When wolves disappeared from Yellowstone National Park, the primary impact was on the deer (elk) population, which increased significantly. The absence of wolves, as top predators, led to overgrazing by deer, which in turn caused a decline in vegetation such as willow trees and other plant species in areas near rivers and streams.
Here’s how the options relate to the situation:
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The willow tree population increases - This is incorrect. The willow tree population actually decreased due to the over-browsing by deer.
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The deer population increased - This is correct. Without wolves to keep their numbers in check, the deer population grew significantly.
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The grassy areas increased - This is partially correct in that overgrazing by deer can lead to reduced grass cover, but the more significant issue was the decline of other vegetation rather than an increase in grasses.
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The bear population increased - This is not directly true. While bears may have had access to more food as a result of the overbrowsing, they do not rely directly on the deer population as their primary food source. The effects on their population would depend on various factors, including food availability and competition.
In summary, the significant consequence of wolf disappearance was the increase in the deer population, leading to various ecological imbalances in Yellowstone.