Given that D&D humans and elves can freely hybridize (and so can humans and orcs), is it really right to call them species?
AIR biologists tend to say that creatures are different species, even if they can potentially interbreed, if they don't normally do so (and produce fertile offspring) when given the opportunity. By that test humans are one species with elves and orcs, but elves and orcs are different species.
Or more accurately, the scientificc term 'species' is not really applicable here, because it's fantasy.