Chapter 7: Food Chains and Food Webs

By the end of this chapter, you will be able to:

a) understand the feeding relationships in an ecosystem, and express them in using food chains, webs and pyramids.

b) appreciate the organisms and processes involved in the carbon dioxide cycle and its role in maintaining the carbon dioxide balance in the atmosphere.

Keywords

carbon cycle,

consumer,

decomposer,

ecological

pyramids,

food chain,

food web,

global warming,

interdependence,

producer,

trophic level.

Introduction

You already know that human beings obtain food from plants and animals. Other animals also obtain food from organisms within their environment; it is only plants that are able to make their own food. Therefore, in nature there is interdependence between different organisms for food.

This is either direct or indirect linkage and can be traced in their feeding relationships which can be represented by food chains, food web, and ecological pyramid of numbers.

In this chapter, you will be able to understand feeding relationships in an ecosystem using food chains, webs and pyramids. You will also appreciate the organisms and processes involved in the carbon cycle and its role in maintaining the carbon balance in the atmosphere.

7.1 Feeding relationships in an ecosystem

Within an ecosystem different living things depend on others for many requirements for survival. One of the requirements is food. This places them in many feeding relationships categorised as food chains and webs.

Activity 7.1 Discussing the meaning of food chain and food web

Key question

What is the meaning of food chain and food web?

What you need •

Computer with internet connection

Pen

Biology textbooks •

Notebook

What to do •

1 In groups, research and discuss the meaning of food chain and food web and the different trophic levels that make food chain and food webs.

2 Make a write-up of your findings and share it with your class.

3 Harmonise your findings and pin up your work on the class noticeboard.

Activity 7.2 Identifying food chains and food webs in an ecosystem

Key question

What food sources do different animals in an ecosystem depend on?

What you need

• Notebook

• Pen

What to do

1 In your group, visit a local ecosystem.

2 Identify and record the organisms in the ecosystem.

3 Draw simple food chains showing the feeding relationships among selected organisms in the ecosystem.

4 Develop the food chains into a food web.

5 Identify the trophic levels of the organisms observed.

6 Share your report with the rest of the class.

7 Pin a copy of your final report on the class noticeboard.

7.2 Ecology pyramids

These are diagrammatic representations of the amount of the energy, biomass and numbers at each trophic, level of a food chain. The ecological pyramid is of three types; pyramid of energy, pyramid of biomass and pyramid of numbers. A pyramid of numbers is a representation of the numbers of organisms at different trophic levels in a food chain. It is used to demonstrate the fact that in most food chains the number of individuals in each link decrease as one progresses along the chain.

This is a true reflection of the decrease in energy at each trophic level due to loss during transfer. In this activity you are to construct a pyramid of numbers and explain its appearance.

Activity 7.3 Constructing a pyramid of numbers

Key question

How can numbers of organisms be used to construct a pyramid of numbers?

What you need Data on numbers of organisms in a grassland ecosystem as given in the Ta

Notebook Pen

Table 7.1 shows the approximate numbers of organisms found in an ecosystem

What to do

1 Place the organisms under the following categories as producers, primary consumers, secondary consumers.

2 Calculate the number of organisms in each trophic level.

3 Use the numbers to construct a pyramid.

4 Explain the appearance of the pyramid of numbers constructed.

5 In your notebook record the conclusions agreed upon.

7.4 Carbon cycle

The carbon cycle is a series of events in which carbon in the form of carbon dioxide and organic carbon is interchanged between the living and non- living components of the atmosphere. In the next activity, you are going to research on carbon cycle and its role in maintaining carbon dioxide balance in the atmosphere.

7.4 Explaining the carbon cycle and its role in maintaining carbon dioxide balance in the atmosphere

Key question

What are the role of processes and organisms in the carbon cycle and its role in maintaining carbon dioxide balance in the atmosphere?

What you need

• Biology textbook/Computer with internet connection

Manila paper

Notebook Pen

What to do

1 In groups, discuss and label the process in the carbon cycle.

2 Explain the role of the process and organisms in the carbon cycle.

3 Discuss the role of the carbon cycle in maintaining carbon dioxide balance in the atmosphere.

4 Present your conclusions to the rest of the class.

Chapter Summary

In this chapter, you have learnt that:

organisms in nature show interrelationship through feeding. This can be represented in form of food chains, food webs and ecological pyramids.

the carbon cycle is important in maintaining the carbon dioxide balance in the atmosphere.

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