Energy Flow, Ecological Pyramids of Energy, Numbers and Biomass for SS 1

Class: SS 1
Term: Second Term
Week: 6
Age: 15 years
Duration: 60 minutes
Subject: Biology
Curriculum Theme: Society and Environmental Biology
Focal competence: Illustrating feeding relationships in an ecosystem
Key competencies/values: Critical Thinking; Environmental Literacy
Skills:

• Measuring ecological factors in an environment

Previous Lesson: Feeding Relationships, Food Webs and Trophic Levels
Topic: Feeding Relationships Among Organisms: Energy Flow
Subject Matter: Energy flow, Pyramid of, Energy, Numbers, Mass

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Identify organisms at different trophic levels.
• Explain how energy flows in an ecosystem.
• Explain why energy decreases across trophic levels.

Affective Domain

• Appreciate the interconnectedness of organisms in an ecosystem.
• Recognise the importance of energy transfer for life processes.

Psychomotor Domain

• Draw and label simple food chains.
• Draw a food web based on given organisms.

Social Domain

• Collaborate effectively in group activities to classify organisms.

Reference Materials

The following resources were used in planning this lesson:

• 2025 New Revised Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Biology textbook for Senior Secondary 1
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)

Instructional Materials

The teacher will teach this lesson with the aid of:

• Charts/posters of food chains
• Diagrams/charts of food webs
• Energy flow charts
• Cardboard papers
• Markers
• Pictures of various organisms

Rationale for the Lesson

This lesson is important for understanding how living organisms obtain and transfer energy within their environment. It helps students grasp the fundamental principles that govern the structure and function of ecosystems. Knowledge of energy flow and ecological pyramids provides insight into the delicate balance of nature and the impact of human activities on these relationships.

Prerequisite/Previous Knowledge

Students should have a basic understanding of living organisms, their habitats, and simple feeding habits from previous lessons.

Lesson Content/Board Summary

Feeding Relationships Among Organisms: Energy Flow

Energy Flow in an Ecosystem

Energy flow refers to the movement of energy through an ecosystem, typically from the sun to producers, then to consumers, and finally to decomposers. The primary source of energy for almost all ecosystems is the sun.

1. Producers (Autotrophs): These are organisms, mainly green plants, that produce their own food using sunlight through photosynthesis. They form the base of the food chain.
2. Consumers (Heterotrophs): These organisms obtain energy by feeding on other organisms.
   1. Primary Consumers (Herbivores): Feed directly on producers (e.g., grasshoppers, rabbits).
   2. Secondary Consumers (Carnivores/Omnivores): Feed on primary consumers (e.g., frogs, snakes).
   3. Tertiary Consumers (Top Carnivores/Omnivores): Feed on secondary consumers (e.g., eagles, lions).
3. Decomposers: These are organisms like bacteria and fungi that break down dead organic matter from all trophic levels, returning nutrients to the soil for producers (e.g., bacteria, fungi).

Food Chain

A food chain illustrates a single pathway of energy flow, showing who eats whom. It starts with a producer and moves through various consumers.

Example:

Grass (rightarrow) Grasshopper (rightarrow) Frog (rightarrow) Snake (rightarrow) Eagle

Food Web

A food web consists of several interconnected food chains in an ecosystem. It shows the complex feeding relationships and multiple pathways of energy flow, reflecting the reality that most organisms feed on, and are fed upon by, more than one type of organism.

Example:

In a typical forest food web, a rabbit might eat grass, while a fox might eat the rabbit. However, the fox might also eat mice, and the mice might eat seeds. An owl might eat both mice and snakes, and snakes might eat frogs. This interconnectedness forms a web.

Trophic Levels

Trophic levels are the feeding positions in a food chain or food web. Each level represents the position an organism occupies in the sequence of energy transfer.

1. First Trophic Level: Producers (e.g., plants, algae).
2. Second Trophic Level: Primary consumers (herbivores) (e.g., deer, grasshoppers).
3. Third Trophic Level: Secondary consumers (carnivores/omnivores that eat herbivores) (e.g., foxes, frogs).
4. Fourth Trophic Level: Tertiary consumers (carnivores/omnivores that eat secondary consumers) (e.g., eagles, lions).

