• A complex system must consist of a sufficient number of individuals. 

The parts of a complex system are its elements. A complex system is more than just a group of individuals. As a whole, it can exhibit patterns, properties, or behaviors that are not shown by its individual components. Therefore, the number of individuals needed to be enough to give rise to the patterns, properties, or behaviors. Depending on the specific system, this number can vary.

• A complex system consists of many similar individuals. 

The word "similar" is the key in the statement above. It means these individuals share many common traits, but they are not identical. Shared traits allow individuals to enact similar behaviors to interact with one another and respond to external stimuli [5]. For example, all bark beetles use host trees for food and shelter and are affected similarly by rising temperatures; all sticklebacks forage, reproduce, and are preyed upon by predators. Of course, individuals in a complex system can belong to different types, such as different species in an ecosystem or different molecules involved in a chemical reaction. However, individuals of the same type remain similar (Figure 20). 

Individuals of the same type may vary in some specific aspects.  For instance, when flying together, starlings may set out in different flying directions at the beginning and pay attention to different nearby birds. These differences will influence how individuals interact with one another, how they respond to external stimuli, and what outcomes are yielded at the system level accordingly.

• Individuals in a complex system are autonomous. 

Autonomy is an essential feature of individuals in complex systems, meaning they can decide, intentionally and unintentionally, what to do and when to do something based on their status and behavioral rules. Therefore, while individuals remain interacting with one another, they enact their own behaviors independently based on the current information and their own rules. For example, starlings decide for themselves when and how to adjust their flying directions based on the surrounding context. Individual ants in a colony carry out their own routines to live.