Does Immerwolf's bonus stay after it leaves the battlefield?
Immerwolf's ability only gives other creatures +1/+1 while Immerwolf is on the battlefield.

Immerwolf's ability is a static ability that creates a continuous effect. The specific rules that covers this scenario are:


  
  611.3. A continuous effect may be generated by the static ability of an object.
  
  
  611.3a A continuous effect generated by a static ability isn’t “locked in”; it applies at any given moment to whatever its text indicates.
  611.3b The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.
  
  


As soon as Immerwolf leaves the battlefield, the ability stops existing. Since it isn't "locked in" to those specific creatures that it was affecting, those creatures lose the power and toughness bonus.