after 70 years of absence, the gray wolf was reintroduced to yellowstone national park in 1995. after 1995, the elk population, which had been exploding, began to decline in numbers. willows and aspen trees began to grow more abundantly on riverbanks. the beaver population began to increase, and songbirds were more abundant in the grasslands and riverbanks. what is the most likely explanation for these events after the wolf was reintroduced?