Unity 4.6 UI Canvas - Some questions about the EventSystem

Hi,

I figured out that when i add a Canvas it automatically add an EventSystem. Are they somewhat hard-linked to each others ?

I know the EventSystem is there and act as an input system for the GUI but in some case it isn't needed. Is the EventSystem optional or required ?

Also, since i reference my Canvas (Which is a prefab) in a UIManager script at the first frame of the game, it is safe to do same with the EventSystem and use DoNotDestroyOnLoad with it ?

Also, what happen if you have two Canvas ? It is only needed to have one EventSystem for this scenario ?

Thanks a lots!