What makes a species?
There are different operational definitions used in different sub-fields of biology. These days, however, having a common gene pool tends to be the fundamental definition.
If they're not interfertile, they're different species.
If the offspring are sterile, different species.
If the offspring are fertile but incapable of surviving for long in the wild, different species.
Even if none of the above are a problem, if they could successfully mate and have offspring that could continue to contribute to the same gene pool but choose not to mate, different species.
All of which gets complicated by ring species and clines, which can exist because gene pools are viscous.
Oh, and then there are rabbits in Australia. They've been artificially separated.. If you brought Australian rabbits back to Britain they're probably still the same species (don't do it the other way around because the Australians will get upset if you bring rabbits into the country) but eventually will no longer be.