Environmental issues in the Niger Delta are caused by its petroleum industry. The delta covers 20,000 km2 (7,700 sq mi) within wetlands of 70,000 km2 (27,000 sq mi) formed primarily by sediment deposition. Home to 20 million people and 40 different ethnic groups, this floodplain makes up 7.5% of Nigeria’s total land mass. It is the largest wetland and maintains the third-largest drainage basin in Africa. The Delta’s environment can be broken down into four ecological zones: coastal barrier islands, mangrove swamp forests, freshwater swamps, and lowland rainforests. Fishing and farming are the main sources of livelihoods for majority of her residents.