ANGELES NATIONAL FOREST – When Glendora rock climber Troy Mayr started scaling the steep granite walls at Williamson Rock 15 years ago, he had the place pretty much to himself.Now, on a peak summer weekend, he shares the crags with 250 to 500 people, who come to climb at what has become one of the premier climbing destinations in Southern California.But this season, Williamson Rock, 6,700 feet above La Ca?ada in the San Gabriel Mountains, will be unusually quiet.Williamson has been closed to protect the mountain yellow-legged frog, whose numbers have been decimated. An estimated 100 adult frogs remained in the Southern California population in 2002, when it was listed as a federally endangered species. Scientists have not fully determined why the frog is struggling, but it may be due to fires, non-native predators and disease.The frogs live just upstream and downstream of the main climbing area in a stream at the base of the gorge. The Forest Service on Dec. 27 closed access to 1,000 acres north of Angeles Crest Highway in the Cooper Canyon area.Once considered a sport for only the most adventurous outdoorsmen, rock climbing has exploded in popularity over the past 10 years, with better equipment and the advent of indoor climbing gyms making the sport safer, more accessible and easier to learn.And Williamson Rock, with its 300 climbing routes, mild summer weather and convenient location off the Angeles National Highway, is a climber’s dream.“Anyone who is a climber in L.A. has been there, goes there, or is planning to go there,” said Kenny Suh, 24, of Pasadena, who teaches and works at an indoor climbing gym in Arcadia.Suh avoids Williamson on summer weekends because of the crowds.“There are dogs running around, kids running around. It gets noisy. Sometimes, it feels like a gym. But you can still find places to climb by yourself if you hike in more,” said Suh, who was dismayed at the closure.“I didn’t even know there were frogs there.”But as more crowds have flocked into the canyon, official management of the area has lagged.Two trails leading into the gorge are unmarked, and climbers and hikers who venture offtrail can land in frog habitat, or on easily eroded scree (rock debris) slopes, the home of a threatened plant species, Johnston’s Buckwheat. Some climbers leave behind human waste and trash.Mayr was among the first to set routes at Williamson, and later put the spot on the map with magazine articles and a climbing guidebook he wrote.He said he warned the Forest Service a decade ago that Williamson was going to be extremely popular, and they needed to develop one clear trail with signs, and find a way to dispose of human waste.There is a sign for Williamson Rock at the trailhead, and a parking lot, but no bathrooms.Cid Morgan, Angeles National Forest ranger, said most rock climbers are environmentally conscious.“You don’t notice the 99 percent of people following traffic rules. You notice the one person weaving in and out of traffic,” she said. “Unfortunately, if we have to close it, we have to close it to everyone.”While the likelihood of someone actually stepping on a frog is low, humans could disrupt egg masses, trample down habitat or degrade water quality with trash and waste, she said.The Forest Service may work out an arrangement to eventually allow climbing at Williamson, such as issuing a limited number of passes to visitors, making access to the area weather dependent, or assigning a ranger to monitor the spot, Morgan said.Mayr is organizing a nonprofit organization, Friends of Williamson Rock, to manage the area and raise funds, possibly to pay for a ranger to monitor the area.“The Forest Service has been very open minded to suggestions,” Mayr said.The length of the closure is unknown, but will probably last through the summer at least, officials said. Researchers are studying the impacts of human activity on the frog during the closure, which comes after an environmental group, the Center for Biological Diversity, sued the U.S. Fish and Wildlife Service for failing to designate critical habitat for the frog.Lori Paul, a certified wildlife manager from Altadena, said the smallest species can often serve as indicators for the rest of the wildlife in an area. Studies of animals with short lifespans can reveal any damage human activity may be wreaking on a habitat long before any signs in longer lived species would show up.Since the frogs live in just a handful of pockets throughout the state, isolated populations can serve as reserves in case of catastrophic disaster in one region.“No one isolated population in one sense is truly expendable,” Paul said.Staff Writer Kimm Groshong contributed to this story. 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