The correct answer is willow. The scarcity of willow as a food source caused the Yellowstone National Park beaver population to decrease. Willow is a significant part of a beaver's diet, and its scarcity in the park impacted the availability of food for the beavers. This scarcity might have been a result of various factors such as environmental changes or disturbances.
The food source whose scarcity caused the Yellowstone National Park beaver population to decrease is willow.
