This picture shows a symbiotic relationship between moss and the tree.  This is a symbiotic relationship due to the fact that the moss flourishes on the tree, and the tree is relatively unaffected, and possibly does profit.  Some bugs may take up residence on the moss, differeent bugs than the tree is used to, but won't be harmed by them.  This was taken in my backyard, on a three-trunked tree also inhabited with some poison ivy.