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(I seem to have run into trouble with my juggling of multiple e-mail accounts; I apologize if somehow this ends up on the list twice, but I don't think it went through before.)

All right, so I'm just starting out with Drupal, so I don't quite know how things work yet, and would appreciate some pointers about how to structure my site. (Excuse the mis-use of terminology that will surely follow.) I'd like to have a small number of pre-defined broad categories (say, Land, Water, and Air), and then parallel to that a hierarchical setup of terms like this:

Animals{Vertebrates[Mammals, Amphibians, Reptiles], Invertebrates[Worms<Earthworms, Tapeworms>, Sponges, Snails]}

Plants{Bushes, Trees[Oak, Pine<Red Pine, White Pine>, Cedar], Green Algae, Sea grasses}

Fungi{Mushrooms[Portabello, Shiitake], Yeasts, Eurotium herbariorum}

Anyway, you get the idea. There will be a variety of categories here, and each of them can have a number of children, which in turn can have children, etc. This set of terms should be able to be created on the fly by the site's users, since clearly I'm not going to be able to pre-create something like that. They should be able to either create just one low-level tag (like Earthworms in the example), or it and all of the parents necessary to properly categorize it. (Additionally, they may need to create parents and move tags under it, in case another user creates Earthworms standalone before its parents exist and don't bother to make them.)

However, the first set will be pre-defined. Only administrators will add to the list of Land, Water, and Air.

As you may have noticed, lowest-level items in the hierarchical set will usually fall within only one of the broad groups (ie, Cedar would only be within Land), and this will often also be true of their parents, but it should be possible when necessary to put items or parents in multiple groups for the few occasions when this makes sense.

Once this is all done, people should be able to view content by any arrangement of these (ie, a page that lists all Water organisms, a page that lists all Trees, and a page that lists all Vertebrates that life in Water), and "drill down" through the hierarchical parts or go to the parent of the current list, either retaining a group limiter or removing it to view all habitat types.

Hopefully someone was able to follow that - any ideas on what I would need to do/use to accomplish that?

- Tony