Well since the biological classification is usually presented in a form of tree, then it's pretty logical to map it to the eZP content object tree model.
As an alternative there could be a solution of having an object relation attribute which would then be used to connect to an object of type "Species". The classification tree would then be implemented as a tree of "Species" objects.
Which solution is best for you depends on how will you use and manage this data. You should ask yourself this questions:
how will you present the data on the site?
do you have any other parallel classification that you have to implement?
Also, note that fetching on attribute filter is very very much slower than just fetching nodes from a subtree. With that in mind I think the first idea would be your best bet.
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Nothing is impossible. Not if you can imagine it!
Maybe you should also consider to use tags to group animals.
With tags you could also make listings later that groups animals according to tags ala "pink". In that way you make collections that maybe children asks for like all animals with a colour no matter if its is Coral-animal or Flamingo.
