4-Food chains and food webs
Organisms need energy to grow and reproduce. Some organisms, such as plants, make their own food, but most feed on other organisms and this creates a interdependence of species that creates the  Food chains , that are made up of
three levels: producers, consumers and decomposers. 

Several food chains interconnected forms a Food web

Levels of a food chain

1-Producers are the base of food chains. Plants and some protists are producers.

2-Consumers feed on other living things. There are three types of consumers:

-Primary consumers, such as grasshoppers or giraffes, feed on plants. Herbivores are primary consumers.

-Secondary consumers, such as snakes, feed on primary consumers.Carnivores and omnivores are secondary consumers. They can be both predators and prey.

3-Decomposers feed on dead organic matter. Most decomposers are fungi and bacteria.