10% Rule.

It states that, on average, only about 10% of the energy at one trophic level is transferred to the next trophic level.

The remaining energy is:

• Used for life processes.
• Lost as heat.
• Not eaten or not fully digested.

Example

The exact percentage varies between ecosystems, but the 10% Rule is a useful model for understanding energy transfer.

 

 

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Figure 3. Only a small fraction of energy is transferred from one trophic level to the next.

Comparing Consumers

Some organisms occupy more than one trophic level.

For example:

A bear may eat:

• Berries (acting as a primary consumer)
• Fish (acting as a secondary or tertiary consumer)

This is why food webs are often more complex than simple food chains.

Analysing Energy Transfer

Scientists often compare the amount of energy available at each trophic level.

Example

Suppose an ecosystem contains:

• Producers: 20,000 kJ
• Primary consumers: 2,000 kJ
• Secondary consumers: 200 kJ
• Tertiary consumers: 20 kJ

Notice that:

• Energy decreases rapidly.
• Far fewer organisms can survive at higher trophic levels.
• Top predators require large hunting territories because so little energy reaches them.

 

 

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Figure 4. Energy decreases at each trophic level because organisms use much of it for life processes.

Why Energy Loss Matters

The decrease in available energy affects ecosystem structure.

It explains why:

• Grasslands contain millions of plants but relatively few wolves.
• Oceans contain vast numbers of phytoplankton but far fewer sharks.
• Large predators require extensive habitats to find enough food.

Energy availability limits the number of organisms that each trophic level can support.

Worked Example

Question

The following organisms form a food chain:

Grass → Grasshopper → Frog → Snake

Identify the trophic level of each organism.

Solution

If the grass contains 5,000 kJ of energy, approximately how much energy would be available to the grasshopper according to the 10% Rule?

Answer: Approximately 500 kJ.

Real-World Connection

Commercial fisheries must consider trophic levels when managing fish populations. Large predators such as tuna and sharks are naturally less abundant because much less energy reaches the highest trophic levels. Overfishing these species can disrupt marine food webs and affect many other organisms within the ecosystem.

Did You Know?

The blue whale, the largest animal ever to live on Earth, feeds mainly on tiny shrimp-like animals called krill. Because krill feed directly on phytoplankton, blue whales feed at a relatively low trophic level, allowing enough energy to support their enormous size.

Key Terms

Apex predator – A predator at the highest trophic level with few or no natural predators.

Consumer – An organism that obtains energy by eating other organisms.

Energy transfer – The movement of energy from one trophic level to another.

Primary consumer – A herbivore that feeds on producers.

Producer – An organism that captures energy from sunlight or chemicals to make its own food.

Secondary consumer – A consumer that feeds on primary consumers.

Tertiary consumer – A consumer that feeds on secondary consumers.

Trophic level – The feeding position an organism occupies in a food chain or food web.

10% Rule – The guideline that, on average, only about 10% of the energy at one trophic level is transferred to the next.

Key Takeaways

• A trophic level is an organism's feeding position in a food chain or food web.
• Producers occupy the first trophic level, followed by primary, secondary, and tertiary consumers.
• Energy decreases at each trophic level because organisms use energy for life processes and lose energy as heat.
• The 10% Rule helps explain why only a small amount of energy is passed to the next trophic level.
• The limited transfer of energy explains why ecosystems contain many producers but relatively few top predators.

5. Energy Pyramids

Introduction

Although food chains show who eats whom, they do not show how much energy is available at each stage. Scientists use energy pyramids to illustrate how energy decreases as it moves through an ecosystem. These diagrams help explain why ecosystems contain many plants, fewer herbivores, and even fewer top predators.

Energy pyramids are one of the most important tools in ecology because they show that the amount of energy available becomes smaller at each trophic level. This decreasing energy supply limits the number of organisms that an ecosystem can support and explains why food chains are usually only four or five trophic levels long.

What Is an Energy Pyramid?

An energy pyramid is a diagram that shows the amount of energy available at each trophic level in an ecosystem.

The pyramid is widest at the bottom because producers contain the greatest amount of energy.

Each higher level becomes narrower because less energy is available.

A typical energy pyramid looks like this:

• Top: Tertiary consumers
• Secondary consumers
• Primary consumers
• Bottom: Producers

The width of each level represents the amount of energy available.

 

 

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Figure 1. An energy pyramid shows that the amount of available energy decreases at each trophic level.

Why Is the Pyramid Wide at the Bottom?

