Shrews are distributed almost worldwide; of the major tropical and temperate land masses, only New Guinea, Australia, and New Zealand have no native shrews; in South America shrews appeared only relatively recently, as a result of the Great American Interchange, and are present only in the northern Andes. In terms of species diversity, the shrew family with its 385 known species is the fourth-most successful mammal family, being exceeded only by the muroid rodent families Muridae and Cricetidae and the bat family Vespertilionidae. In terms of population size, it is probably the most successful mammal family, with a typical population of a few shrews per forest acre, adding up to an order of magnitude of 100 billion shrews in the world.