Diversity of organisms is far off of the type of diversity my post was refering too. I am though, quite impressed with Navarre's description of algae use. You are right too, that the best marine tanks make the viewer look at the scape itself (the algaes, corals, etc.) and go . . . WOW. I also like the notion of countless algae species being capable of creating a different feel, even when the same ones are used depending on the scape.
Even so, the type of "diversity" I was referring to has not been addressed (well, except by me). Despite the description Navarre just gave, in every salt water tank I've ever seen (and trust me, that's not a meager handful) has relied on the same princible: Arrange the tank, choose the inhabitants, place the softscape, with the purpose of making an impact by bringing attention to the colors, the shapes, the beauty of individuals, or several types together. It's difficult to put into words, but this factor is limiting. I'll put it this way: Tthe beauty of the aquarium is ruled by the the subjects of the tank, instead of by the artistic talent/vision of the aquarist. Perhaps it is because we are a terrestrial species, but human viewers, and perhaps scapers as well, cannot transcend the fact that it is a marine tank.
I think it is because the corals and algaes and fish live in a world that seems to strange, to alien, to different to a human. However, a planted tank is different. Even though man is not a fish, man can see a field of eleocharis, and the memories of running through open fields well up in his mind, or emerge from some primal instincts. A person looks at a mound of sparkling microsorium dominating the tank, and he imagines the towering mountains in the same way, or the dense jungles, or petches of ferns, or river sides. These things are things that man can easily relate to. Because of this, the planted aquaria can choose, whether to show man something aliend to him, or to transcend the media, and touch a part of him that is in no-part a river, or pond or lake, but something man can understand.
That is closer to the diversity I'm talking about, but there are times when words cannot describe the meaning. I just have to trust that other people can grip at what I'm trying to say themselves. Marine scapers are normally smart people out of necessity-- I think you can understand.