Feeding relationships

This is an oak tree - a producer. It is also a food source for other creatures living in the tree (see below)

Green plants are known as PRODUCERS because only they carry out photosynthesis, and are capable of producing organic compounds (carbohydrates, which are then converted into fats and proteins), starting with simple compounds (CO₂ and water), using the energy of sunlight, trapped by the pigment chlorophyll.

Animals are known as CONSUMERS because they eat and process these compounds, either incorporating them into their own bodies (assimilation*) or using them (as “fuel”) to provide energy (by respiration) for their activities.
*”converting like to like”

This shows lots of aphids (greenfly) feeding on the sap within a leaf from an oak tree, so they are primary consumers

Some animals (herbivores – “vegetarians”) eat plants directly, and are known as PRIMARY (1st) CONSUMERS. Other animals (“carnivores”) which [usually kill and] feed on these animals are known as SECONDARY CONSUMERS.

Here is a ladybird (secondary consumer) feeding on an aphid (primary consumer)

Animals which feed on these are known as TERTIARY (3rd) CONSUMERS and QUATERNARY (4th) CONSUMERS, and these are much rarer.

Of course, both the organic materials in each of these animals, and the energy used by them, originate in the plant eaten by the primary consumer in the first place.

These descriptions are known as TROPHIC LEVELS.

This is the basis for food chains, food webs, and food pyramids.