El Carmen is one of the most unique and biodiverse ecosystems in North America, home to at least 400 plant species, 220 bird varieties, more than 50 mammals and with species of 64 reptiles and amphibians also recorded.

The McKinneys, who worked for the Texas Parks and Wildlife Department, crossed over the river a year ago to join an unusual project to manage and preserve El Carmen.

It's unusual because it is not the government but one of Mexico's largest corporations - the world's third largest cement maker Cemex - that is driving the plan that will effectively create a private national park.

"We had a great life in the Big Bend National Park, but we used to dream of working in these mountains. They are so beautiful, so unique; so when we got the chance, we jumped at it," said Virginia-born Bonnie McKinney.

Cemex, whose industry is not the most environmentally friendly, is giving something back to Mother Earth.

El Carmen, a collection of huge private ranches in Coahuila state stretching over 800 square miles (2,000 square km), was designated a national park by Mexico in 1994, which put an end to logging and mining.

Cemex has about a quarter of the national park under its control after buying up two huge tracts and agreeing to manage and preserve another 50,000 acres (20,000 hectares) with another local landowner.

The company, based in the northern industrial city of Monterrey, would love to look after all of El Carmen, which is so remote the endangered Mexican black bear has found a last refuge there and is healthily reproducing.

FOCUS ON WHOLE ECOSYSTEM

The McKinneys lead a dozen young environmentalists living permanently at a base camp at the foot of the mountain range and just outside the El Carmen national park.

"So many wildlife projects focus on one species. This one focuses on the whole ecosystem," said Bonnie McKinney at the camp, a seven-hour drive across desert from Monterrey.

"This whole area has been mined, logged and overgrazed. No one had the resources to set up a conservation project. It took a different line, like the input from Cemex, to get this project started," she added.

The first stages of the project were to clean up El Carmen - tearing down barbed wire and fence lines from the area's bygone cattle ranching days. Massive overgrazing of delicate desert grasses has meant vast areas have lost their seed banks - rebuilding the seed bank is also on the project agenda.

The project workers currently are making an inventory of all the fauna and flora in the national park, counting all the species of mammals, birds, reptiles and plant life they find at El Carmen.

Many species are endemic such as the Carmen white-tailed deer and the cliff chipmunk. Others are threatened, such as the northern harrier, jaguarundi and the Texas horned lizard.

In the future, a permanent exhibition about the national park will be set up and further research on its flora and fauna will be encouraged.

There are plans to allow Cemex clients to visit the park - throwing in horse riding and hiking tours - and low density ecotourism might also be allowed for the general public, said Armando Garcia Segovia, Cemex's executive vice president of development.

"It cannot be a place for very intensive visits because ecosystems are fragile," Garcia said. "We have to find a way of allowing visits but with a certain degree of care."

"The end result should be a world class place for people to visit," said Billy Pat McKinney.

HUNTED TO EXTINCTION

Plans also include the reintroduction of animal species, starting with the desert big horn sheep, hunted to extinction in Coahuila and not seen wild in the state since the 1940s.

And if there is evidence that El Carmen also used to be home to elk, buffalo, grisly bears and wolves, these animals all could follow in the reintroduction plans, Garcia said.

Reintroducing lots of different large animals at El Carmen is key to make sure there is a balance of wildlife. If only sheep were reintroduced, the mountain lions would have a feast. "We'd be just releasing food for the lions," Garcia said.

To reintroduce the desert big horn sheep, Cemex has constructed a 15,000 acre (6,000 hectare) reserve and brood park on land it owns just outside the national park, with high, predator-proof electric fencing.

Desert big horns from western Mexico have been released into the reserve - which is so big the sheep have no clue they are in an enclosed area - and after this year's breeding season, now number 53.

"Before we released the sheep, we captured two bears and one mountain lion that were inside the reserve and released these big predators back into wild," said Bonnie McKinney during a tour of the enclosure.

"When the big horn numbers are big enough we'll start wild releases of them in El Carmen," she added.

Mexico, where environmental concerns are often not a high priority of governments or companies, has close to 150 national parks, but the federal budget only provides sufficient cash to run about a dozen of them adequately.

Cemex, with operations in more than 30 nations spanning four continents, would not reveal the amount of investment in El Carmen.

But it hopes other Mexican firms will follow its example.

"We are trying to inspire other companies to do the same because we believe we are making an important contribution to the world, to the patrimony of mankind," said Garcia.