MIAMI (Reuters) - A Florida real estate developer is putting on hold plans to build a Walmart store and hundreds of apartments after federal officials last week moved to protect two species of endangered butterflies on the proposed site, a rare tract of forest brimming with wildlife.

"The last thing these butterflies need is another strip mall smack in the middle of some of their most important habitat," said Jaclyn Lopez, a Florida attorney for the Center for Biological Diversity, a national conservation group.

The U.S. Fish and Wildlife Service last week added two butterfly species to its endangered species list, and designated 11,000 acres of pine rockland forest in seven separate parcels as critical habitat, which includes the site of the proposed Walmart development.

The designation has posed a challenge for Boca Raton-based Ram Realty Services, which closed on the $22.1 million, 88-acre tract in early July with plans to build a Walmart, a gym and 408 apartments in a southwestern suburb of Miami.

Ram said it will set aside 43 acres for a nature preserve and will survey the land to determine how best to comply with the designation, which goes into effect Sept. 11.

"We have committed to refrain from any on-site work, other than hand removal of invasive species, until the issues raised by Fish & Wildlife have been resolved," Ram Chairman Peter Cummings said in an emailed statement.

Local conservationists have already launched a petition that has garnered more than 78,000 signatures calling on federal wildlife officials to stop the project, noting that Walmart, the world's largest retailer, already has several Miami-area stores.

Once the listing of the butterflies becomes formal next month, environmentalists will have more weapons to use, "if not against the developer then Miami-Dade County for illegally permitting things which could result in the harm of endangered species," said Dennis Olle, a Miami attorney and board member of the Tropical Audubon Society.

Lopez said Ram could be required under the Endangered Species Act to seek a federal permit to mitigate any potential harm to the butterflies, a process which is subject to public comment. "We are prepared to engage in this process moving forward," she said.

Walmart said in a statement it expected Ram would cooperate with regulators to protect "a rich and unique nature preserve for generations."

The University of Miami has been criticized for selling the land to Ram after being responsible for it since the 1940s, when the federal government divvied up a decommissioned naval base.

The land was deeded to the university for free in 1981 with a 30-year requirement to use it for educational purposes and to preserve it.

"The university has always been committed to the protection and preservation of our community's natural and historic resources," spokeswoman Margot Winick said.

Olle criticized the university for what he described as neglect of the rare forest during its 33-year ownership.

"Shame on UM. Big piles of shame," he said.


(Additional reporting by Barbara Liston; Editing by David Adams and Leslie Adler)