Here's what I do, I add cubes to a new layer, and i shape the cubes so the cover all that is not into the camera's view, that is the sides and the front of the camera view, ( the front but far from the camera) And use some decay near foreign objects to a value of 2, so the ecos won't populate where the camera can't see. If some other objects are in your scene, they will prevent ecos to populate too, if you don't want this, in the object manager, click the 2nd little icon from the right under the material preview, so these objects will be ignored when populating.
The alpha plane option is a very good trick for distant ecosystems.