I've managed to get ecosystems which are populated above or below a certain altitude using simple functions:
 

The terrain is 100 units high and in the altitude filter min effect is 30 and max 100.

Here's the simple function which populates the terrain below 30 units altitude:
 
What I would like to do is be able to place the rocks (or whatever) between two altitudes, but I can't make the function do this. Anyone have any thoughts - there must be a simple answer to this?

Steve

I'm sorry I can't help you; I don't have Nobel prize in Mathematics! :-D

Hope this makes sense. Play with the numbers in the first 2 Map nodes to adjust where things are applied.

As applied to a standard terrain.

Makes perfect sense! And works extremely well - many thanks.

Steve