When introduced into locations where rats previously did not exist, they can wreak an enormous degree of environmental degradation. Rattus rattus, the black rat, is considered to be one of the world's worst invasive species. Also known as the ship rat, it has been carried worldwide as a stowaway on seagoing vessels for millennia and has usually accompanied men to any new area visited or settled by human beings by sea. Rats first got to countries such as America and Australia by stowing away on ships. The similar species Rattus norvegicus, the brown rat or wharf rat, has also been carried worldwide by ships in recent centuries. The ship or wharf rat has contributed to the extinction of many species of wildlife, including birds, small mammals, reptiles, invertebrates, and plants, especially on islands. True rats are omnivorous, capable of eating a wide range of plant and animal foods, and have a very high birth rate. When introduced to a new area, they quickly reproduce to take advantage of the new food supply. In particular, they prey on the eggs and young of forest birds, which on isolated islands often have no other predators and thus have no fear of predators. Some experts believe that rats are to blame for between forty percent and sixty percent of all seabird and reptile extinctions, with ninety percent of those occurring on islands. Thus man has indirectly caused the extinction of many species by accidentally introducing rats to new areas. Rat-free areas Rats are found in nearly all areas of Earth which are inhabited by human beings. The only rat-free continent is Antarctica, which is too cold for rat survival outdoors, and its lack of human habitation does not provide buildings to shelter them from the weather. However, rats have been introduced to many of the islands near Antarctica, and because of their destructive effect on native flora and fauna, efforts to eradicate them are ongoing. In particular, Bird Island (just off rat-infested South Georgia Island), where breeding seabirds could be badly affected if rats were introduced, is subject to special measures and regularly monitored for rat invasions. As part of island restoration, some islands' rat populations have been eradicated to protect or restore the ecology. Hawadax Island, Alaska was declared rat free after 229 years and Campbell Island, New Zealand after almost 200 years. Breaksea Island in New Zealand was declared rat free