I was able to reach Mark Chase, and he's given me permission to post
his reply:

"In Australia, it appears that their native orchids form exclusive
relationships with fungi, and I believe that some new species have
been recognised due to the line of argument that you mentioned (these
two similar species of orchids must be different because one forms
relationships solely with fungus X, while the other depends solely on
Y). You could contact Peter Weston who is an expert on Australian
orchids if you wanted more information on this subject. In European
terrestrial orchids, we think that each species can have multiple
fungal associations over its range, so there is no one-to-one pattern
as in Australia. This could be due to
our taxonomic concepts not recognising biological entities, but at
this point we think that it is that these orchids are pluralists
(opportunists)."

I've also been sent (by various correspondents) what is turning out to
be a long list of references. If anyone is interested, I can collect
and post them.

The natural world is wondrous strange.

Monique Reed
Texas A&M