melikia, multiple ecosystem layers are extremely useful and that's how nature works, in most cases. If you use just one ecosystem with, say, pebbles and grass, in each spot you either have pebbles or grass, not both. But that's not how nature works. In nature you have everything together. The way to simulate this is by having multiple ecosystem layers, one with pebbles and one with grass, in my example.

I suggest that you take a look at my video tutorial, called "Making of Sic-Transit" in the Classroom of Cornucopia3D. I show an example of these things.