Step 1: Understand Trophic Levels

In ecological pyramids, the first trophic level is occupied by autotrophs (producers like plants), the second by primary consumers (herbivores), the third by secondary consumers (small carnivores), and higher levels or decomposers like bacteria operate across levels.

Step 2: Evaluate Each Pair

Autotrophs correctly form the first trophic level, Primary consumers correctly form the second trophic level, and Small carnivores form the third trophic level. Bacteria act as decomposers operating at all levels rather than being restricted solely to the fourth trophic level.