You can either do it the way I have it with the black parts being the ones I wanted populated and the filter going down, or have the filter going up BUT click the little button on the top left in the FE (next to where it says "image"), that will inverse the colors, the parts that gt populated will be the white ones.
This is the official vue definition : white=high density. But I dont like it, my brain associates white with 'nothing' and black with 'lots', so I reverse it. 
Anyhow, glad it helped.
By the way - if you need it to populate accurately you may need to up the sampling quality - as you can see I'm up to 500% in my example. It takes longer to populate though.
Another way to do it is to use mixed material  - one with eco population and one without, and control the distribution of the two materials with a map. Some gurus say this goes faster, I havnt tested it myself yet. (and if what you are populating is a terrain you can paint the populated material in the terrain editor)

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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