Chapter 3 : The web of Life
Food is an important factor affecting the survival of organisms.
The populations in a habitat are linked by their food. These links are called food chains.
Food chains can be interconnected to form food webs.
Pupils trace the energy conversions from the Sun through a food chain. They are
introduced to the main components of food webs - producers, consumers, decomposers,
predator, prey and the roles these play in food chains and webs.
Pupils will be exposed to food webs around us (garden, marine and rainforest food webs)
and learn more about the organisms in these habitats.
A food chain consists of a series of organisms, each depending on the next for food.
 The source of energy for all food chains is the Sun.
 Energy conversion: light energy  chemical energy (plant)  chemical energy
(herbivore)  chemical energy (carnivore)  chemical energy (carnivore’s body)
 Not all chemical energy is transferred to the consumer’s body (e.g. some energy is
lost as heat from the organism’s body)
 Most food chains have around 3-4 links.
 A food web consists of several food chains interconnected or linked together.
Producers
 They begin any food chain as they are able to photosynthesise - trapping light energy
from the Sun and converting it into chemical energy.
 Examples of producers - green plants, algae, plant plankton
Consumers
 They cannot make their own food.
 Consumers which eat plants only are called herbivores (e.g. grasshoppers, caterpillars,
beetles)
 Consumers which eat other animals only are called carnivores (e.g. tigers, eagles,
sharks)
 Consumers which eat both plants and animals are called omnivores (e.g. orang utans,
humans)
Decomposers
 Fungi and bacteria are decomposers and their role is to recycle nutrients.
 They obtain their energy for survival by breaking down dead matter such as the
remains of plants and animals, and wastes from animals. Decomposers are also called
saprophytes.
 The breaking down or decaying process releases nutrients trapped in them. The
nutrients are natural fertilisers. Once the nutrients are returned to the soil, they are
taken up by plants. Hence the nutrients are recycled.
 Note that not all fungi are saprophytic. Some fungi grow on living things that are still
alive.