NEW YORK -- Multitudes of feral cats roam New York City's concrete jungle, and some now have a practical purpose: They're helping curb the city's rat population.

A group of volunteers trained by the NYC Feral Cat Initiative traps wild cat colonies that have become a nuisance or been threatened by construction, then spays or neuters and vaccinates them. The goal is to return them to their home territory, but some end up in areas rife with rats.

Feline rat patrols keep watch over city delis and bodegas, car dealerships and the grounds of a Greenwich Village church. Four cats roam the loading dock at the Jacob K. Javits Convention Center, where food deliveries and garbage have drawn rodents for years.

"We used to hire exterminators, but nature has a better solution," said Rebecca Marshall, the sustainability manager at the 1.8-million-square-foot centre. "And cats don't cost anything."

About 6,000 volunteers have completed workshops where they've learned proper ways to trap cats.

The program is run through the privately funded Mayor's Alliance for NYC's Animals, a coalition of more than 150 animal rescue groups and shelters. It estimates as many as half a million feral and stray cats roam New York's five boroughs.

The life of a street cat is a tough one. Some are former pets, abandoned by owners. Plenty die of disease and malnutrition or are hit by vehicles. Others ingest poisoned cat food -- set deliberately to get rid of them, cat advocates say.

Many of the animals are displaced as a result of New York's development, with new construction creating perilous conditions for those that once inhabited the city's nooks and crannies, from vacant lots, decaying factories and empty warehouses.

One colony of two dozen cats living in a lot on Manhattan's West Side are about to be displaced by construction on a new $3 billion office tower. A City Council member is working with residents and developers to make sure the creatures are moved to a safe location.

The Javits Center's quartet of cats -- Sylvester, Alfreda, Mama Cat and Ginger -- were lured to its 56 loading docks about two years ago with pet food brought by animal-loving employees. On a recent fall morning, Sylvester stationed himself next to a commercial truck, ready to pounce if needed.

The cats are predators but don't necessarily kill rats. Instead, experts say the feline scent and droppings repel the rodents.

"A mother rat will never give birth near a predator because the cats would eat the babies," said Jane Hoffman, president of the mayor's alliance.

The cat population is controlled through spaying and neutering, provided free of charge by the Humane Society of New York and the ASPCA. In most cases, adoption is out of the question for feral cats because they are just too wild to be domesticated.

Thanks to the volunteers, says Marshall, "we're protecting wildlife in the city, and the cats get a second chance at life."