•why did europeans in general, and the portuguese in particular, take the lead in navigation and exploration in the late 1400s/early 1500s?

•what was the "columbian exchange," and what were some of its consequences (both positive and negative)?

Europeans like Spain and Portuguese took the lead for navigating and exploring because they were searching for the new route to find wealth and spices. The exploration began because of the high prices of the imported goods and spices brought by the Silk Route. Europe decided to find a new route by voyaging through seas to Orient/ Far East for spices, silk and other materials. The period came to be known as Age of Exploration because Europeans were engaged in voyages and exploration to become rich.

Columbian Exchange began after  Christopher Columbus discovered America. Spanish settlers made trading connections with the natives and brought animals (horses, pigs, goats, and chickens) and new goods in America as well as took many plants and other goods to Europe. European not only brought goods and settlers but brought diseases in the New World. Majority of the Native American died after coming connecting with smallpox, chickenpox, cholera, malaria, measles, scarlet fever, etc.