Final answer:
Organisms in an ecosystem are categorized by trophic levels: primary producers (like plants), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers (carnivores that eat other carnivores).
Step-by-step explanation:
In the context of trophic levels within an ecosystem, organisms can be categorized into different roles based on what they consume. The primary producers are the base of the food chain and include photosynthetic organisms like plants and phytoplankton that make their own food. Primary consumers are herbivores that eat the primary producers. Secondary consumers are usually carnivores that feed on the primary consumers. Tertiary consumers, also known as apex consumers, are carnivores that eat other carnivores, and they are often at the top of the food chain, such as the Chinook salmon in the Lake Ontario ecosystem.
For example, in an aquatic food web, algae would be primary producers, small fish that eat algae would be primary consumers, larger fish that eat the small fish would be secondary consumers, and a predator such as a bear that eats the larger fish would be a tertiary consumer. Humans are unique in that they can operate at multiple trophic levels; they are primary consumers when they eat plants, secondary consumers when they eat animals like cows, and tertiary consumers when consuming animals such as salmon.