The term “species” has a clear definition in biology: a group of organisms that breed only among themselves and do not breed with members of any other group. Thus, as far as we can tell, humans are all members of the same species, Homo sapiens.
The lesser black backed gull and the herring gull of Britain, however, act like separate species, yet are connected to each other by a ring of subspecies that extend all around the Northern Hemisphere and can interbreed with their neighbors. So in the sense I stated above, the definition of species breaks down.
The issue of species also fails when asexual life forms are considered, including bacteria, most protists, a few populations of beetles, a population of lizards, and an entire class of rotifers called Bdelloidea. The lizards, beetles and rotifers in question are all females, while among the single celled organisms the issue of gender identity is meaningless.
Suppose we have a population of 400 asexually reproducing lizards which are genetically and physically almost identical. One at a glance would assume they are members of the same species. But because the lizards do not swap genes via sexual reproduction, they would just as well be considered 400 separate species.
The issue of “species” becomes meaningless when one considers extinct organisms that are dug up as fossils. Fossils cannot breed among themselves and so the designation of certain fossils as Homo hablis, Homo egaster, and Homo sapiens is entirely arbitrary, based on the structure of the fossils and nothing more. The same is true of all other organisms in the fossil record, including dinosaurs.
I would therefore argue that the term “species” is really useless and should be abolished completely, because it is a source of unnecessary confusion.