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Difficulty: EasyArgument Development and Evidence Sequencing

The following passage is adapted from an article about ecology in Yellowstone National Park.

In 1995, conservationists embarked on a historic initiative to reintroduce gray wolves to Yellowstone National Park, aiming to restore ecological balance to an ecosystem that had suffered in their absence. For decades, the elimination of these apex predators had allowed the park’s elk population to multiply unchecked. The overabundant elk grazed heavily on young willow, aspen, and cottonwood trees, particularly along riverbanks. This overgrazing eroded the soil and degraded the habitats of numerous other species, demonstrating the profound disruptions that occur when a key predator is removed from its environment.

The return of the wolves initiated a powerful ecological phenomenon known as a trophic cascade, starting with immediate changes in prey behavior. With wolves back in the landscape, elk could no longer graze leisurely in open areas. Instead, they began to avoid places where they could be easily trapped, such as valleys and gorges. Consequently, the vegetation in these recovered zones began to regenerate rapidly. In some areas, trees quintupled in height in just a few years, establishing new forests that provided nesting sites for migratory birds and wood for beavers to build dams.

These physical and biological shifts eventually transformed the entire physical geography of the park. The beaver dams created ponds that became habitats for fish, amphibians, and reptiles. Moreover, the recovering trees and shrubs stabilized the riverbanks, reducing soil erosion and causing the rivers to flow in more defined, stable paths. The successful reintroduction of wolves demonstrates that restoring a single native species can trigger a sequence of positive environmental changes, proving that apex predators are essential for maintaining the structural integrity of their native ecosystems.

Based on the passage, arrange the following ecological events in the correct chronological order, from the earliest state of the ecosystem to the latest stage of the trophic cascade.

  1. 1The unchecked multiplication of elk leads to severe soil erosion and tree degradation along riverbanks.
  2. 2Gray wolves are reintroduced to the park, causing elk to avoid grazing in open valleys and gorges.
  3. 3Willow, aspen, and cottonwood trees rapidly regenerate, providing material for beavers to construct dams.
  4. 4Beaver dams form ponds that establish new habitats for fish, amphibians, and reptiles.

Answer

The correct chronological sequence of ecological events is: the unchecked multiplication of elk leading to erosion and tree degradation; the reintroduction of gray wolves altering elk grazing locations; the regeneration of riverbank trees providing beaver building materials; and the creation of beaver ponds establishing new habitats for fish and reptiles.
The sequence correctly reflects the chronological progression of the Yellowstone ecosystem before and after the reintroduction of wolves: the initial state of elk overgrazing, the reintroduction of wolves, the consequent regeneration of vegetation, and finally the creation of new habitats via beaver dams constructed using the recovered wood.

Step-by-Step Solution

1
Identify the historical baseline state of Yellowstone National Park described in the text.
The first paragraph states that before 1995, the absence of wolves allowed elk to multiply unchecked and overgraze the trees, eroding the soil.
This establishes the starting point of the sequence before any restorative actions were taken.
2
Locate the event that initiated the environmental restoration process.
The text states that in 1995, gray wolves were reintroduced, which immediately altered where the elk chose to graze.
This reintroduction is the trigger for the entire trophic cascade.
3
Determine the direct biological consequence of the change in elk grazing behavior.
With elk avoiding open areas, the vegetation (willow, aspen, and cottonwood trees) regenerated rapidly, providing wood for beavers.
This vegetation recovery is a direct result of reduced grazing pressure.
4
Determine the final geological and ecological shift that resulted from this cascade.
Beavers used the wood to build dams, creating ponds that served as new habitats for fish, amphibians, and reptiles.
This is the final consequence of the cascade detailed in the text, completing the sequence of events.

Key Concept

Understanding the chronological sequence of cause-and-effect relationships that develop an argument in an explanatory text.
Estimated Time:1m 30s
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Topics

Argument Development and Evidence Sequencing

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