When wolves were removed from Yellowstone National Park, the deer population increased significantly due to the lack of natural predators. This led to overgrazing and a decline in vegetation, particularly in mountain streams, which impacted other species and the overall ecosystem.
Therefore, the correct answer is:
A. the deer population increased.
While the increased deer population had various effects on vegetation, including the reduction of willow and other plant species, the most direct impact was the significant increase in deer numbers.