A line of Congolese park rangers raised their guns in salute on Monday as the coffin bearing their colleague Burhani Abdou Surumwe, a 30-year-old father of four, was buried in the black volcanic soil of a Muslim cemetery outside Goma. Abdou was one of six rangers killed on Sunday in an ambush in Democratic Republic of Congo’s Virunga National Park, a sanctuary for endangered mountain gorillas that is also home to dozens of armed groups. The rangers, heavily armed and well-trained, are frequently attacked. In April, a dozen of their number from the Congo Institute for Nature Conservation (ICCN) were…