Why Energy Decreases Across Trophic Levels

Energy decreases significantly at each successive trophic level due to the following reasons:

1. Metabolic Activities: Organisms use a large portion of the energy they consume for their own life processes such as respiration, movement, growth, and reproduction. This energy is lost as heat to the environment.
2. Incomplete Consumption: Not all parts of an organism are consumed by the next trophic level (e.g., bones, fur, roots).
3. Indigestible Matter: Not all consumed matter is digested and assimilated. A significant portion is egested as waste.
4. Heat Loss: Energy is lost as heat during metabolic processes, following the second law of thermodynamics. Only about 10% of the energy from one trophic level is transferred to the next; the remaining 90% is lost. This is known as the “10% Law.”

Ecological Pyramids

Ecological pyramids are graphical representations that show the relationship between different trophic levels in an ecosystem. They can be pyramids of energy, numbers, or biomass.

Pyramid of Energy

This pyramid shows the total amount of energy at each trophic level. It is always upright because energy decreases at each successive level, with the largest amount of energy at the producer level.

Structure: Broad base (producers) tapering upwards (primary, secondary, tertiary consumers).

Example: A large amount of energy in grass supports a smaller amount of energy in grasshoppers, which in turn supports an even smaller amount of energy in frogs, and so on.

Pyramid of Numbers

This pyramid shows the number of individual organisms at each trophic level.

1. Upright Pyramid: Common in grassland or pond ecosystems where producers are numerous and support fewer primary consumers, which support even fewer secondary consumers.

   Example: Many blades of grass (rightarrow) fewer grasshoppers (rightarrow) even fewer frogs.

2. Inverted Pyramid: Occurs when a single large producer supports many smaller primary consumers.

   Example: One large tree (rightarrow) many insects feeding on it.

3. Spindle-shaped Pyramid: Occurs when a few producers support a larger number of primary consumers, which in turn support a smaller number of secondary consumers.

   Example: Few trees (rightarrow) many birds feeding on fruits (rightarrow) fewer predatory birds.

Pyramid of Biomass (Mass)

This pyramid shows the total dry weight (biomass) of organisms at each trophic level.

1. Upright Pyramid: Typical for most terrestrial ecosystems where the biomass of producers is greater than that of consumers.

   Example: Large biomass of plants (rightarrow) smaller biomass of herbivores (rightarrow) even smaller biomass of carnivores.

2. Inverted Pyramid: Common in some aquatic ecosystems where producers (phytoplankton) have a very short lifespan and reproduce rapidly, but their total biomass at any given time is less than that of the longer-lived primary consumers (zooplankton).

   Example: Small biomass of phytoplankton (rightarrow) larger biomass of zooplankton (rightarrow) smaller biomass of fish.

Teaching Methods/Instructional Techniques

Discussion, Explanation, Question and Answer, Group Work, Demonstration, Guided Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Questioning/Recall

Teacher’s Activity: The teacher asks students about what they eat and what those organisms eat, leading to a discussion on feeding relationships. The teacher then introduces the topic: Energy Flow and Ecological Pyramids.

Pupils’ Activity: Pupils respond to questions and participate in the discussion.

Learning Point: Basic feeding relationships

Step 2: Energy Flow and Trophic Levels

Time: 10 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains the concept of energy flow, starting from the sun, through producers, consumers (primary, secondary, tertiary), and decomposers. The teacher introduces and explains trophic levels using a chart of a food chain.

Pupils’ Activity: Pupils listen, ask questions, and identify organisms at different trophic levels from the chart.

Learning Point: Energy flow concept

Step 3: Food Chains and Food Webs

Time: 10 minutes

Teaching Skill: Demonstration/Guided Practice

Teacher’s Activity: The teacher draws a simple food chain on the board and guides students to draw and label their own simple food chains. The teacher then presents a list of organisms and guides students to draw a food web based on these organisms, highlighting the interconnectedness.

