1. Food chains and food webs
Ena Haniff

2. Food Chains
A food chain is a simple sequence which shows the flow of energy through an ecosystem (in the form of food)
Energy from the sun is converted to plant matter by producers during the process of photosynthesis
Energy is transferred as the nutrients in food
Each organism is a link and each link is called a feeding level or “trophic level”

3. Food Chains
A food chain can be represented by a simple diagram as follows:
Grass Deer Tiger
Arrow denotes “ is eaten by”
Arrows are used to show the flow of energy from one trophic level to another

4. Food Chains Feeding relationships are shown in a food chain
Food chains begin with a plant or producer (usually)
All other organisms in a food chain are animals or consumers

5. Questions How is the sun’s energy transferred to plants?
Plants use the sun’s energy during photosynthesis to produce carbohydrates which is sugar and is a store of energy
How do animals (consumers) obtain their energy?
Animals obtain their energy by eating plants and other animals and use the energy from the carbohydrates. They also build more complex substances (such as proteins) which also stores the energy they obtained from producers

6. Questions Why do organisms need energy?
Organisms need energy to carry out all the functions of living things:
Growth
Repair
Reproduction
Movement (animals)

7. Activity
From the list of organisms, choose three suitable organisms and construct a food chain.
Crab, butterfly, shark, fish, cat, snake, grass, bird, rat, flower, octopus, pig, tiger, deer, lettuce, cow, human, seaweed,
You can construct more than one food chain
Can you spot the producers?
Which organisms are herbivores?
Omnivores?
Carnivores?

8. Activity Producers Consumers Herbivores Omnivores Carnivores
Grass, flower, lettuce, seaweed
Crab, butterfly, shark, fish, cat, snake, bird, rat, octopus, pig, tiger, deer, cow, human
Herbivores
Omnivores
Carnivores
Butterfly, deer, cow
Pig, human, bird, crab, fish, rat,
Shark, snake, tiger, octopus

9. A little more on producers and consumers
Producers make their own food using the sun’s energy
They include green plants, mosses, aquatic plants, phytoplankton in water and certain bacteria
Phytoplankton are microscopic algae that live in water that use photosynthesis to produce their own food and which forms the base of marine food chains

10. A little more on producers and consumers
Consumers cannot make their own food and need to eat other organisms to obtain energy
There are many types of consumers in a food chain:
Primary
Secondary
Tertiary
Quaternary

11. A little more on producers and consumers
Type of consumer
Symbol
Description
Primary 1°
Organisms that consume producers (green plants). They are the first set of consumers and they occupy the 2nd trophic level
Secondary 2°
Organisms that consume primary consumers. The are the 2nd set of consumers and they occupy the 3rd trophic level
Tertiary 3°
Organisms that consume secondary consumers. They are the 3rd set of consumers and they occupy the 4th trophic level
Quaternary 4°
Organisms that consume tertiary consumers. They are they 4th set of consumers and they occupy the 5th trophic level.

12. An example Grass Deer Human Tiger
In this food chain, the grass is the producer.
The Deer is the primary consumer
Can you identify the secondary and tertiary consumers?
Human: Secondary
Tiger: Tertiary
Can you name the 2nd feeding or trophic level?
The deer

13. An unusual food chain
Recall that in the beginning of the lesson I said that food chains usually begin with producers? Well, here is an exception:
Leaf litter Earthworm Bird
In this food chain, a producer is not at the beginning.
This food chain begins with detritus (decaying organic matter)
The Earthworm is a detritivore here (decomposer)
It fills the role of returning organic material to inorganic material where it can be used again by plants

14. Food webs
Ena Haniff

15. Introduction
In the pop up chart, what would happen if I introduced a bird, a rabbit and a snake into the Jungle?

16. Food webs In reality, organisms have more than one source of food
Organisms can be a link in more than one food chain
For example the tiger can eat both the deer and the rabbit
And plants are eaten by both the deer and the butterfly
This makes the feeding interactions more complicated

17. Food webs

18. Activity From the food web shown
Construct a food chain with three links and one with four links

19. Food web A food web shows a network of interconnected food chains
It shows the transfer of energy through an ecosystem
It uses arrows to show the transfer of energy

20. Question
What would happen if all the rats were suddenly removed from this food web?

21. Activity

22. Possible results
There will be an explosion in the populations of all the organisms that the rat feeds on including grasshoppers, caterpillars, grubs and beetles
These are herbivores; they will consume more plants
Food supplies could be affected
The amount of oxygen produced and carbon dioxide removed from the air could be affected
The owl population would be affected because rats were half of their food supply

23. Equilibrium
Predators limit the number of prey in an ecosystem and prey limit the number of predators as well.
There is a delicate balance in an ecosystem that can be easily upset if the population of the various organisms are disturbed
An example: swarms of locusts happen under conditions which are favourable to their reproduction; and it wreaks havoc on the food crops when this occurs

24. Assessment activity
Using any of the organisms provided, construct a simple food web.
Crab, butterfly, shark, fish, cat, snake, grass, bird, rat, flower, octopus, pig, tiger, deer, lettuce, cow, human, seaweed