In simple words, Many habitats were disrupted by colonisation, which brought in new species thus eradicating some. The Europeans carried with themselves a slew of illnesses that wiped out Native American societies. Both colonists as well as Native Americans turned at new plants for medicinal purposes.
Discovery in Europe is motivated by three factors. For the benefit of their wealth, piety, and fame, they exist. They desired to develop their economies by purchasing more herbs, jewelry, and easier and quicker trade routes, for example.
Explorers like Columbus took on voyages from Europe to discover the New World. The discovery of North America allowed Europeans to migrate and set up colonies. More explorers explored the lands beyond colonies and made contacts with native Americans. The native American taught them survival skills. Further exploration greatly extended the colonists' reach across the plain; finally to the Rocky Mountains and beyond.
Without those explorers, America would have been a very different country.
