Rats in the Big City

Some creatures, such as rats, never really left the city. Today, an estimated 28 million rats—which are non-native, like much of the city's human population—inhabit New York. The greater New York area is home to eight million people, which means there are more than three rats to every person. Matthews explains how it happened.

"Rats are smart," she writes. "Although a fast-forward version of natural selection has made rats in many big cities immune to nearly all conventional poisons, they still may press one pack member into service as a taster; if the test rat dies, the others resolutely avoid the bait."

Matthews says the strong adaptive ability of non-native species has begun to change the definition of wilderness. Rats were introduced into U.S. cities in the 1700s after arriving as stowaways on merchant ships. Zebra mussels, which have caused major problems in the Great Lakes by clogging intake pipes, were imported in the ballast water of international ships.

"The most important thing is to realize that a city is wilder than we tend to imagine and the land we think of as untouched or wild really isn't," says Matthews. "There has been so much human interference and reshaping that we really don't know what a pristine planet is."

Rats return to the Big City...they never left