DESERET RANCH, Fla. - Under one of the biggest land development plans ever proposed in Florida, a tract more than six times the size of Manhattan could be transformed from a home for cows and alligators into new housing developments for half a million people.

Over the next six decades, the plan being developed by the Mormon church-owned Deseret Ranch promises to convert the largest undeveloped section of metro Orlando into more than a dozen bustling neighborhoods. In the process, it would radically reshape Osceola County, a suburb that has been transitioning from cowboy culture into a major destination for Puerto Ricans moving to the mainland.

Opponents say the plan to convert cattle pastures to cul-de-sacs could destroy tens of thousands of acres of important habitat near the headwaters of the St. Johns River flowing north and the Kissimmee River flowing south. They also worry that Osceola County has been doing the bidding of Deseret Ranch, one of the nation's largest ranches. County commissioners approved the ranch's plan unanimously in September and now it awaits a state review.

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"There are no checks and balances happening," said Karina Veaudry, a landscape architect, who is with the Florida Native Plants Society. "The county has been bending over backward pretty much to do whatever they say."

But other environmentalists who once opposed Deseret's proposal have dropped their objections, saying the plan shifts population growth to east of the county's urban core, rather than to its south where development might pose a greater threat to the headwaters of the Everglades. They also say it's better to have a comprehensive plan rather than breaking off 1,000-acre tracts at a time for piecemeal development. Osceola County currently is about three-quarters rural.

"The future of that property is better with this plan, than the plan not being there," said Charles Lee, director of advocacy for Audubon Florida. "Doing it on a large scale gives you a lot of possibilities for preservation."

The plan looks ahead six decades for property that the Church of Jesus Christ of Latter-day Saints began purchasing more than six decades ago. The church began acquiring ranchlands in 1950 to have resources for food production in times of need. The ranch now has 42,000 cattle and more than 80 workers, and straddles three counties in an area nestled between two of central Florida's iconic institutions: Walt Disney World to its west and the Kennedy Space Center to its northeast.

"This is the way we're going to grow for the next 60 years, maybe 100 years," said Commissioner Fred Hawkins before the vote.

Deseret Ranch general manager Erik Jacobsen called the development plan a way to meet growth pressures smartly in what's expected to be one of the nation's fastest growing regions in the coming decades. He said it will be bicycle- and-pedestrian-friendly and an economic boon, connecting Orlando's medical research hub south of the airport with tech companies along the Space Coast. The county, one of the nation's fastest-growing, currently has a population of 310,000, expanding by 15 percent since 2010.

"It provides a very important framework for the future," Jacobsen told commissioner before the vote.

The first shovel of dirt won't be turned for another two decades and the 133,000 acres of pastures, citrus groves, and woodlands slotted for development make up less than half of the ranch's nearly 300,000 acres.

Deseret would likely rely on outside developers. By 2080, the former ranch land could accommodate up to half a million residents, which would make it Florida's 10th largest metropolitan area in today's population. In a best-case scenario, the plan could lead to a quarter million jobs and have an economic impact worth hundreds of millions of dollars, if not more, said Jeff Jones, the county's economic development chief.

An independent environmental review commissioned by Osceola County recommended setting aside 19,000 acres more land for conservation on top of the current 56,000 acres planned for conservation and agriculture. But the final written plan doesn't include it, to the dismay of Veaudry.

Audubon Florida originally opposed the plan but dropped its opposition after Deseret tweaked some of the more controversial proposals. Some 19,000 acres are being set aside for parks and trails. Language greenlighting the construction of a causeway through a conservation tract was made more tentative. A proposal to allow two creeks to be dammed to build a second reservoir on the property now requires approval from state and federal regulators.

Veaudry isn't buying the compromises, especially the proposal of a second reservoir.

"Why do they need another reservoir?" she said. "They're trying to get future water rights."

Deseret already is in six-year a legal battle with state regulators over whether it can charge public utilities in central Florida to use the water from a reservoir on its property.

"Deseret has had a vision for a long time of being a producer of potable water to sell to others," Lee said.

Hardly any of today's planners likely will be around when the land is built out by 2080. Because of that, observers say it will be an ambitious task to make sure that what is proposed in 2015 is carried out by 2080.

"It's a big, complicated project on a tremendous scale," said Jay Exum, an ecologist who co-wrote the independent environmental review. "The size, scale and location of the project justifies a lot of attention."