In today's evolutionary biology there is a large discussion of how to define "species"(Jerry Coyne wrote a great blogpost about that here http://whyevolutionistrue.wordpress.com/2011/01/03/more-on-species-part-1/). But most people don't really have this fine appreciation of the difficulties in using the concept. In most places, the equivalence of species to one of the "platonic" (in quotes, because I don't think people use it consciously) types is what people use. So a dog is a dog is a dog. People don't realize that there is a large degree of variation, and lots of dogs are more similar to a wolf than other, the same way that a lot of wolves are more similar to a dog than other, depending on the population.
And things like this symbiosis between a bacteria and a whitefly makes it even more confusing, as you could always argue that the whiteflies that have the bacteria are a completely different species, to the point that they replaced almost all the ones without a bacteria. But then you would have to say that just by symbiosis you changed your species! This obviously would make even more interesting (conceptually at least) the study of the microbiome (the ecosystem of microorganisms) of the human gut, to see if you have subpopulations that have a significant different microbiome to the others, to the point that they have different functions. How interesting it would be if we could manipulate this to give ourselves extra abilities (Yes, sir, take an enema of this microbiome, and your cholesterol level will drop by half).
I think that as the thinking species in our world, we have to start becoming more open to use the symbiosis that we have available to modify our health and our intrinsic characteristics as the "human" species.