Or there is no real boundary. Species is, at least to some degree, a human-invented arbitrary concept. When two organisms become different enough, we call them different species. One definition of "too different" is inability to interbreed. This definition doesn't always work too well, though, such as with organisms that reproduce asexually.
"Species is, at least to some degree, a human-invented arbitrary concept."
Genesis 1:25
God made all sorts of wild animals, livestock, and small animals, each able to reproduce more of its own kind. And God saw that it was good.