Yes, that actually was my fault for not providing an example.
You need a
tag page, like for example
tag:animals:reptiles:quelonids to exist.
Use the following as the content:
==== Quelonids =====
{{topic>:animals:reptiles:quelonids}}
From there on, you can use the index that comes integrated with DokuWiki (which
will show the tree up to the "quelonids" level) or the syntax of the
indexmenu plugin that specifies a namespace, something like this:
{{indexmenu>:tag #5|js}} (show tree with JavaScript, up to 5 levels)
I hope that helps.