But an international approach to preserving a unique storied area along Texas-Mexico border has bucked the trend helping preserve, and even revives, at least one park, according to the association.
Big Bend National Park, a breathtaking expanse of West Texas, is a sliver of a larger desert ecosystem, a major part of which exists in Mexico, south of the Rio Grande.
In 2003, the conservation association rated the park abysmally in terms of its ecosystems, visitation rates and cultural resources.
Since then, however, the park has seen improvements that have revived the region and protected wildlife such as black bears, bats and peregrine falcons.
"I see it on an upward trend. I think it's getting better," James Nations, head of the association's Centre for the State of the Parks, which put out the report, told Reuters.
Experts point to a push from US and Mexican leaders for preservation along both sides of the border as a driving force behind that improvement.
US President Barack Obama and Mexican President Felipe Calderon released a joint statement in May urging cooperation in the region, which the statement said is "one of the largest and most significant ecological complexes in North America."
In January, the United States took a first step that could create a binational park, which has been called for as far back as President Theodore Roosevelt, and as recently as last year when Interior Secretary Ken Salazar visited.
The United States announced it would reopen the informal border crossing at Boquillas that had long provided an important source of income to Mexican villagers by allowing American tourists to easily visit and buy souvenirs. The crossing was closed after the Sept. 11, 2001, attacks.
Conservationists believe the reopening will recharge cooperation between the two communities, and encourage ownership of a single park by the two countries.
"I think [the village] will benefit, and economic benefit will show them conservation is a positive thing, and that's an incentive for protection," Nations said.
Big Bend National Park Superintendent William Wellman said the reopening will ease research and restoration projects that have to be conducted on both sides of the river.
"It's made it much harder," Wellman said of the closing. "You have to do everything sort of remotely, even though you're trying to work with people just across the river from you."
The bi-national push, of which the reopening was an integral part, has paid off in the revival of wildlife, Big Bend park wildlife biologists said. Mexican attempts to restore its half of the ecosystem also have played an important role.
It's especially evident in the black bear population, which has made a comeback in Big Bend from the few endangered bears that roamed the region some 20 years ago.
The park's population, now relatively stable at about 20 bears, was founded in the late 1980s when a female found her way across the Rio Grande from Mexico, met a male and formed a mating pair.
"The future of bears in Big Bend depends on there being a healthy population in Mexico," said Raymond Skiles, Big Bend wildlife biologist.
"If they were completely isolated from the Mexican bears it's just really highly unlikely that such a small population could persist on its own."
The black bears are not the only species on the rebound.
In Maderas del Carmen, a 500,000-acre (202,343-hectares) preserve in Mexico, one of the nation's largest cement companies, Cemex, has spent roughly a decade reviving grasslands which feed grazers like desert bighorn sheep, now making a comeback in the Big Bend region. Those grazers provide food for predators like mountain lions.
Small populations of peregrine falcons have historically lived in the park's cliffs, but relied on the more extensive mountain ranges in Mexico for food, shelter and mates.
Endangered bats migrate from Mexico to Big Bend, their livelihood also dependent on both sides of the Rio Grande.
"It's never going to be what it was 100 years ago. There's just not enough water and too many exotic species, but we're getting a lot closer to that," Wellman said.
"You can only do that if you work with both sides."
Copyright Reuters, 2011