Or you could just do a view-source on that page to see where that dropdown comes from and what URL gets sent when you click on the link

There's only one <select> box where one of the options is "species".
The select box has the id "tax_level" which is referenced only once,
where the value gets assigned to "new_level"
which is likewise assigned only once,
which then gets you to a single "execute_ajax()" call,
which looks to be a fairly generic routine that builds a URL.

My guess is if you look through the .js files you'll find where execute_ajax() is defined to see just how it builds the URL.

Note, by the way, that unless you're running something like greasemonkey, you're never actually getting to pass anything to J[ava]script yourself.