I don't know about all the human-esque sentients being bascially the same species. That Men and Elves are biologically the same (and thus the same species) is stated by Tolkien, as is the fact that the Hobbits are an off-shoot of Men. Thus, we seem to have three sub-species of
Homo Sapiens:
Homo Sapiens Sapiens,
Homo Sapiens Quendi, and
Home Sapiens Hobbitus. However, I would say that the orks, what with Morgoth's heavy hand in changing their biology, are probably a different species, although the same genus, since they can interbreed with humans and Elves (although there is no definite indication that these offspring are fertile, as they are in the case of Man/Elf offspring). Let us call them
Homo Orkus. I would say that the Dwarves are a similar case,
Homo Khazadus.
And, while I'm having fun with scientific classification, where would we put the Ents on the tree of life? It's pretty easy to give them a scientific name, such as
Onodrimus Fangornus, but where do they belong? Are they plant or animal?
Same with the trolls, which seem to come in at least two species:
Trollus Lapis (Stone Troll) and
Trollus Ologgus. However, the question of where they belong on the tree of life is a bit uncertain. They are certainly not plant, but are they animal? Does that make them some sort of gigantic bacteria!
Anyways, there are my scientific names for the races of Middle-earth. Take them or leave them as you like. At least I had fun making them.