Rodentia is an order of mammals (Mammalia). Members of the order Rodentia are called rodents.
Size and range of order
In terms of number of species — although not necessarily in terms of number of organisms (population) or biomass — rodents make up the largest order of mammals, with over 40 percent of mammalian species belonging to the order. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods.(Lambert, 200)
There are between 2000 and 3000 species of rodents, which are found in vast numbers on all continents except Antarctica (they are the only placental order other than bats, (Chiroptera), to reach Australia without human introduction), most islands, and in all habitats except for oceans.