Yellowstone National Park, the USA’s most flamboyant nature reserve, is home to a plethora of some of the nation’s most exquisite wildlife. It is also the place of one of the natural world’s most prominent ecological phenomena in recent times – the trophic cascade associated with the reintroduction of Gray Wolves into the park in 1995. A trophic cascade refers to the rippling direct and indirect impacts caused by a change in the population of a keystone species. Contrary to popular beliefs, a thriving wolf population is essential for the survival of countless smaller species. One such example is the Yellowstone wolf reintroduction.
History of Yellowstone’s Wolves
Wolves have historically been a keystone species of the Yellowstone National Park, but due to rampant trophy hunting and conflict with ranchers, the species became locally extinct in the 1920s. With the removal of the most successful carnivore of the park, the population of ungulates, the Yellowstone Elk in particular, skyrocketed. This became a matter of concern for the local park authorities as the unsustainable population of ungulates had resulted in excessive grazing. Extensive habitat degradation became prevalent as early as 1930. Numerous attempts to control the elk population through trophy hunting had proven cumbersome and ineffective. Fortunately, at the time, the Gray Wolf was considered as an “Endangered Species” nationally and conservationists were tirelessly working to revive the world’s most widespread canine species.
As a part of this conservational effort, 14 wolves were originally translocated from Jasper National Park in Alberta, Canada. The translocation took place in the form of a soft release where the 14 wolves were temporarily kept in three makeshift acclimation pens. Elk carcasses were periodically placed in the enclosure to aid with the familiarization of the habitat. The wolves were then released to freely roam the reserve. This reintroduction was a part of a far larger project, expanding across the Greater Yellowstone Landscape and the neighbouring state of Idaho. By the end of 1996, an accumulation of 66 wolves were released in the said area. The project has been overwhelmingly successful with the region’s wolf population crossing 1,000 individuals in two decades. Meanwhile, Yellowstone National Parks seems to have reached it carrying capacity. The population of wolves within the park’s perimeter has been oscillating between 83 and 108 since 2009.
Impact on Ungulate Population
The impact of this exponential growth of the wolf population was nearly immediately evident. Elk which are the primary prey of the wolves saw a drastic reduction in numbers. Between 1995 and 2008, the elk population halved from 17,000 individuals to 8,000. The population has since stabilized at this point. The sheer reduction of ungulates directly reduced browsing pressure on the local flora such as aspen and cottonwood significantly and reduced habitat degradation immensely. In fact, a study by the U.S. Geological Survey in Fort Collins, showed that unbrowsed willows had 10 times greater biomass than their browsed counterparts. The wolf kills were also vital for nutrient recycling. A study of 36,000 moose kills spread over 50 years in Michigan’s Isle Royale National Park, showed that the area around the kills had 12 times increase in nitrogen deposition.
Furthermore, due to constant fear of predation, elk herds would be constantly migrating to newer areas, vacating former feeding grounds. This would allow degraded habitats to regenerate properly. One such habitat was the riverbeds. The regenerated vegetation was essential in stabilizing the riverbeds, which improved the river dynamics drastically. The channels became narrower, meanders lost their prominence and there was an increase in pools on the banks. These changes provided for excellent wildlife habitats.
Habitat Restoration
However, the rivers were impacted by more than one way. The increase in willow on the river beds allowed for the revival of local North American Beaver (Castor canadensis) populations. At the time of reintroduction, there was only one beaver dam. This can be attributed to the lack of willow caused by the overgrazing by ungulates in the pre-wolf era.
The lack of willow severely hindered the capability of beavers to cope with the harsh winters. Fortunately, two decades on, the number of beaver dams has increased to 12. Of course, the increase in the number of natural dams had a rippling impact on the riverine ecosystem, providing habitat to species such as muskrats, ducks and fish.
The increased vegetations was also essential for Black Bears who were provided with an increase in berry supply. Similarly, songbirds and frugivores also witnessed a population increase.
The increased hunting of ungulates also had indirect impacts on numerous other species. Species like the American Bison saw a steady increase in population following the due to lower competition by elk and lack of predation by wolves. The increase in carcasses was also beneficials to scores of scavengers such as Golden Eagles, Chickadees, Masked Shrew, Great Gray Owl, and 445 species of beetle. In fact, predators such as Mountain Lion, coyotes and American Black and Brown Bears also feast on these carcasses.
