Every living organism is involved in a food chain. For example, a snail eats leaves, a bird eats the snail, a cat eats the bird. Every food chain begins with some kind of plant. It is because plants are the only organisms which can make food by photosynthesis in the presence of sunlight.
Hence all animals are dependent on plants. Food chains are also known as energy chains, because it is through the various links in the chain that animals and human beings get their energy.