It’s an interesting conundrum. More middle-class people are moving back to the city these days, after years of “white flight.” And it’s squeezing out the poor:

Mayor Michael R. Bloomberg is closing in on a milestone: building or preserving 165,000 city-financed apartments and houses for low-, moderate- and middle-income families, the goal of a $7.5 billion housing plan he announced in 2002 and expanded in 2005.

It has already financed the creation or preservation of 94,000 units, including 72,000 for low-income households, city officials say.

But those efforts have been overwhelmed by a far larger number — the 200,000 apartments affordable to low-income renters that New York City has lost over all, because of market forces, during the mayor’s tenure.

The shrinking supply of these apartments, highlighted by researchers at New York University, illustrates not only the increasing strain that housing costs have had on this city of renters, but also the limits of the mayor’s success in providing the city’s poor with reasonable places to live. While the mayor’s plan has put thousands of low-income families in new or rehabilitated buildings and helped stabilize neighborhoods, it has been nearly drowned out by the twin waves of gentrification and rent deregulation.

The trends of city life are reversing, due to economic factors, lower crime, successful urban planning, and the like. It’s making cities a more attractive place to live, because you can get the upsides of city life – diversity, cultural excitement, shorter commutes – without the historic negatives. Except, squeezing out all of the diversity and culture through gentrification seems to reverse that.

Mayors like Bloomberg are trying to keep up, but this is clearly a bigger problem than classic development planning schemes can solve.