Introduction to Ecology (link)
Ecosystem Ecology (link)
The trophic level of an organism is the
position it holds in a food chain.
Primary producers (organisms
that make their own food from sunlight and/or chemical energy from deep sea
vents) are the base of every food chain - these organisms are called autotrophs.
Primary consumers are
animals that eat primary producers; they are also called herbivores (plant-eaters).
Secondary consumers eat
primary consumers. They are carnivores (meat-eaters)
and omnivores (animals
that eat both animals and plants).
Tertiary consumers eat
secondary consumers.
Quaternary consumers eat
tertiary consumers.
Food chains "end" with top
predators, animals that have little or no natural enemies.Food Chain (link)