Pupils’ Activity: Pupils draw food chains and food webs in their notebooks, identifying the organisms and their feeding relationships.

Learning Point: Drawing food chains/webs

Step 4: Why Energy Decreases

Time: 5 minutes

Teaching Skill: Explanation/Reasoning

Teacher’s Activity: The teacher explains the reasons why energy decreases across trophic levels, focusing on metabolic activities, incomplete consumption, indigestible matter, and heat loss (the 10% law).

Pupils’ Activity: Pupils listen and ask questions to clarify their understanding of energy loss.

Learning Point: Reasons for energy decrease

Step 5: Introduction to Ecological Pyramids

Time: 5 minutes

Teaching Skill: Explanation/Visualisation

Teacher’s Activity: The teacher introduces the concept of ecological pyramids as graphical representations of trophic levels and their relationships, mentioning the three main types: energy, numbers, and biomass.

Pupils’ Activity: Pupils listen and mentally prepare for the different types of pyramids.

Learning Point: Ecological pyramid concept

Step 6: Types of Ecological Pyramids (Group Activity)

Time: 10 minutes

Teaching Skill: Group Work/Facilitation

Teacher’s Activity: The teacher divides students into groups and provides each group with cardboard paper and markers. The teacher guides students to sketch energy flow charts and pyramid templates. Each group is assigned to illustrate one type of pyramid (energy, numbers, or biomass) with appropriate examples, explaining its shape (upright, inverted, spindle-shaped) and why.

Pupils’ Activity: Pupils work in groups to sketch and discuss the different types of ecological pyramids, assigning organisms to appropriate trophic levels and explaining the shapes.

Learning Point: Types of ecological pyramids

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Identify the producers in a given food chain.
2. Explain how energy flows from producers to primary consumers.
3. Draw a simple food chain involving a plant, a goat, and a human.
4. Why does the amount of energy decrease as you move up a food chain?
5. Describe the difference between a pyramid of numbers and a pyramid of biomass.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding energy flow

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on energy flow, food chains, food webs, trophic levels, and ecological pyramids into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson notes

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Consolidation

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reinforcing the concept that energy flows through ecosystems in a structured way, diminishing at each step, and that ecological pyramids illustrate these relationships. The teacher encourages students to observe feeding relationships in their environment.

Pupils’ Activity: Pupils listen and ask any final questions.

Learning Point: Consolidating energy flow concepts

Continuous Assessment/Further Study

Type: Homework/Further Reading

Instruction: Answer the following questions in your biology notebook:

1. Draw a food web for a typical pond ecosystem, including at least two producers, three primary consumers, two secondary consumers, and one tertiary consumer.
2. Explain the “10% Law” of energy transfer in an ecosystem.
3. Research and give an example of an ecosystem where the pyramid of biomass is inverted. Explain why it is inverted.
4. List five organisms and identify their trophic level in a typical terrestrial ecosystem.

Lesson Keywords

• Energy Flow – The movement of energy through an ecosystem.
• Food Chain – A linear sequence showing who eats whom.
• Food Web – Interconnected food chains in an ecosystem.
• Trophic Levels – Feeding positions in a food chain.
• Producers – Organisms that make their own food.
• Consumers – Organisms that feed on other organisms.
• Decomposers – Organisms that break down dead organic matter.
• Ecological Pyramids – Graphical representations of trophic levels.
• Pyramid of Energy – Shows energy content at each trophic level.
• Pyramid of Numbers – Shows the number of individuals at each trophic level.
• Pyramid of Biomass – Shows the total dry weight of organisms at each trophic level.

Differentiation

For students who grasp concepts quickly, encourage them to research the impact of human activities (e.g., deforestation, pollution) on energy flow and ecological pyramids. For students needing additional support, provide simplified diagrams and extra guidance during group activities, focusing on identifying basic producers and consumers before moving to complex food webs.

Suggested Lesson Videos

Search on YouTube for: “Energy flow in ecosystems SS1 Biology” or “Ecological pyramids explained for secondary school”