Re: What they really need...
Actually, the only real definition of species, even back then, was that they were able to interbreed. If you can make a child with another animal, you were the same species (but not necessarily the same animal) as them. Hence dogs are all one species, even if they look vastly different. Technically, there's nothing to stop a chihuahua impregnating a Great Dane successfully.
That definition, though, still isn't "perfect" because things change as we discover more about animals. You are not one species. Just you. You contain probably hundreds to thousands of distinct species, genii, etc. of life. Similarly, even down to a cellular level, your mitochondria (without which you would have no decent way to give cells energy) are "captive" parts of a complete different organism inside your own cells. They can even have different DNA to you. That's before you even GET into what's in your stomach from birth and (we believe) replenished by your appendix in the case of a stomach illness. None of them have even remotely similar DNA to you, are the same species of you, etc. but without them you would die (even "boy in a bubble" cases have stomach bacteria etc.).
So any definition of species will be long, like Wall-E trying to categorise a "spork" into his collection of kitchen utensils. But the longest-standing, easiest-to-categorise, easiest-to-observe, most common-sense definition is "what you can successfully mate with, is your species". Let's not get into the complications of the other groupings within species, etc. because then it gets horrendously complicated.
Biting the hand that feeds IT
