(a) What is meant by the term Food chain? (b) Draw a food chain involving four trophic levels which can be found in a terrestrial habitat. (c) Explain (i) t...

(a) Meaning of the term "Food chain"

A food chain is a linear feeding relationship among the organisms in a habitat, showing the transfer of food energy in one direction from the producer (green plant) through a series of organisms, each of which feeds on the one before it (the preceding organism) and in turn provides food for the one after it (the succeeding organism). The energy passes from one trophic level to the next along a straight line.

(b) A food chain with four trophic levels in a terrestrial habitat

In this chain:

• Grass – producer (1st trophic level)
• Grasshopper – primary consumer / herbivore (2nd trophic level)
• Lizard – secondary consumer / carnivore (3rd trophic level)
• Hawk – tertiary consumer / top carnivore (4th trophic level)

The arrows point from the organism that is eaten to the organism that eats it, showing the direction in which food energy flows.

(c)(i) Flow of energy through the food chain

The energy flow is unidirectional (non-cyclic). The original source of energy is the Sun. Light energy from the sun is absorbed by the chlorophyll in the grass and used in photosynthesis to build carbohydrates, so light energy is converted into chemical energy stored in the plant's tissues. When the grasshopper feeds on the grass, this chemical energy is passed on to it; when the lizard eats the grasshopper the energy is passed along to the lizard; and finally to the hawk when it eats the lizard. Thus the chemical energy is passed down the chain in one direction only from producer to top consumer; at each level the energy is either utilised (for life activities) or stored in body tissues, and it is not recycled back along the chain.

(c)(ii) How energy in the chain is lost to the environment

Energy is lost at each trophic level, so that only a small fraction (about 10 percent) is passed on to the next level. Energy is lost in the following ways:

• Not all parts of the organism are eaten (roots, bones, uneaten and undigested parts), so not all the energy at a level is consumed by the next.
• Energy is released as heat during respiration.
• Energy is used up in movement, growth and other metabolic (life) activities.
• Energy is lost in excretory products and faeces (egestion and excretion).
• Energy in dead bodies and uneaten remains passes to decomposers rather than to the next consumer.

Because of these repeated losses, the amount of usable energy decreases sharply along the chain, which is why a food chain rarely has more than four or five trophic levels.