During the late 16th and early 17th centuries, Spain concentrated its colonial efforts on the more prosperous mainland North, Central, and South American colonies. With the advent of the lively Bourbon Dynasty in Spain in the 1700s, the island of Puerto Rico began a gradual shift to more imperial attention. More roads began connecting previously isolated inland settlements to coastal cities, and coastal settlements like Arecibo, Mayaguez, and Ponce began acquiring importance of their own, separate from San Juan. By the end of the 18th century, merchant ships from an array of nationalities threatened the tight regulations of the Mercantilist system, which turned each colony solely toward the European metropole and limited contact with other nations. U.S. ships came to surpass Spanish trade and with this also came the exploitation of the island's natural resources. Slavers, which had made but few stops on the island before, began selling more enslaved Africans to growing sugar and coffee plantations. The increasing number of Atlantic wars in which the Caribbean islands played major roles, like the War of Jenkins' Ear, the Seven Years' War and the Atlantic Revolutions, ensured Puerto Rico's growing esteem in Madrid's eyes. On April 17, 1797, Sir Ralph Abercromby's fleet invaded the island with a force of 6,000–13,000 men, which included German soldiers and Royal Marines and 60 to 64 ships. Fierce fighting continued for the next days with Spanish troops. Both sides suffered heavy losses. On Sunday April 30 the British ceased their attack and began their retreat from San Juan. By the time independence movements in the larger Spanish colonies gained success, new waves of loyal creole immigrants began to arrive in Puerto Rico, helping to tilt the island's political balance toward the Crown.