The islands that make up Cape Verde were uninhabited when the Portuguese arrived in the mid-15th century, and in 1495 Cape Verde became a Portuguese colony. By the 16th century Cape Verde had become a shipping center for the African slave trade. Portugal had ruled Cape Verde and Guinea-Bissau as a single colony. In 1975, Cape Verde became independent, a year after Guinea-Bissau won freedom from Portugal. Plans to unify Cape Verde and Guinea-Bissau were abandoned following a coup in Guinea Bissau in 1980. A one-party system was established following independence and maintained until multi-party elections were held in 1990, and Cape Verde enjoys one of sub-Saharan Africa's most stable democratic systems. Cape Verde has few natural resources and suffers from poor rainfall and limited fresh water. Drought is a frequent problem, and the country is heavily dependent on food imports – over 80 percent of the food must be imported. Cape Verde’s economy is service-oriented, in particular tourism, which accounts for over 65 percent of GDP.