Was talking about this earlier with the s.o., we've both got pretty substantial biology training (phds, ms, bs etc). We both agreed that "dominant species" is a bit of a term looking for a definition, as in, it's not something extending from biology or ecology but rather something being imposed upon them. We were between nostoc and rhizobium, with fungi capable of digesting lignin in third place, for the most "world dominating" species, in the sense that those species, through their biology, have carved the planet into a place much more suited for themselves.

It strikes me that humans aren't even really doing that, but rather, we're selecting for an environment less suitable to our own survival. So I don't know that humans would even rank for dominance over the environment because we really don't have any sense of control over the matter, whereas, some other species clearly do.