For a wikispecies, I would expect to be able to click on the Kingdom (Animala), Phylum (Chordata), Subphylum (Vertebrata), and so on so I can narrow down progressively to homo sapiens, homo neanderthalensis, homo erectus, etc., and on the each further down level (i.e. Chordata, then Vertebrata), the most notable species would have higher links on that page. Is that what you mean by category?
James
Yes, partly. A taxonomic order should be one very important aspect, but there are plenty of other ways to set up a order for species; by size; by distribution; by behaviour, etc. The organisation of a database is ideal for a cross-linked, highly specific search.
Benedikt