The best transition to connect the sentences and synthesize the information in the passage is:
"In the 19th-and 20th-centuries, widespread efforts were made to reduce the number of wolves at Yellowstone National Park. As a result, grazing herds sharply rose."
This choice effectively shows the cause-and-effect relationship between the reduction of the wolf population and the increase in grazing herds, which is consistent with the information provided in Source 1.