Nigeria has been a member of the Organization of the Petroleum Exporting Countries since 1971. The mission of the organization is to coordinate and unify the petroleum policies of its members and ensure a stable oil market. Nigeria has the second largest oil reserve in Africa and as of the 1980s oil revenue has provided 90% of their foreign exchange earnings. Yet, in the past 50 years, an estimated 9 to 13 million barrels of oil have been spilled in the Niger Delta ecosystem. The oil spills can have massive effects on the large mangrove swamps along the coastline. Crude oil can destroy mangroves by coating the roots and from direct absorption. The coating of the roots can hinder salt exchange and the oil can further contaminate the soil. The contaminated soil can cause huge problems for agriculture. The stress of living near crude oil production sites is also an environmental stressor that can cause effects on health, well being and the quality of life.