The Wolf's Ripple Effect: Yellowstone's Keystone Comeback - Summarizing and Paraphrasing Informational Texts (Grade 8)

According to the passage, what was a direct and immediate consequence of the elimination of wolves from Yellowstone in the 1920s?

• The collapse of stone arches in the park

• A significant increase in the elk population

• The disappearance of complex pond ecosystems

• An increase in the songbird population

Answer explanation

The passage explicitly states, 'With no wolves to hunt them, the park's elk population exploded.' The disappearance of ponds was an indirect, later effect, and the songbird population decreased, not increased. The stone arches are part of an analogy, not a real event in the park.

Which statement best explains the concept of a 'trophic cascade' as described in the passage?

• A process where predators are removed from an ecosystem to help other species.

• A chain reaction of indirect effects triggered by a change at the top of the food chain.

• The method used by conservationists to reintroduce animals to a habitat.

• The way elk herds graze on trees and change the landscape.

Answer explanation

The passage defines a trophic cascade as 'a chain reaction of indirect effects that rippled through the ecosystem' which was 'initiated' by the absence of the top predator, the wolf. The other options misinterpret or describe different concepts mentioned in the text.

Which piece of evidence from the passage most strongly supports the inference that the reintroduction of wolves improved river health?

• Elk began to avoid open areas like river valleys.

• Beaver populations recovered and built new dams.

• Songbirds returned to the flourishing forests.

• Stabilized riverbanks led to less erosion and narrower river channels.

Answer explanation

The passage directly links the recovery of riverbanks to improved river health, stating that 'stabilized riverbanks narrowed and deepened the river channels, leading to healthier aquatic environments.' While the other options are positive effects of the wolves' return, this one provides the most direct evidence of improved river health.

Based on the passage, sort the following ecological effects into the correct category: effects caused by the wolves' absence or effects caused by the wolves' presence.

Groups:

(a) Caused by Wolf Absence

,

(b) Caused by Wolf Presence

• Increase in songbird populations

• Erosion of riverbanks

• Decline in beaver populations

• Overgrazing of willow trees

• Creation of new marshland habitats

• Recovery of aspen groves

Answer explanation

The passage describes a series of negative effects when wolves were absent (overgrazing, erosion, beaver decline) and a series of positive effects that occurred after they were reintroduced (tree recovery, new habitats, return of songbirds). This question requires students to distinguish between the two sets of consequences.

Match each term to the phrase that best describes its role in the Yellowstone ecosystem as explained in the passage.

• Structures that created new habitats for fish and amphibians

• Elk Population

• A series of indirect effects triggered by a change in a top predator

• Keystone Species

• The primary herbivore whose behavior was altered by wolves

• Trophic Cascade

• A species with a disproportionately large effect on its environment

• Beaver Dams

Answer explanation

The passage defines a keystone species as having a 'disproportionately large effect.' It explains the trophic cascade as a 'chain reaction of indirect effects.' It identifies the elk as the herbivore whose population 'exploded' and behavior changed. Finally, it notes that beaver dams 'created marshland habitats that enriched the park's biodiversity,' including for fish and amphibians.

Analyze how the author organizes the information in the passage to explain the concept of a keystone species. How does this structure—moving from the wolves' absence to their reintroduction—help you understand their importance? Cite specific evidence from the text to support your analysis.