Explain why producers are always the first tropic level in a food chain or food web?

According to my Biology text book a producer is defined as: an organism that uses energy from light (or inorganic chemicals) to convert carbon dioxide to organic compounds. They are the first tropic level because they are able to make their own food through photosynthesis, meaning they do not depend on others for food; all other organisms in the community depend on them for food, either directly (an animal eating them) or indirectly (by eating an organism that ate a producer).