__________________ were most negatively affected by westward expansion and the new fencing methods.

Answer:
Plains Indians (Plains Native Americans)
Explanation:
As the colonists moved west, they wanted more land. They wanted to establish more cities/villages in the plains and also in the west where the Native Americans lived. The Native Americans also depending on the migration of the animals - deer, buffalo, etc. - for hunting which provided them with food and clothing. However, when the colonists moved west, they put up more fences which prevented animals from freely moving among the plains. The fences also interrupted and changed the animal migration routes.