This was evident in a study which found that 84% of wolf-coyote interactions in Yellowstone occurred at kills. Similar trends were observed in bear and wolf interactions. The increase in carcasses would increase the food supply for the various predators in the park, hence contributing to the increase in their populations. This in turn would reinforce the ecological impacts the wolves have had on the reserve.
Impact on Scavengers
The wolf populations also acted as a buffer against climate change in Yellowstone. Due to climate change, the winters would be shorter and the snow levels lower. This allows for the quicker regeneration of the herbaceous plant and hence providing easier foraging options for the elks. The decrease in energy expenditure would reduce the chance of an elk succumbing to the harsh winter. This could prove devastating for the scavengers, who are heavily dependent on these natural deaths for food. Fortunately, wolves have diminished these impacts significantly, as elk composed of at least 90% of the wolf diet during winters. This was proven by a 50-year study of carrion availability in the winter months in Yellowstone. In areas where wolf presence was unknown, there was a 66% reduction in carrion availability in the late winter months of April. In comparison, the densely populated areas of the park only saw a 11% reduction in carrion availability. While the wolves certainly cannot mitigate the impacts of climate change in totality, the species does provide the opportunity for scavengers to adapt to the changing ecosystem.
Influence on Coyotes
However, there is one particular species which shares a unique relation with wolves due to their small size, coyotes are only able to hunt elk calves and smaller rodents. Elk calves and rodents’ populations are larger during the Spring months. This means that during the winter months, coyotes are heavily dependent on wolf kills as hunting is impractical. However, the predation of coyotes by wolves has also been well documented. In fact, there has been a 90% reduction in coyote population recorded within the range of the wolves. This in turn has proven beneficial for Pronghorns, who are heavily predated upon by coyotes. A Study published in the Journal Ecology by the WCS showed that the fawn survival rate in areas with higher coyote populations was a mere 10%. This is compared to a 34% survival rate in areas where wolves were abundant. Moreover, Pronghorns have been observed to give birth near wolf dens on numerous occasions. This explains the 50% increase in Pronghorn population following the reintroduction of wolves in 1995. There was also an increase in rabbits and mice amongst other small rodents due to the reduction in coyote populations. This would allow for an increase in population for smaller predators such as weasels, foxes and badgers and raptors.
Challenges
However, as fabulous as the ecological restoration has been, the reintroduction of wolves met significant resistance from ranchers and hunters alike. Trophy hunters were disappointed due to the increased hunting restrictions. They also argued that wolves would wipe out the entire elk populations. However, a study between 1995 and 2001 in the park showed that 57% of the elks predated upon by wolves were old and weak adults with the mean age of 14. This would not cause a significant demographic imbalance and hamper population growth as these individuals were past their reproductive age. Meanwhile, trophy hunters outside the park usually target female individuals with a mean age of 6. This is the prime reproductive age of female elks. The removal of healthy reproducing individuals is more destructive for the future generations of elk.
Ranchers feared that the reintroduction of wolves would increase human-wildlife conflict due to increased livestock mortality. Wolves have been responsible for 569 cattle and sheep deaths in Western USA, although this represents less than 1% of livestock deaths in the region. Increased stress levels amongst cattle due to the increased harassment by wolves also resulted in a 30-50% reduction in biomass. Retaliation killing is also fairly prevalent. However, no concrete statistics were found regarding human wolf conflict in Yellowstone National Park. Moreover, the wolves have proven to be a major attraction for tourists, increasing ecotourism revenue to $35 million annually. Majority of the revenue would be distributed to the local community in the form of jobs and increased business outreach, hence increasing tolerance of wolves in the neighbourhood.
Conclusion
To conclude, while translocation of keystone species has been challenging, the success of the wolf reintroduction augurs a sense of hope for the future of conservation. Projects such as Rewilding UK and India’s Cheetah Reintroduction Plan can take a lot of inspiration from the success of Yellowstone National Park. Also, India might be encouraged to protect her population of Grey Wolves to save her dying grasslands.
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