why did the ecosystem in the yellow stone river valley change so much from the mid-1920s to the mid-1990's?

Explanation:

A sequence of flowing lava, pyroclastic flows, volcanic eruptions, caldera-forming events, and earthquakes changed the landscape of Yellowstone forever. The pressure exerted by the magma chamber has resulted in measurable ground deformation in certain parts of Yellowstone known as resurgent pressure domes.

Fire Activity also causes the Yellowstone to change. As the number of very large wildfires and total acres burned annually increases, there is an increasing frequency of warm spring and summer temperatures, reduced winter precipitation, and early snowmelt in the Western United States during the last 20 years.

In 1995, the wolf population increased in Yellowstone, causing the deer population to decrease and to change their behavior. When threatened by wolves, deer graze less and move about to aerate the soil.

In the 70 years since the wolves left Yellowstone, the entire ecosystem had collapsed into chaos, with coyotes running riot, and elks overgrazing willows and aspens. Without those trees, songbirds declined, beavers could not build their dams, and riverbanks eroded.