Energy Flow in Ecosystems

Understanding how energy moves through food chains, food webs, and trophic levels

CAPS Grade 10 Life Sciences

Energy in an ecosystem starts with the sun and moves from one organism to another. Learners need to know how food chains, food webs, and trophic levels show this movement of energy.

1. The Source of Energy

The Sun is the primary source of all energy in an ecosystem. It provides radiant energy, which is captured by producers, mainly plants, through the process of photosynthesis.

During photosynthesis, radiant energy is converted into chemical potential energy stored in glucose. This transformation is the foundation of energy flow in ecosystems, as it initiates the food chain.

Sun → Radiant Energy → Photosynthesis → Chemical Energy (Glucose) in Plants

Figure 1: Energy flow from the sun to producers

2. Food Chains

A food chain is a simple linear sequence that illustrates who eats whom in an ecosystem. The format of a food chain can be represented as follows:

Producer (Plants) → Primary Consumer (Herbivore) → Secondary Consumer (Carnivore) → Tertiary Consumer (Top Predator)

In a food chain, arrows are used to represent the direction of energy flow, pointing towards the "eater." This visual representation helps to clarify the relationships between different organisms and how energy is transferred from one level to another.

Example Food Chain:

Grass → Zebra → Lion

3. Food Webs

Food webs are more complex than food chains, consisting of interconnected food chains that illustrate the various feeding relationships within an ecosystem. In reality, most animals consume multiple types of food, making food webs a more accurate representation of ecological interactions.

Martial Eagle
↑ ↑ ↑
Snake Lion
↑ ↑ ↑
Mouse Zebra Rabbit
↑ ↑ ↑
Grass Shrubs Seeds

Key idea: If one species disappears, some organisms may still survive because they can feed on something else. That is why food webs are usually more stable than simple food chains.

Quick Check: Food Chain Levels

Question 1/4

1. In a food chain, what does the arrow represent?

AWho eats whom
BDirection of energy flow
CBoth A and B
DSize of the organism

4. Trophic Levels

Trophic levels refer to the position an organism occupies in a food chain. The levels are categorized as follows:

Trophic Level 4
Tertiary Consumers (Top Predators)
Trophic Level 3
Secondary Consumers (Carnivores/Omnivores)
Trophic Level 2
Primary Consumers (Herbivores)
Trophic Level 1
Producers (Plants)

Each trophic level shows where an organism gets its energy. Learners should use these levels to explain feeding relationships clearly.

5. Ecological Pyramids

Ecological pyramids are graphic representations that illustrate the relationships between trophic levels. There are three main types of ecological pyramids that are important to understand:

Pyramid of Numbers

Shows the total number of individual organisms at each trophic level. It can sometimes be inverted; for example, a single tree may support many insects.

Pyramid of Biomass

Represents the total dry organic mass of organisms at each trophic level, providing insight into the amount of living material available at each stage.

Pyramid of Energy

Displays the total amount of energy available at each trophic level. Unlike the other pyramids, the pyramid of energy is never inverted, as energy diminishes at each successive level.

Key Point

Pyramid of Energy is always upright - energy decreases at each trophic level.

6. Energy Loss (The 10% Rule)

One of the critical concepts in understanding energy flow is the 10% Rule. This rule states that only about 10% of the energy at one trophic level is passed on to the next level.

Producers (10,000 J) 100%
Primary Consumers (1,000 J) 10%
Secondary Consumers (100 J) 1%
Tertiary Consumers (10 J) 0.1%

The remaining 90% of energy is lost primarily as heat during respiration, through movement, or in undigested waste (faeces).

This significant energy loss explains why food chains are rarely longer than four or five links. There simply isn't enough energy available to support additional levels, which limits the complexity of food chains and webs.

Matching Game: Trophic Levels

Match each trophic level with its correct description and example.

Matches: 0/4
Trophic Levels
Trophic Level 1
Trophic Level 2
Trophic Level 3
Trophic Level 4
Descriptions & Examples
Producers - Plants, algae
Primary Consumers - Herbivores (Zebra)
Secondary Consumers - Carnivores (Lion)
Tertiary Consumers - Top predators (Martial Eagle)

True or False Quiz

1. The sun is the primary source of energy for most ecosystems.

2. In a food chain, arrows point away from the eater.

3. The pyramid of energy can sometimes be inverted.

4. According to the 10% rule, only about 10% of energy is transferred to the next trophic level.

5. Food chains are typically longer than 5 or 6 links because energy is abundant.

Fill in the Blanks

1. The primary source of energy in an ecosystem is the _______________.

2. Plants convert radiant energy into chemical energy through the process of _______________.

3. According to the _______________ rule, only about 10% of energy is transferred between trophic levels.

4. A _______________ is a complex network of interconnected food chains.

Exam Tip

Exam questions often ask what will happen if one organism is removed from a food web. For example, if frogs die out, the grasshopper population may increase because one of its predators is gone.

Key Terms

Producer Consumer Food chain Food web Trophic level Pyramid of numbers Pyramid of biomass Pyramid of energy 10% rule Photosynthesis Primary consumer Secondary consumer Tertiary consumer

What To Remember

Energy starts with the sun and moves through producers to consumers. The most important exam idea is that only a small part of the energy moves to the next trophic level.