From a biological standpoint, a species is the largest unit of potentially and actually interbreeding organisms (i.e. two different species never interbreed, if they did, they would be one species).
A subspecies is a group of organisms inside a species that interbreed and share some unique characteristics that differentiate them from the others in a species (i.e. benthic and limnetic stickelback fish).
So what are the salamandar ensatina? Aren't they still salamandars? And apparently interbreeding produces offspring with characteristics not beneficial so it isn't in their best interests to breed? If sub-species don't interbreed, isn't it usually because it would not benefit them?