I'm assuming you have a plug-in called "Ambience" or something....
Put it on an effects bus and automate the volume.
Put it on an effects bus and automate the send from the track.
Automate the output level of the effect or the wet/dry ratio.
Put only the portions of your tracks that you want to have the effect on a separate track and don't worry about automating it.
That's just a couple ideas I can think of.