Okay, I upgraded to Pro, mainly so I could do Eco-systems.

But now I am running across something I don't know if it is a bug, or working as intended... and maybe there is a work around or a setting I am missing.

I am trying to make a grassy hill with trees. I want the whole object to be covered with grass/weeds/clover. I can make an eco-system that does that, but the problem is as soon as I add a tree to the ecosystem, the other plants thin out.

In the Density tab of the ES, I have overall density set to 100 (have even tried 1000 with minimal difference). Both Placement options are unchecked, and sampling quality is set to 100%. Offset from surface is set to absolute, with the slide at 0. Variable density is turned off and so is Decay near foreign objects.

On the Environment tab, everything is still set to the default.

If I take out the tree from the ES, and populate, I get 18k instances. If I put the tree back in and populate, I get 5701 instances.

Am I doing something wrong here, or is this a known issue?

Thanks

I'd use a layered material and keep the tree eco on it's own layer and the other plants on a second layer. 

In the General tab of the material editor, you get extra Distribution controls to play around with how the eco layers attract (affinity) or repel each other.  Works really good, but does take some practice to get right. 

Layered material?

Can I do that in Pro?

Yes, you can.
But to answer your question more precisely, in the density tab, by default,  dacay near foreign object is checked. And your tree is a foreign object, so the density is adjusted to leave 5% space around your tree.
You can change this behaviour by unchecking the decay box, but there will be absolutely no dacay near ANY object, which you may want or not, or, in the aspect tab of the object manager ( under the preview sphere), with your tree selected, click the tiny icon taht looks like a branch. This icon means Ignore this object for ecosystem population. So this tree will no longer be an obstacle to populate ecosystems, but any other object you want decay  around it will still be able to do it.

I had already turned off the decay near foreign objects, and it made no difference as to the density on the ground... just the density around the trees.

I see now how to do layers with Eco-systems, but I can't see where you are talking about with the aspect tab of the object manager... what screen are you refering to here?

Upper right corner of the screen.  There are three tabs, the first one is the Aspect tab.  There are 4 tiny icons.  The one that looks like a branch/weed is the ignore from ecosystems. 

This works great when you have a single tree, and don't want it to mess with the ecosystem or for example, you have a waterplane, and want plants to populate under the water.  Not sure how it will help if the tree is part of the ecosystem. 

Oh, the tree is part of the ecosystem? Hadn't read well enough, sorry. Trees are big, weeds are small, if both have the same presence settings, you will have 50%weeds, 50% trees, so itis normal weeds thin out when trees are involved. Change the presence setting for each plant, or works with layers, as Peggy suggested.