LHASA, June 7 (Xinhua) -- Tibet's Ali Prefecture has seen its population of protected animals increase because of 20 years of conservation work, local forestry authorities claimed Thursday.

The Forestry Bureau of Ali Prefecture said Class-I protected animals were doing well, including the Tibetan antelope with a population that has grown from around 20,000 to 60,000. The Tibetan wild ass population is close to three times what it was 20 years ago.

The number of snow leopard doubled during the period.

At an average altitude of more than 4,500 meters, Ali Prefecture in western Tibet, covers more than 300,000 square kilometers. There are 14 nature reserves and three national forest and wetland parks.

The prefecture is home to seven Class-I and 12 Class-II protected animals.

Increased investment, more staff, better management and appropriate law enforcement, have led to increased wetlands and greater biodiversity, according to the forestry bureau.