John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Debunking the total-world-domination myth of the common rat.
A couple days ago, a map hit Reddit that purported to show the distribution of brown rats, aka sewer or "common" rats. With their territory marked in crimson, the rats were seemingly everywhere, infesting every continent save for Antarctica and only absent in certain northern territories:
Many folks appear to be taking this map at face value: "Just because you haven't seen them doesn't mean they aren't there," explains one commenter. A querulous few have poked at its veracity, though. Asks one, "So there are rats in the middle of the Sahara desert?" Wonders another, "What about the Himalayas and other mountainous regions where there's little more than rock and ice?"
People are right to be skeptical. As with so many things generated on Wikipedia, this map is "grossly oversimplified/exaggerated," says Kelly Carnes, a press officer for the Smithsonian's American Museum of Natural History. Rattus norvegicus has not conquered the world (not yet, anyway). Its range remains closely tied to cities and human activity, so you're not likely to spy a set of twitching whiskers at the top of Kilimanjaro, say, or in the sand dunes of Saudi Arabia.
After consulting with the Smithsonian's rodent experts, Carnes has more to add to myth-bust this baby:
In much of the world, especially the tropics, the Brown/Norway rat is unknown outside of large cities and coastal areas, and it is associated especially with port cities and human-dominated, especially urban, habitats. It thrives especially where there is water and disturbed habitats, and is more at home in colder climates than hotter. It is, generally speaking, not an animal encountered in the interior of Africa, South America, tropical Asia, or Australia.
The map did get interesting detail correct, however – the big rat black hole inside Canada. When the tip of the westward rat invasion hit Alberta shortly after WWII, the Canadian province responded with a harsh eradication program that decimated the rodents. The Global Invasive Species Database has the skinny on this rare success story:
Since 1950, Alberta Agriculture has supervised and co-ordinated a rural-based Norway rat control program that has essentially kept the province rat-free. Success is achieved by eliminating invading rats within a control zone 600 km long and 30 km wide along the eastern border of the province. A systematic detection and eradication system is used throughout the zone to keep rat infestations to a minimum. Strong public support and, citizen participation was developed through public education and a sound awareness effort. Although rat infestations within the interior are minor, a rat response plan is in place to deal with a large or difficult case. Government preparedness, legislation, climate, geography, effective rat baits and close co-operation between provincial and municipal governments have contributed to program success (Bourne, 2006).