The fast-growing population of blue sheep, a once endangered species of wild sheep in northwest China, has created new problems for conservationists.

The average population density of the sheep was 40 to 50 per square km of forest in Qilian Mountain, a 900,000-hectare nature reserve in Gansu Province, said Ni Ziyin, a forestry official in the reserve.

"It's much higher than the upper limit of 10 per square km," said Ni.

The fast-moving species reproduces rapidly, causing the population to multiply by almost 10-fold every year.

Hunting was banned in the reserve and the sheep had almost no natural predators, as wolves and other large carnivores had disappeared decades ago as a result of hunting and other human activities, said Ni.

"The blue sheep far outnumber domestic cattle and are rapidly eating away the grassland," said Ren Jin, a villager in Pingshanhu Village,Zhangye City.

A sharp increase in the sheep's population has also harmed the ecological balance of the nature reserve they inhabit.

Wherever the blue sheep go, grass disappears, exposing the sandy topsoil of the arid region.

"Excessive numbers of blue sheep have made it hard for the vegetation around Qilian Mountain to survive," said Ni. "They eat not only grass, but also shrub leaves, which has done great damage to the vegetation."

This had led to the decline in the number of other herbivorous animals including wild donkeys and deer, he said.

"Unless something is done to curb their growth, the sheep will destroy all the pastureland," said Ren.

The blue sheep was hunted in large numbers before 1983, causing its population to fall sharply.

Central and local governments started protecting the animal in 1988, when nature reserves were established in Qilian Mountain and Mount Helan, in the neighboring Ningxia Hui Autonomous Region.

Overpopulation and damaged vegetation have been reported at both sites.

The Qilian Mountain Nature Reserve used to operate an international hunting ground in the 1980s. But that was stopped after China promulgated a law on wildlife protection in 2004.