The best transition to connect the sentences and synthesize the information in the passage would be:
"Consequently, grazing herds sharply rose."
So the full sentence would read:
"In the 19th and 20th centuries, widespread efforts were made to reduce the number of wolves at Yellowstone National Park. Consequently, grazing herds sharply rose."
This transition clearly indicates that the increase in grazing herds is a result of the reduction in wolves.