British Soldier lichens
I'm intrigued by the range of ecological relationships that exist. We like to make neat distinctions between mutualist and parasite that apply to all individuals in a given relationship, but there's no intrinsic reason why they'd all be the same.
While identifying these British Soldier Lichens I ran across this:
Some scientists think Cladonia cristatella is a parasite of Trebouxia erici, using up its food but not giving much back to the algae. Since Trebouxia erici can live on its own, or with another lichen, it doesn't really need Cladoina cristatella. Other scientists believe the two organisms have a mutualistic relationship, since they both seem to do better with each other, rather than on their own.