The components of the ecosystem in this scenario are the stream, willow trees, deer population, mountain trout, and mountain lion.
If the mountain lion population vanishes, the deer population would likely increase due to the absence of their primary predator. This increase in deer population would lead to overgrazing of the willow trees, which would negatively impact the habitat of the mountain trout. Without the shade provided by the willow trees, the mountain trout population would decrease, which would have a ripple effect on the entire ecosystem. The decrease in mountain trout population would impact the food chain, as other predators that rely on them for food would also be affected. Overall, the disappearance of the mountain lion population would have a significant impact on the balance of the ecosystem.