While there’s been evidence of significant shifts in U.S. population growth over the past decade, the decennial census performed earlier this year gives us an opportunity to learn precisely what’s been happening and end some of the “speculation.”

And now, with the U.S. Census Bureau releasing its preliminary population reports, we’re seeing how this has played out in cities across the country. While it’s true that the American population has grown pretty steadily at about 2.5 million people per year, some areas have grown much faster than others as a result of being better positioned through the education of their workforce and/or their business- and technology-friendly environments.

Alas, other areas haven’t merely stagnated, but actually lost residents because of failing industries and unattractive business climates, sparking net out-migration of their residents.

Interestingly, many of the cities in the “industrial heartland” of America have managed to stay on the positive side of population growth – even if just barely. But some cities have experienced such hardship that their populations have dropped dramatically in the past decade.

New Orleans tops the list … and who’s surprised about that? After all, Hurricane Katrina effectively robbed the city of one-third of its residents – with most of them electing not to return after establishing new livelihoods in Houston, Shreveport, and other localities further yon.

But New Orleans surely represents a “special case” if ever there was one. Other cities have suffered greatly due to their dependence on industries that took a beating over the past decade. And really, any city with a major focus on traditional manufacturing saw thousands of jobs disappear.

According to the U.S. Bureau of Census report on the nation’s largest cities — ones with 100,000+ population — the seven experiencing the biggest percentage declines in population over the past decade are:

1. New Orleans, LA – Dropped by ~129,000 to ~355,000 (-27%)
2. Flint, MI – Declined by ~13,000 to ~112,000 (-11%)
3. Cleveland, OH – Fell by ~45,000 to ~431,000 (-10%)
4. Buffalo, NY – Dropped by ~22,000 to ~270,000 (-8%)
5. Dayton, OH – Declined by ~12,000 to ~154,000 (-7%)
6. Pittsburgh, PA – Dropped by ~22,000 to ~312,000 (-7%)
7. Rochester, NY – Declined by ~12,000 to ~207,000 (-6%)

[I was a bit surprised to see Detroit missing from this list. After all, it’s the poster child for urban decay and depopulation. But Detroit’s population percentage decline was actually smaller than the cities above, and it remains the nation’s 11th largest city. However, the 2010 census will likely show that its population has fallen below 800,000 for the first time in nearly a century – and the figure is even more startling when you realize the city’s population was nearly 2 million as late as the 1950 census.]

Unfortunately, the negative implication of population declines in these proud American cities go far beyond the loss of social prestige and political clout.

Once decline sets in, it can go on for years. The loss of residents contributes to a drop in tax receipts and the subsequent curtailing of social services ranging from police and sanitation to schools and recreation. Home vacancy rates say volumes about the precarious position in which the cities above find themselves – they’re above 15% in every single case (and sometimes dramatically higher).

Confronted with such a reality, too often the result is more people fleeing the urban core, creating a continuing downward spiral that seemingly has no bottom. Representative examples of where this sorry state of affairs can end up can be found in two smaller but particularly grim urban communities: Camden, NJ and Chester, PA.

From the outside looking in, it’s difficult to accept these population reports … and it seems like people should step in and do something – anything – to arrest the decline.

And in the abstract, it’s only natural to feel that this is what should happen. But in the “real world,” who are going to be the ones to step up to the plate and expose themselves (and their families) to the harsh reality of urban pioneering?

Would I do it? Would you?

For most of us, the answer to that question falls into the “life’s too short” category.