To answer this question, we have to look inside our ecosystem. First of all we need to have a basic idea on food chain and food web. First thing first, what is a food chain ? Lets consider a forest ecosystem . Grass is consumed by a rabbit . Rabbit has to consume the grass so that it can survive. The rabbit may fall as a prey to other organisms like fox and the fox may even become the dinner of some tiger or lion. This chain is known as food chain. The web created by this chain is known as food web. For example, not only rabbit but deer too consume grass. The whole chain is compiled together and known as food web.The different levels on this chain is known as trophic levels. Primary trophic level is always a producer . In the example it is grass , secondary trophic level consists consumer/consumers. As the pyramid goes up, the number of organisms decreases. This maintains the equilibrium of the ecosystem .