Wolves are keystone species and top predators. As such, they are important to the balance and biodiversity of the ecosystem. 

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The impact of the extirpation of wolves in the 1920s is widely acknowledged by the scientific community. What was noted in Yellowstone was that without the impact of human hunting, there was an elk population explosion, a decimation of vegetation, specifically willows and aspens, due to overbrowsing, and  the disappearance of beavers due to loss of needed vegetation, among other impacts. 

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Would bringing wolves back reverse this scenario, restoring the ecosystem to the healthy state it was in pre-extirpation?  Were wolves the "magic solution" to Yellowstone's ecosystem imbalances? 

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Indeed, with the wolves' return,  the wolves predation on elk helped reduce overbrowsing by elk. This likely contributed to willow and aspen recovery in some areas and overall habitat diversity. Wolves were part of a tropic cascade-ecological effect that rippled through the ecosystem. This is the story that the provided activities in this website teach. 

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However, following thirty years since the wolf reintroduction, Yellowstone's ecosystem has not recovered to it's pre-extirpation state. Recent scientific research suggests that due to the complexity of the ecosystem, the story of Yellowstone ecosystem recovery isn't so simple. The reintroduction of wolves alone cannot will not quickly repair the impacts resulting from their removal. Other factors have also contributed significantly.

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Elk population decline: While wolves certainly played a role, perhaps of more consequence were other predators (bears and mountain lions) and human hunting outside the Park. In addition, other factors including  weather, drought, harsh winters also contributed to the elk herd decline. 

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Vegetation recovery - After the return of wolves, an increase in willow height began to be observed. This brought improved beaver habitat and the beaver numbers began to climb. (In 1995, 100 beavers had been reintroduced outside of the Park. Within the Park, in 1995, there was one beaver colony. In 2015, there were nineteen.) With beavers came dams that raised the water table, providing the water that trees need to grow. Experimental studies have found that a reduction in overbrowsing alone is not sufficient to recover willows along some small streams. What is additionally needed is the adequate water supply, like that which the beaver dams provide. It seems that river ecosystems that have wolves, along with cougars and bears, decreasing elk overbrowsing, together with beavers providing water, are key to riparian habitat recovery.

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Has the Yellowstone riparian ecosystem recovered?  Unfortunately, overbrowsing continues in some areas - not by elk, but by bison. While elk populations have gone down, bison populations have gone way up. While some wolves predate on bison, the size of bison limits wolves' ability to have a substantial impact. Bison eat much more than elk do. There remains a problem of overgrazing, which continues to limit the restoration of riparian ecosystems.

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Ecosystems are very complex and challenging to fully understand. The indirect impacts from the return of wolves did much to begin healing the degraded ecosystem.  However, when an ecosystem is degraded to the extent that it was following the wolf extirpation, whether full recovery over time is possible remains to be seen.   We do know that many areas of the Yellowstone ecosystem are healthier and more biodiverse following the return of wolves, thanks to the wolves and many pieces of this puzzle. ​

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Resources - Colorado State University - Ecological Effects of Wolves  

                    The Royal Society Biological Sciences Abstract -  Stream hydrology limits recovery of riparian ecosystems after wolf
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                    National Parks Traveler - https://www.nationalparkstraveler.org/

                    

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