Fossils show us that species are not fixed; they evolve. Over a period of millions of years, certain land-dwelling mammals evolved into dolphins or whales. Sometimes species "split" in EVOLUTION, with groups of members diverging into different patterns. There are also evolutionary cases in which originally quite different species have converged toward similar forms.
Knowing species names give us a precise vocabulary for recognizing and discussing them. It's much more fun to identify that sociable bird with its lovely trickling song as a linnet than merely as "that pinkish bird."
Species are very important and I hope that a careful reading of the defintion of SPECIES has lead you also to a better understanding of the definition of SUSTAINABILITY by Ernest Callenbach.