In a food chain, organisms are classified into three groups: (i) producers, (ii) consumers, and (iii) decomposers. While producers form the first trophic level of the food chain, consumers form the three subsequent trophic levels; at times, four. At the second trophic level, there are primary consumers; at the third, there are secondary consumers; tertiary consumers occupy the fourth level, and in rare cases, there is a fifth level which is occupied by quaternary consumers. Besides producers and consumers, the food web also has decomposers, which are assigned the task of breaking down dead plant and animal matter, and releasing it into the environment for recycling. Simply put, a primary consumer, or first-order consumer, is any organism that occupies the second trophic level of the food chain, and feeds on producers, i.e., plants, which form the first trophic level. In essence, primary consumers ... or consumers in general, are heterotrophs, i.e., organisms that cannot produce their own food and thus, have to rely on autotrophs (organisms that produce their own food by using light, water, carbon dioxide, or other chemicals) for the same. The easiest way to identify primary consumers in any given ecosystem is to identify the herbivores in the said ecosystem. These include grass-eating species found in the savannas, leaf-eating and fruit-eating species from tropical rainforests, and even freshwater species which feed on aquatic plants. No wonder primary consumers are also called herbivores. As for omnivores, they can be both, primary and secondary consumers. As most secondary and tertiary consumers in the food chain―carnivores in particular―can neither resort to photosynthesis to produce their own food, nor ingest plants, they are dependent on primary consumers for their energy requirements. The problem though is that, only 10 percent of energy is transferred from one trophic level to another during the process of energy transfer, which is why there are less top-level predators and more primary consumers in the ecosystem. It is this arrangement that gives the energy transfer diagram it's characteristic pyramid shape.