About 38 million years ago many of the families that live today bcame recognizabe. Two and a half MYA the Muidae family was the first and only family of the order to invade Austrailia. Presently rodents live abundantly on every continent. Because of their high reproductive rates and ability to invade many habitats, rodents are able to spread and multiply quickly. Ocasionally population explosions occur, sometimes on a regular basis as in the leming populations in the artic tundra.
Population soon crash because of predation, disease and food shortages.
Each year rodents consume 42.5 tonnes of food worth an estimated 30 billion U.S. dollars. Rodent may eat stored food directly, kill plant crops or contaminate stored food. Rodent pests can transmit more than 20 pathogens to humans. The bubonic plague was responsable for the death of 25 million Europeans in the 14-17th cenuries. The three main rodent pests to human are the Common rat, Roof rat, and the House mouse.