Soru

### Passage

In the early twentieth century, gray wolves (*Canis lupus*) were systematically eliminated from Yellowstone National Park due to predator control programs. By the 1920s, the park’s wolf population was completely eradicated, leaving the ecosystem without its primary apex predator. For decades, scientists and park managers observed a gradual decline in the health of Yellowstone’s valleys, though the precise cause remained a subject of intense debate. It was not until the landmark reintroduction of gray wolves in 1995 that the full ecological significance of the species became clear. The reintroduction of gray wolves to Yellowstone National Park has proved that restoring a top predator can completely reshape and revitalize an entire ecosystem through a cascade of ecological changes. This phenomenon, known to ecologists as a trophic cascade, demonstrates how influence at the top of the food chain trickles down to affect every level of the environment.

With the wolves absent for seventy years, the population of elk (*Cervus canadensis*), the wolves' primary prey, had exploded. Unchecked by predators, large herds of elk gathered along river valleys, heavily browsing on young woody plants. This constant grazing prevented trees such as willows, aspens, and cottonwoods from growing beyond seedling height. The valleys became barren, and the lack of shade and young timber threatened other wildlife. However, the return of the wolves quickly disrupted this pattern. By reducing and dispersing the elk populations, wolves allowed overbrowsed streamside vegetation like willows and aspens to regenerate. As wolves hunted, elk were forced to move more frequently and avoid open valleys where they were vulnerable. Consequently, the vegetation in these valleys began to recover, growing several meters high in just a few years.

This botanical recovery initiated a surprising change in the physical landscape of the park. When the streamside trees and shrubs were stunted by elk, their shallow roots could not hold the soil in place. As a result, riverbanks rapidly eroded, causing streams to become wider, shallower, and muddy. Once the wolves returned and the vegetation recovered, the plants' growing root systems bound the soil along the water's edge. The regrowth of trees stabilized riverbanks and altered the physical geography of Yellowstone's waterways by reducing erosion. Rivers began to run cleaner and established more stable, meandering paths. Deep pools formed, creating critical microhabitats for aquatic life. Thus, the presence of a predator indirectly reshaped the very paths of the rivers.

The recovery of the forests and rivers in turn sparked a chain reaction among Yellowstone’s animal communities. The rejuvenated willow and aspen trees provided essential building materials and food for North American beavers (*Castor canadensis*), which had nearly vanished from the park's streams. Beavers returned in large numbers, building dams that created deep, cool ponds. These ponds became habitats for fish, frogs, and waterfowl. Additionally, the taller trees offered nesting sites for migratory birds, leading to a rise in bird diversity. The resurgence of vegetation and wolf-provided carrion created new habitats and food sources, boosting populations of beavers, birds, and other carnivores. Scavengers such as grizzly bears, bald eagles, and ravens also benefited from the remains of wolf kills, particularly during harsh winters when food was otherwise scarce.

Today, Yellowstone stands as a premier example of ecological restoration. The return of the gray wolf has shown that an ecosystem is not merely a collection of isolated species, but a complex, interconnected web where the loss or restoration of a single key player can have profound effects. While the reintroduction initially faced skepticism from local ranchers and communities, the scientific data gathered over the past three decades has solidified the project’s success. It serves as a powerful model for conservation efforts worldwide, proving that active restoration of trophic levels is a viable strategy for repairing damaged landscapes.

### Question
Based on the passage, match each of the specified paragraphs to its explicitly stated main or sub-main idea.