Paul, of Sawston Village College, asks :-
Why are there so many species of animals?
Edward Minot, an ecologist at Massey University, responded.
To date, taxonomists have described about 1.35 million species of animals. Estimates of the true total range from three million to 100 million. Even the low estimate represents a lot of different species.
How did so many species come about and what maintains this great diversity of life? To survive, an individual must be well suited to the environment in which it lives. Those individuals best able to live and compete will be most likely to breed. Because many traits are heritable, parents pass their successful traits on to their children through their genes. Environmental conditions vary even over very short distances; so different characteristics will best suit the individuals in different places. Hence the individuals in one location will become genetically distinct from those living elsewhere. For individuals with a common ancestor, those characteristics and genes may vary sufficiently over their range such that they become separate species.
The world is big, but there are not enough different habitats to account for all of the species. Within each habitat, however, there are many ways to live successfully. Herbivores get their energy and nutrients from eating plants. Carnivores eat herbivores or other carnivores. Decomposers feed on dead plants and animals. Thus, each habitat will have many species and every one will make its living in a different way.
So the diversity of species depends on the great global variety of habitats, the breadth of opportunity for specialisation within the habitats, and the heritability of traits that allow individuals to live and reproduce successfully.
While not complete, this explanation indicates that there should be many, rather than a few, species in the world.