Kinshasa, Congo ? Farmers have overrun thousands of acres in Congo’s oldest national park, the latest threat to more than half the world’s 700 remaining mountain gorillas, conservationists and park workers say.

Stacking lava rocks, 200 workers are building a wall at the Rwandan border of Virunga National Park in a desperate effort to stop farmers, fighters and refugees from sweeping into the home of the endangered primates.

After a decade of conflict, militia forces still roam Virunga’s forests, said Eugene Rutagarama, head of the International Gorilla Conservation Program. Officials suspect the fighters in the killings of three park workers in the past month.

“We want to have this wall built as soon as possible,” Rutagarama said Thursday.

The 3-foot-high wall, however, will be little more than a “symbol to show the limit of the forest,” he acknowledged. “I don’t think it can really stop people.”

The gorillas live on the misty, green tops of volcanoes along Congo’s and Rwanda’s border, nesting in the forests as farmers work terraced fields on the volcano sides below.

Gorillas strip bamboo forests for food, clutching their infants. Occasionally, they venture into the farmers’ fields in search of crops.

In Virunga live 380 of the mountain gorillas. Their only other known home is the Bwindi Impenetrable Forest, a national park in Uganda.

A female mountain gorilla called Mugeni, 15, and her 5-month-old son, Bonane, are seen in the Kahuzi Biega Nature Park near the town of Bukavu, Democratic Republic of Congo.