3, i.e. Primary consumers.The series or organisms taking PART at VARIOUS biotic levels FORM a food chain.Each step or level of the food chain forms a trophic level.The autotrophs or the producers are at the FIRST trophic level.They fix up the solar energy and make it available for heterotrophs or the consumers.The herbivores or the primary consumers come at the second, small carnivores or the secondary consumers at the third and larger carnivores or the tertiary consumers form the fourth trophic level.