A functioning ecosystem is made up of the living and non living components which not only interract among themselves but are closely linked by various biological, chemical and physical processes.

The biotic (living) component of an ecosystem comprises of the Autotrophs but the abiotic (non living) component consists of the non living materials like carbon dioxide, oxygen, mineral salts and water which are inorganic materials, food substances which are organic materials and energy.

In a given ecosystem, energy and nutrients are transfered sequentialy (step by step) among organisms in a feeding pathway (trophic level). This feeding pathway always begins with primary producers which are Autotrophs, followed by one or more consumers then ends with decomposers. A linear feeding relationship among organisms in the same community in which each organism feeds on the one before it in the sequence is called a food chain. In nature, there are more complex feeding relations than food chain shows. A complex feeding relationship consisting of a network of interrelated food chains is called food web. Each level in a feeding path way is called trophic level. A given feeding relationship (food chain or food web) shows energy and nutrients transfer from organism to organism in a feeding pathway.

Energy flow and energy changes

In an ecological system, energy source is usually from the sun .Energy flow takes place unidirectionally starting from the producer e.g.. grass which traps energy from the sun. st this energy flows right from the producer (1 trophic level) to the secondary or tertiary consumer. But as it flows it is only a certain amount of the original energy that is transferred from one trophic level to the next. Part of the energy in a given trophic level is lost as heat energy during the orgainsms metabolic activities. Thus we say that there is an inefficient energy transfer process from the producer to the tertiary consumer which is in line with the laws of thermodynamics.

This means that for the survival of the biotic component of an ecosystem, there is need for a constant supply of energy from outside it. This outside energy source is the sun. Without this energy source life on earth will cease.

Ecological Pyramids

Feeding relationships in an ecosystem can be represented graphically using ecological pyramid of numbers, pyramid of biomass and pyramid of energy.

Pyramid of numbers: The pyramid of numbers represents the number of individuals in a given (each) trophic level of a food chain at a given time. The length of each bar in the pyramid gives appoximate measure of the number of individuals at that particular trophic level. Generally in most ecosystems the number of individual organism decreases up the trophic level (from the producer)in the food chain while the size of the individuals increases. However equal status accorded to each individual without mindng their great variations in sizes and wide ranges in figure between producers and consumers introduces odd-shape pyramids of numbers. These bring in a limitation and disadvantage to the method.

Pyramid of biomass: This represents the sum of the wet or dry masses of the organisms in each trophic level. Biomass gives a more accurate representation of the relatioships between the organisms at the various trophic levels in the food chain than a pyramid of numbers. This can be bacause biomass takes both the size and the numbers into consideration. Inconsistences that can arise due to equal status accorded to small sized producers with short life span and large sized consumers with long life span introduces odd shaped biomass.

Pyramid of energy: The rate of energy flow through each trophic level of a food chain can be represented with pyramid of energy. The pyramid of energy gives the best way of representing relationship between organisms in the various trophic levels of a food chain. Hence there is always a great loss of useful energy as food is transferred from one trophic level to the next one up, Apyramid of energy is always upright.

Practical Activities

1. A decay process yields carbondioxide, hydrogen sulphide and ammonia. Write briefly one after the other, how you can identify each of these gases that occur during a decay process.

The diagram above is an illustration of a community. Draw two possible food chains that occur in the community.

(4a) Write three habitat you can identify in the diagram (i).

(b) What is the trophic level of the toad in the community

General Questions

1. The role of an autotroph in a food chain is

2. What name can you call a unit which comprise of a living community and its physical factors

3. Every food chain must begin with a

4. An ecological describes the occupation of an organisms in an ecosystem.

5. What is the ultimate source of energy in a food chain

6. A unit of the biosphere in which organisms interract with themselves and their physical environment is called

7. is a measure of the total mass of the dry organic matter produced in a given area.

8. A can be used to graphically illustrate the transfer of energy in a food chain.

9. In a functional ecosystem, pyramid of numbers, energy flows, mineral salts in the soil, humus and trophic level are associated with food chain or food web except

10. In the food chain illustrated here:
Guinea grass → grasshopper → Toad → snake. Which of the organisms is the secondary consumer and what trophic level does it belong

11. Examine the food chain drawn here:
Diatom → paramecium → water flea → fish.
What type of habitat can this food chain be found? What is the role of the diatom in the food chain?

12. A complex feeding relationship which consist of interrelated food chains is called

13. Identify the diagram in figure 11.2

State two limitations in using the diagram to represent food relationships as answer to questions 14 and 15
14.

15.

16. Energy transformations in nature are in line with the first and second laws of

17. Name three nutrients cycling in nature

18. Write the function of nitrogen fixing bacteria during Nitrogen cycle

19. Which of the ecological pyramids gives the best way of representing relationship between organisms in the various levels of a food chain.

20. The rate at which primary producers manufacture food per unit area per unit time is called