text: which group of living things has the greatest population?why do you think so?   this depends on what you mean by "group" and "greatest population."  in terms of biomass: probably plants.  in terms of disperate diversity: probably the bacteria.  for smaller groups, i agree with the insects, especailly beetles.  edit: don't listen to dollar below.  you can find more rats in 1 city than humans on the earth.