The spellcasting rules explains that a spellcaster can lower his effective caster level if he wishes so, but not below the minimum caster level needed to cast the spell for his class.

Does this mean that you can't cast a spell of a given level if your effective caster level is below the minimum needed (through some effect that reduce caster level, like, for example, the Alternative Source Spell metamagic feat, or Uncanny Forethought)?

And you can't cast any spell if your effective caster level is lowered to zero?