© 2009 Marshall Cavendish International (Singapore) Private Limited

Questions to think about… What are the roles of various organisms? What is the source of energy? What is a food chain and a food web? © 2009 Marshall Cavendish International (Singapore) Private Limited

Energy from the Sun All living things depend directly or indirectly on the Sun for energy. Plants need light from the Sun to make food through photosynthesis. Some animals get their energy from these plants. Other animals get their energy from eating these plant-eating animals. Without the Sun, life cannot exist. © 2009 Marshall Cavendish International (Singapore) Private Limited

Producers Plants are the only organisms that can use energy from the Sun to make their own food. Plants are called producers as they are able to produce their own food. © 2009 Marshall Cavendish International (Singapore) Private Limited

Consumers Animals cannot produce their own food. They eat plants and other organisms to get the energy they need. Animals are thus called consumers. © 2009 Marshall Cavendish International (Singapore) Private Limited

Consumers Animals eat plants only herbivores eat animals only eat plants and animals carnivores omnivores © 2009 Marshall Cavendish International (Singapore) Private Limited

Predators and prey Animals that hunt and feed on other animals are called predators. Animals that are being hunted and eaten are called prey. All herbivores are prey. All carnivores and omnivores are predators, however, they can also be the prey of other carnivores and omnivores. © 2009 Marshall Cavendish International (Singapore) Private Limited

Predators and prey plant herbivore or omnivore carnivore or omnivore carnivore or omnivore © 2009 Marshall Cavendish International (Singapore) Private Limited

Predators and prey Predators often kill the weak and sick animals. By killing the prey, predators control the number of prey in the community. If the number of prey gets too big, there will not be enough food for all the prey to feed on. a hawk eating its prey © 2009 Marshall Cavendish International (Singapore) Private Limited

Predators and prey The number of predators in a community is in turn dependent on the number of prey. When the number of prey decreases, predators will have less food. Thus, their numbers will decrease. © 2009 Marshall Cavendish International (Singapore) Private Limited

Food chains When a plant is eaten by an animal which is in turn eaten by another animal, we have a food chain. A food chain shows the food relationship between different organisms. An example: plantconsumer eaten by plantcaterpillarbird © 2009 Marshall Cavendish International (Singapore) Private Limited

Food chains The caterpillar eats the leaf of the plant, and the bird eats the caterpillar. In a food chain, energy is transferred from one organism to another. The energy stored in the plant is passed on to the caterpillar, and then to the bird. plantconsumer eaten by plantcaterpillarbird © 2009 Marshall Cavendish International (Singapore) Private Limited

Food webs An organism usually eats more than one type of food. An organism can also be eaten by more than one type of organism. Thus, an organism can belong to more than one food chain. In a community, food chains are linked together to form a food web. © 2009 Marshall Cavendish International (Singapore) Private Limited

Food webs An example: aphid plant fruit fly bee caterpillar ladybird spider bird © 2009 Marshall Cavendish International (Singapore) Private Limited

Food webs There are food webs in other communities too! Here is an example of a food web in a forest community. squirrel grass deer tiger fox rabbit owl fruit © 2009 Marshall Cavendish International (Singapore) Private Limited

Food webs Here is an example of a food web in a mangrove swamp community. monkey frog mangrove plant fish crocodile snake insect kingfisher © 2009 Marshall Cavendish International (Singapore) Private Limited

Keeping a balance In a food chain or food web, the populations of all the producers and consumers are kept in balance. plant caterpillarbird For example, what will happen if many of the caterpillars in the food chain above are killed? The birds will have less food. Some birds will starve to death. So the number of birds in the population decreases. When the number of birds decreases, the number of caterpillars will increase because there are fewer birds to eat them. With more caterpillars, more plants will be eaten up, leading to less food for the caterpillars. So the number of caterpillars will decrease with time. From this example, we can see that when there is an increase or decrease in the number of a predator or prey, the number of other organisms in the food chain will be affected. Eventually, the number of all organisms in the food chain stabilises as a balance is reached. © 2009 Marshall Cavendish International (Singapore) Private Limited

What happens when we remove a population of organisms from a community? A group of farmers discovered that sparrows were eating up a lot of their crops. So they tried to kill all the sparrows in their village. With the killing of large numbers of sparrows, insects that used to be sparrows’ prey grew and multiplied. The farmers’ efforts failed because eventually, the insects ate a far larger portion of their crops than the sparrows did. Thus, removing a population from a community is not always a good thing! Let us examine a case study. © 2009 Marshall Cavendish International (Singapore) Private Limited

What happens when we add a population of organisms to a community? In 1869, a few gypsy moth eggs were brought to North America for the first time from Europe. These silk moths were in demand as they are able to produce better silk. But these moths turned out to be poor silk producers, and some escaped from where they were housed. What can happen if the opposite occurs? Let us take a look at another example. A hundred years later, the moths had spread throughout the north-east of the United States. This was because the moths had no natural predators in North America. The moths ate up all the leaves of the trees, leaving no place for birds to build nests. Baby birds and other animals could not be hidden, and were easily spotted and eaten by predators. © 2009 Marshall Cavendish International (Singapore) Private Limited

Decomposers When plants and animals die, they become food for organisms known as decomposers. Bacteria and fungi are decomposers. Decomposers feed on dead matter and animal waste by breaking them down into simple substances such as mineral salts and carbon dioxide. Most of these simple substances are returned to the soil and air to be used again by plants. © 2009 Marshall Cavendish International (Singapore) Private Limited

Decomposers animal wastedead animal decomposers fungi bacteria Decomposers break down dead matter and animal waste. dead plant nutrients for plants mineral salts in soil © 2009 Marshall Cavendish International (Singapore) Private Limited

Decomposers Decomposers play an important role in nature by ensuring that the Earth would not be covered by dead matter and animal waste. At the same time, they provide the soil with nutrients for plants to grow. mould breaking down the nectarines © 2009 Marshall Cavendish International (Singapore) Private Limited

Animals that feed on dead organisms Some animals are not decomposers. Instead, they help decomposers to break down dead organisms to simple substances more quickly. They do this by biting or breaking up dead organisms into smaller pieces. © 2009 Marshall Cavendish International (Singapore) Private Limited

Animals that feed on dead organisms Animals that feed on dead plant matter include snails, earthworms, millipedes, termites and woodlice. Animals that feed on dead animals include vultures, blowflies and flesh flies. Female flies usually lay their eggs near or in the flesh of dead organisms. When the eggs hatch, the maggots feed on the decaying flesh. Decomposers like bacteria then complete the breakdown of the dead matter. © 2009 Marshall Cavendish International (Singapore) Private Limited