Answer: Food chain
Step-by-step explanation:
Food chain can be defined as a complex system as it has many variable (different types of animals) and different parts (different tropic level).
The food chain starts from the producers and then the food is eaten by primary consumers. The primary consumers are eaten by secondary consumers.
There are many animals at different tropic levels and there is a unidirectional flow of energy from the producers to the consumers.
Hence, a food chain can be considered as a complex system with many parts and many variables.