Food Web


1. One of the common causes of interaction between organisms is the need for food.


2. Living organisms can be classified into three groups according to their role in the ecosystem:

    (a) Producers

         (i) Producers are organisms that produce their own food from their non-living

        (ii) Green plants are producers. They can produce sugars and starch from carbon
             dioxide and water, using energy from sunlight by a process called photosynthesis.

    (b) Consumers

         (i) Consumers are organisms that eat other organisms or their products.

        (ii) Animals cannot make their own food. They depend on plants or other animals for
             food. Therefore, all animals are consumers.

       (iii) A primary consumer is usually a herbivore and it feeds directly on plants.

       (iv) An animal which feeds on a primary consumer is a secondary consumer.
             Secondary consumers can be carnivores or omnivores. These animals are bigger in

        (v) Tertiary consumers are even larger animals which feed on the secondary consumers.

    (c) Decomposers

         Decomposers are organisms that break down dead animal and plant materials into
         simpler substances which can be used again by green plants. Examples of decomposers
         are bacteria and fungi.


3. The feeding relationship between producer and consumer
organisms can be written in a series of levels called food chain.


4. In a real ecosystem, feeding relationshop among organisms are complex.


5. Most organisms eat more than one type of food. Therefore, food chains are interconnected.


6. When food chains interlink, a food web is formed.


7. Food web helps to maintain a balanced ecosystem by controlling the number of organisms at each level of a food chain.



Pyramid Of Numbers

1. Food relationship between organisms in a food chain can be represented in the form of a pyramid of numbers.


2. A pyramid of numbers is a diagram that shows the relative number of
organisms at each level of a food chain.


3. As you can see in Figure 4.10, there is a progressive drop in the number of organisms from a lower level to a higher level.


4. The animals at the higher levels tend to be larger in size.


Energy Flow In A Food Web And The Pyramid Of Numbers

1. A food chain or a food web also shows the energy transfer from one organism to another.


2. In an ecosystem, the main source of energy is sunlight.


3. Green plants trap and change solar energy into chemical energy which is stored in food during photosynthesis.


4. When a primary consumer eats the producers, some of the chemical energy stored in the plants is passed on to the primary consumer, which would later pass its energy to a secondary consumer.


5. In this way, energy flows through a food web and pyramid of numbers.



6. However, not all of the energy is passed on at each level. Some energy is lost as heat.


7. Food webs are not permanent in any community.


8. Some animals and plants may die. Some animals may leave the community while some may move into the community.


9. When one population in a food web is missing of decreases in number, it will affect the other organisms. The food web becomes imbalanced.


10. If new organisms are introduced into a food web, the food web will become imbalanced too.


11. It would take a long time for the ecosystem to become balanced again.


12. Suppose the snakes in Figure 4.12 move out of the paddy field for some reasons. The number of rats will increase because they have less predators (snake). The number of hawks that feed on the snakes will decrease because there is less food for them.



13. After some time, we can even predict that the large number of rats will eat up most of the paddy plants.


14. As a result, there will be little food left for the rats in the long run. So the rats will eventually have to leave the community or they will die of starvation.


15. Therefore, it is important to maintain a balanced number of organisms at each level of a food chain so that there is sufficient food supply.


16. When one component of the organisms is missing or disturbed, it will affect the balance of the entire ecosystem.