Producers capture energy directly from the Sun through photosynthesis.

Because they receive energy first, producers contain the largest amount of available energy.

Examples of producers include:

• Grass
• Trees
• Algae
• Phytoplankton

All other organisms depend directly or indirectly on the energy stored by producers.

Why Does Energy Decrease?

When organisms consume food, they do not pass all of the stored energy to the next trophic level.

Energy is used for:

• Movement
• Growth
• Reproduction
• Respiration
• Repair of body tissues
• Maintaining body temperature (in mammals and birds)

Much of this energy is eventually released into the environment as heat.

Some energy also remains in:

• Bones
• Hair or fur
• Feathers
• Waste products
• Uneaten body parts

Only a small fraction becomes available to the next trophic level.

 

 

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Figure 2. Most energy is used by organisms or lost as heat, leaving only a small amount to pass to the next trophic level.

The 10% Rule

Ecologists often use the 10% Rule as a simple model of energy transfer.

It states that, on average:

Only about 10% of the energy at one trophic level is transferred to the next trophic level.

Approximately:

• 90% is used or lost.
• 10% becomes available to the next consumer.

Example

This dramatic decrease explains why ecosystems cannot support large numbers of top predators.

 

 

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Figure 3. The 10% Rule illustrates the rapid decrease in available energy between trophic levels.

Interpreting Energy Pyramids

When reading an energy pyramid, remember:

• The bottom contains the most energy.
• The top contains the least energy.
• Each level represents a trophic level.
• The pyramid narrows because available energy decreases.

For example:

If producers contain 50,000 kJ, the approximate energy available to:

• Primary consumers = 5,000 kJ
• Secondary consumers = 500 kJ
• Tertiary consumers = 50 kJ

Energy Availability and Population Size

The amount of available energy influences how many organisms can survive at each trophic level.

Generally:

• Many producers
• Fewer herbivores
• Even fewer carnivores
• Very few apex predators

For example:

A grassland may contain:

• Millions of blades of grass.
• Thousands of grasshoppers.
• Hundreds of frogs.
• Dozens of snakes.
• Only a few hawks.

As available energy decreases, fewer organisms can be supported.

 

 

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Figure 4. As energy decreases, ecosystems can support progressively fewer organisms at higher trophic levels.

Why Food Chains Are Short

Most food chains contain only four or five trophic levels.

If food chains became much longer:

• Very little energy would remain.
• Top consumers would not obtain enough food to survive.

This is why:

• Large predators require extensive hunting territories.
• Apex predators are usually rare.
• Food chains are naturally limited in length.

Energy Pyramid vs Food Chain

Both diagrams are useful and complement one another.

Worked Example

Question

An energy pyramid contains:

• Producers = 40,000 kJ

Using the 10% Rule, estimate the amount of energy available at each trophic level.

Solution

Only about 10% of the energy is transferred from one level to the next.

Real-World Connection

Energy pyramids help farmers, fisheries managers, and conservationists make informed decisions about sustainable resource use. For example, raising herbivorous fish generally requires less energy than raising large predatory fish because fewer trophic levels are involved. Similarly, protecting producers such as forests, grasslands, and phytoplankton is essential because they provide the energy that supports entire ecosystems.

Did You Know?

Although phytoplankton are microscopic, they capture enormous amounts of solar energy every day. They form the foundation of nearly all marine food chains and support some of the largest animals on Earth, including whales. Without these tiny producers, most ocean ecosystems would collapse.

Key Terms

Apex predator – A predator at the highest trophic level with few or no natural predators.

Energy pyramid – A diagram showing the amount of energy available at each trophic level.

Energy transfer – The movement of energy from one trophic level to another.

Primary consumer – A herbivore that feeds directly on producers.

Producer – An organism that captures energy from sunlight or chemicals to make its own food.

Secondary consumer – A consumer that feeds on primary consumers.

Tertiary consumer – A consumer that feeds on secondary consumers.

Trophic level – The feeding position an organism occupies in a food chain or food web.

10% Rule – The guideline that, on average, only about 10% of the energy at one trophic level is transferred to the next.

Key Takeaways

• An energy pyramid shows the amount of energy available at each trophic level.
• Producers form the broad base of the pyramid because they contain the greatest amount of energy.
• Only about 10% of energy is transferred from one trophic level to the next, while the rest is used by organisms or lost as heat.
• The decreasing availability of energy means that higher trophic levels can support fewer organisms.
• Energy loss limits the length of food chains, so most ecosystems have only four or five trophic levels.