I think I answered my question @
http://www.the-art-of-web.com/sql/lookup-table/
It sounds like the field taxon_id is similar to the field id - it's just
automatically populated by a numerical key. So if I add a new taxon on row
5, then the taxon_id for the following row with change from 5 to 6 - right?
It therefore sounds like the field parent_id is useless for this particular
exercise and could be deleted - unless I find some other use for it.