Plants are producers. The herbivores or the primary consumers come at the second trophic level. Small carnivores or the secondary consumers come at the third trophic level. Larger carnivores or the tertiary consumers form the fourth trophic level.

The fourth trophic level in an ecosystem consists of the top carnivores. Let's break down the trophic levels and their components to better understand why top carnivores belong to the fourth trophic level.

Trophic Levels:
1. First Trophic Level: Plants
2. Second Trophic Level: Herbivores
3. Third Trophic Level: Small carnivores
4. Fourth Trophic Level: Top carnivores

Explanation:

1. First Trophic Level (Plants):
Plants make up the first trophic level in an ecosystem. They are called producers because they are capable of producing their own food through photosynthesis. Plants convert sunlight, water, and carbon dioxide into glucose (sugar) and release oxygen into the atmosphere.

2. Second Trophic Level (Herbivores):
Herbivores are the primary consumers in the ecosystem and make up the second trophic level. They consume plants as their main source of food. Examples of herbivores include cows, deer, rabbits, and many insects.

3. Third Trophic Level (Small Carnivores):
Small carnivores are the secondary consumers in the ecosystem and belong to the third trophic level. They feed on herbivores for their energy and nutrients. Examples of small carnivores include snakes, foxes, and small birds of prey.

4. Fourth Trophic Level (Top Carnivores):
The top carnivores are the tertiary consumers in the ecosystem and constitute the fourth trophic level. They are at the highest level of the food chain and feed on other carnivores. Top carnivores have no natural predators and play an important role in controlling populations of other animals. Examples of top carnivores include lions, tigers, wolves, and apex predators like great white sharks.

In summary, the fourth trophic level consists of top carnivores, which are the highest level of consumers in an ecosystem. They play a crucial role in maintaining the balance of the ecosystem by controlling the populations of other animals.