You are obviously driving at the commonly understood notion of sexual reproduction being the standard. If two creatures are capable of producing fertile offspring they are of the same species. Even if they look wildly different but can produce fertile offspring. Yes?
So if ring thing 'A' can not reproduce with ring thing 'Z' they are different species according to this definition.
The fact that ring thing 'A' can reproduce with, say, 'D' through 'S' or something along those lines, whereas 'Z' also can do the same thing, indicates that 'A' and 'D' through 'S' are the same 'species' whereas 'Z' and 'D' through 'S' are.....in any case it would appear that 'A' AND 'Z' can reproduce with at least ONE common link in the ring orgy. Are they then different species, or is something else going on here?