Although the Dutch colonizers initially were motivated by the prospect of trade in the Caribbean, their possessions became significant producers of crops. The growing importance of agriculture was indicated by the export of 15,000 kilograms of tobacco from Essequibo in 1623. But as the agricultural productivity of the Dutch colonies increased, a labor shortage emerged. The indigenous populations were poorly adapted for work on plantations, and many people died from diseases introduced by the Europeans. The Dutch West India Company turned to the importation of enslaved Africans, who rapidly became a key element in the colonial economy. By the 1660s, the enslaved population numbered about 2,500; the number of indigenous people was estimated at 50,000, most of whom had retreated into the vast hinterland. Although enslaved Africans were considered an essential element of the colonial economy, their working conditions were brutal. The mortality rate was high, and the dismal conditions led to more than half a dozen rebellions led by the enslaved Africans.