Mackinac Island -- News of Dan Gilbert’s hospitalization for an apparent stroke is a jarring reminder to Detroit of just how much of its well-being is invested in one man.

No other city in America is as heavily dependent on the commitment of a single individual as Detroit is on Gilbert’s.

More: Dan Gilbert hospitalized with stroke-like symptoms

The head of the privately owned Quicken Loans/Bedrock/Rock Ventures empire has over the past decade invested nearly $6 billion in rebuilding the Motor City. He has rehabilitated, or is in the process of doing so, roughly 100 buildings across downtown, transforming the central city into one of the hottest development prospects in the country.

His family of companies employs 17,000 people, most of them located in the downtown buildings Gilbert has restored. He is now both the largest employer and the biggest taxpayer in the city.

No one can make a credible case that Detroit would have come as far as it has as quickly as it has had Gilbert not made it such an intense personal mission to restore it to a place among America’s premier cities.

So what happens should he no longer be able to lead his enterprise at the same level of personal involvement?

You can be sure that will be the major topic of conversation on the Grand Hotel porch as Michigan’s business, civic and political leaders gather on Mackinac this week for the Detroit Regional Chamber’s annual policy conference.

Bedrock has been able to pour so much money into Detroit because it is wholly owned by Gilbert; he doesn’t answer to stockholders and doesn’t have to worry about Wall Street’s reaction to quarterly earnings reports. His mortgage company has been a giant cash cow, generating revenue he can spend as he pleases.

I asked him once why he’s putting so much of his fortune into Detroit. “Because I can,” was his answer, meaning he has the rare freedom to make investments without the need for immediate returns.

That’s critical because Detroit is still a long bet. In a conversation we had a few weeks ago, Gilbert said, “I don’t know when some of this stuff is going to pay off. It could be decades, it could be 100 years.”

That liberty to be an altruistic investor has allowed Gilbert to take on dozens of projects at once and put Detroit on a faster comeback track.

At the same time, it has made Detroit overly dependent on Gilbert’s checkbook, and that could disappear from Detroit as quickly as it arrived in 2010, when he moved his headquarters from Livonia to downtown.

The succession plan for the companies post Gilbert is unclear. No other family members are actively involved in the company. Should he be unable to run them, the worst case scenario is that the empire will be broken up and the individual pieces sold, as happened to the Guardian Industries umbrella of companies when Bill Davidson died.

While there are other major developers active in Detroit, none comes close to having Gilbert’s impact or influence.

And the truth is, despite this decade of mind-boggling progress, Detroit still has a long way to go. On Gilbert’s plate this summer, for example, is the start of the 900-foot Hudson Tower skyscraper and the Monroe Block project, both designed to fill longstanding holes in the downtown footprint.

Regardless of the weather, the question of what might happen to Detroit without Gilbert will hang over Mackinac Island this week like a big, black cloud.