Explanation

All animals depend on plants directly or indirectly for their food needs. They are hence, called consumers and also heterotrophs. If they feed on the producer, the plants (belonging to the first trophic level), they are called primary consumers. Obviously the primary consumers will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic system. Thus, primary consumers belong to the second trophic level.