What are primary consumers? Can a food chain present quaternary consumers without having secondary or tertiary consumers? Can a tertiary consumer of one chain be a primary or secondary consumer of another chain?

The Primary consumers are living beings that eat autotrophic beings that is they eat the producers. The Primary consumers always belong to the second trophic level of a chain.

A food chain can't have consumers of superior orders without having the consumer of the inferior orders. A consumer though can participate in several different chains not always belonging to the same consumer order in each of them.