An unsettling urban legend claims that there are as many rats as people in New York City. But that's simply not true, according to a statistician who found that a generous estimate for the rat population would actually be 2 million — far fewer than New York's 8 million humans.
To estimate animal populations, ecologists often use a "capture–recapture" method. First, researchers capture and mark a random sample of a species, and then later, they round up a second random sample of the animals. Based on the percentage of marked animals recaptured in the second batch, ecologists can estimate the species' total population in a given area. But Jonathan Auerbach, a doctoral student in the statistics department at Columbia University, said that the city's Department of Health and Mental Hygiene was not keen on having a large-scale rodent-releasing experiment in New York.
Instead, Auerbach turned to a wealth of data on rat sightings collected by the 311 information and service hotline, where New Yorkers can file complaints about noise or rodents, get updates on parking regulations, or even request a tree to be planted on their block. [What Really Scares People: Top 10 Phobias]
The researcher looked at rat sightings on New York's 842,000 property lots, during the first half of 2010 and then during the first half of 2011. The percentage of "recaptured" lots (or lots that had rat sightings in both 2010 and 2011) helped Auerbach determine that 40,500 (or 4.75 percent) of all New York's lots were rat infested. Assuming a typical rat colony has 40 to 50 members, he estimated the total rat population in the city is 2 million. But even that number might be an overestimation, Auerbach said.
"This is because we treat every lot with at least one rat sighting as evidence that a full colony of rats inhabited the lot for the duration of the study period," Auerbach wrote in the journal Significance. "However, it is possible that the territory of one rat colony encompasses several lots. Since we do not know the extent to which a rat colony has established itself on each reported lot, we proceed as though each reported lot is associated with a full colony of rats. This overestimation allows us to be confident that there are not 8 million rats in NYC."
News about New York City rats is rarely so comforting (if the idea of 2 million rats, which is still about equivalent to the human population of Houston, can even be considered comforting). Last month, a report found that rats in Manhattan are carrying a host of diseases — and not just familiar stomach bugs like E. coli and Salmonella. Some rodents even harbor Seoul hantavirus, which can cause Ebola-like hemorrhagic fever. Days before that study was published, New York City Comptroller Scott Stringer unveiled the findings of an audit, showing that rodent complaints were on the rise and health officials were not adequately responding to pest problems.
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