Biologists care about the survival of populations, as well as species, because of the unique genetic information contained within populations. Each population contains a distinct set of genetic instructions for how the species might adapt to threats. Nature, in other words, figured out long ago not to put all its eggs in one basket. Finally, there is species diversity, which is what most people mean when they talk about biodiversity. Consider people, whose species name is Homo sapiens.
Homo is the genus, and sapiens designates the species.
We happen, now, to be the only living species in the genus Homo . There are about one and a half million named species on earth, but we know that many unnamed species exist, and the total number is probably between 5 and 15 million. There are several hundred classes and several dozen phyla. Individuals in different classes differ from each other far more than do those in the same class but in different orders, families, or species. And individuals in different phyla differ from each other far more than those in the same phylum but in different classes.