Rodents are classified by a single pair of continuously growing incisors in both their upper and lower jaws; the length of their incisors are maintained by gnawing, which is how they eat. The diet of Rodents usually consists of seeds and plants, but some species do have more varied diets. Historically, Rodents have been keep as pets, used as laboratory animals, and treated as pests. Rodents accidentally introduced as invasive species to new ecosystems, most notably islands, have been seen to wreak havoc and cause extinction of naturally-occurring species. Rodents are found on all continents, except Antarctica, and are the only species, besides Bats and Sea Lions, to reach Australia without human introduction.