The passage identifies each of the following as a pressure on elk numbers over the period in question EXCEPT

predation by a growing grizzly population.

increased hunting quotas outside the park.

disease spreading through the northern range herd.

The observation that willow recovery has been strongest where the water table was already high functions primarily to

suggest that a factor other than reduced browsing helps explain where recovery occurred.

establish that willows cannot recover without wolves.

show that the park's streams have deepened since 1995.

argue that beaver dams were the original cause of the recovery.

The author notes the absence of an untouched comparison chiefly to indicate that

the reintroduction should not have been attempted without one.

observations from a single park cannot be trusted at all.

the wolf effect cannot be separated from everything else changing at the same time.

Yellowstone differs from other parks in ways that make it unrepresentative.

It can be inferred that the author regards the wolves' contribution to the changes along the streams as

one cause among several rather than the single decisive one.

impossible to establish on present evidence.

confined to the reaches where willows recovered.

According to the passage, the feature that makes the popular story quotable is also what makes it

difficult to reconcile with cascades observed in kelp forests and lakes.

unpopular among the ecologists who study the northern range.

dependent on elk population figures that remain disputed.

a poor guide to management, where causes interact and cannot be acted on singly.

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Trophic Cascades and the Yellowstone Story

PRACTICE

100:00+4 −1MCQ Single Answer

Reading Passage

Few accounts of ecological restoration have travelled as widely as the story of wolves in Yellowstone. Wolves were reintroduced to the park in 1995 after an absence of seventy years. Elk, no longer able to browse in safety, moved away from open riverbanks; willow and aspen along those banks recovered; beavers returned to the regrown willow and built dams; and the streams themselves, stabilized by roots and dams, began to run in narrower and deeper channels. Told this way, the sequence is a demonstration that a single predator can rebuild an ecosystem from the top down. The individual observations are sound. What is not settled is the attribution. Elk numbers in the northern range fell by a large margin over the same period, but wolves were not the only pressure on them: a growing grizzly population took calves, hunting quotas outside the park boundary were raised substantially, and the region passed through a run of unusually dry years that reduced forage. Willow recovery, meanwhile, has been patchy rather than general, and it has been strongest in exactly those reaches where the water table happened to be high enough to support it. Nor is there an untouched comparison. Yellowstone is one park in one landscape, changing in many ways at once, and no control exists against which the wolf effect alone could be measured. The objection here is to the strength of a claim, not to its direction. Trophic cascades are a real phenomenon, documented in systems from kelp forests to lakes, and the balance of evidence does place wolves among the causes of what happened along those streams. What the popular version adds is a promise the science does not make: that pulling one lever produces one predictable outcome. That promise is what makes the story so quotable, and it is also what makes it a poor guide to management, where the levers are many, act on each other, and are rarely available to be pulled one at a time.

The main point of the passage is that the popular account of wolves in Yellowstone