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“Within a generation, it would almost certainly reduce the genetic diversity even more,” Mr. Cothran said. “They would be at a pretty high risk to be unable to survive another severe bottleneck.”

That would be disastrous for horse lovers and the local economy, which has capitalized on the herd as a tourist attraction.

Six companies give guided tours of the horses’ habitat to thousands of tourists every year, bringing in hundreds of thousands of dollars.

“There’s the lighthouse and there’s the horses, and aside from that there’s no other kind of economic activity,” said Kevin Jackson with Bob’s Off-Road Wild Horse Adventure Tours.

The wild horse fund has put its hopes in a bill introduced in Congress by Rep. Walter B. Jones, North Carolina Republican. The bill would set a target population of between 120 and 130 horses and allow the introduction of mares from the herd at Shackleford Banks, at the other end of the Outer Banks, to enhance the genetic diversity of the Corolla horses.

In the 1990s, Mr. Jones authored legislation that brought similar federal protection to the wild horses at Shackleford Banks, in the Cape Lookout National Seashore.

The Fish and Wildlife Service commissioned a two-year study that began in June to measure the effects on the region of nonnative species such as the horses.

“We don’t really know what’s going on over there, and we’d like to get the science together so we can better manage our resources there,” said Mike Hoff, manager of the Currituck refuge.

But the federal government agrees with the horses’ advocates that booming development is a challenge to all wildlife, native and nonnative alike.

“The county says there are 3,195 developable lots up there,” Mr. Hoff said. “At that point, what habitat is left?”