When using ecosystems you have several options available to you to control plant count. 
1.  If you reduce the density just a single percentage or two sometimes it has a huge decrease in plant count WITHOUT affecting the overall quality of the image---the land still looks heavily populated.
2.  After you use the populate command, use your erasure brush and delete the plants that are over the horizon and those that are out of the boundary lines of the camera---if you cant see them, why plant them?

3. If you increase the scale size of the trees by just a single percentage or two, it doesn't make a noticeable diffence in visuals, but it decreases plant numbers and therefore drops your poly count down considerably.  You can use this in conjunction with tip #1

These tips might help you control your poly count and still furnish acceptable results.