Trophic Cascade: The Case For Wolves

Laramie – Could anyone be unfamiliar with the expression “Man’s best friend?”

But does everyone know that the beloved household dog is descended from wolves? Indeed, recent genetics research suggests that all domestic dogs originated from wolves in East Asia about 15,000 years ago. (Leonard et al., 2002)
Could wolves, too, be man’s friend?

The suggestion probably seems ridiculous, considering “wolves were hunted and killed with more passion and zeal than any other animal in U.S. history.” (U.S. Fish and Wildlife Service)

Like nearly all animals other than pets, wolves have been “objectified.” They have been “objects of scientific inquiry” and “objects of interest to people bound up in the natural world with them,” as well as “objects of hatred for livestock raisers.” (Barry Lopez, Of Wolves and Men, 2004: 203). Wolf advocates romanticize them as symbols of the wild.

But if we prize our dogs in part because of how useful they are – for herding, guarding, retrieving, guiding, and so forth – then our view of wolves must change. It seems, in fact, that we need wolves.

The scientific evidence is that ecosystems unravel when wolves and other “keystone” predators are removed.

The term for this phenomenon is a “trophic cascade,” defined as the “progression of indirect effects [caused] by predators across successively lower trophic levels.” (Estes et al., Encyclopedia of Biodiversity, 2001). “Trophic” is defined as “the food relationship of different organisms in a food chain.”

Trophic cascade theory can be traced to Aldo Leopold’s observations more than 60 years ago in the Southwest:

I have lived to see state after state extirpate its wolves. I have watched the face of many a new wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every edible bush and seedling browsed, first to anemic desuetude, and then to death. I have seen every edible tree defoliated to the height of a saddle horn. (A Sand County Almanac 1949)

A consensus is growing: “Questions about trophic cascades have shifted from whether to when, where and how often.” (Pace et al. 1999). There is “increasing evidence that the absence of large carnivores can initiate cascading perturbations through the trophic webs.” (Soulé and Terborgh, 1999:). In the past ten years, field studies supporting this conclusion have appeared in leading journals and been reported by the National Research Council.

Trophic cascades involving wolves or cougars have now been demonstrated in national parks in five different North American ecosystems – Yellowstone, Yosemite, Wind Cave, Jasper (in Alberta), and Zion. Similar findings from Olympic National Park will be published later this summer.

In each study area the long absence of wolves or cougars resulted in a similar “cascade” of effects:

• Large herbivores, such as elk or deer, increased in number and foraging behavior changed significantly.
• These animals over-browsed preferred plants, especially deciduous trees and shrubs like cottonwood, aspen, willow, and oaks, and spent more time in riparian areas.
• As a consequence, “recruitment” of cottonwood and aspen (i.e., the growth of seedling/sprouts into tall saplings and trees) was drastically reduced, and uncommon plants became rare or were disappeared completely.
• Long-term loss of streamside vegetation caused major changes in channel morphology and floodplain function.
• Loss of berry-producing shrubs, and young aspens and cottonwoods, led to changes in the diversity and abundance – and sometimes the outright loss – of other species, including beaver, amphibians, and songbirds.
• The disappearance of top predators triggered an explosion of smaller “mesopredators,” such as coyotes, which led to further cascading effects.

Two of the principal research scientists, Oregon State University ecologist William Ripple and forest hydrologist Robert Beschta, summarize (2005: 764) the unraveling of the northern ranges of Yellowstone National Park:

[T]he extirpation of the gray wolf – a keystone predator in this ecosystem – is most likely the overriding cause of the precipitous decline and cessation in the recruitment of aspen, cottonwood, and willow across the northern range. This hiatus in recruitment of woody species is also directly linked to the loss of beaver and the decline in food availability for other faunal species.

Other environmental factors were excluded as possible causes:

[T]he loss of recruitment occurred despite long-term variations in winter weather, snowpack, and other climate variables, with or without the occurrence of fire, and independent of efforts by the [Park Service] to control ungulate numbers inside the park (pre-1968) or to let them increase by ceasing control efforts (post-1968).

The evidence is “compelling” and the conclusions “inescapable”: similar, undesirable ecological conditions in each of the parks can be traced to the absence of keystone predator(s).

Restoring degraded ecosystems

The exciting news, however, is that reestablishing top predators could help restore entire ecosystems.

This phenomenon is playing out vividly on Yellowstone’s northern ranges, where wolves were reintroduced in 1995-96. Ripple and Beschta (2007: 514) report “reduced browsing and increased heights of young aspen during the last 4–5 years, particularly at high predation risk sites (riparian areas with downed logs).” Both willows and aspen “appear to be growing much taller” (Ripple and Beschta, 2005: 120), and cottonwood seedlings were “generally widespread” in 2001. (Beschta and Ripple, 2003: 1307; see also Ripple and Beschta, 2004)

Scientists who had predicted that wolves would lead to the return of aspen are elated. (Morrell, 2007: 349)
The OSU scientists conclude that return of wolves “may provide the greatest promise for renewed cottonwood recruitment and the eventual recovery of riparian plant communities” (Beschta, 2005: 402) and “may be integral to the long-term recruitment” of aspen. (Halofsky and Ripple, 2008: 204) Hydrologist Beschta further predicts, based on the initial recovery of willows in parts of the Lamar Valley, that the badly degraded Lamar River will eventually recover. (Morrell, 2007: 349)

More than 500 miles to the south lies Zion National Park. Its cougar/mule deer system provides compelling evidence that what’s happening in Yellowstone is not a fluke.
Ripple and Beschta (2006) compared conditions in Zion Canyon, where cougars had been essentially absent for about 50 years because of heavy human use, and nearby North Creek, where cougars are present.

In the North Creek watershed, deer browsing levels were “relatively low, as indicated by long-term patterns of cottonwood recruitment. Furthermore, streambank erosion was low, channels relatively narrow and deep, and relative abundances of hydrophytic plants and other indicator species of biodiversity were high….” (Ibid., 407)

Zion Canyon was a stark contrast: deer browsing had prevented cottonwood recruitment, streambank vegetation and riparian biodiversity were reduced, banks were eroding, and the stream was wider and shallower. (Ibid. See the article and an illustration here.)

As in Yellowstone and the other parks, the scientists were able to eliminate other environmental factors, such as climate or fire, as possible causes.

“Unless changes occur at the top of the food chain,” they concluded, “cottonwoods in Zion Canyon may ultimately disappear.” (Ibid.)

Lessons for managers

Trophic cascade studies in national parks have broad implications. Aspen habitat is declining throughout the West, and riparian areas are widely degraded.

It is well known that both communities can be severely damaged by wild ungulates and domestic livestock. More than 250 million acres of public lands are foraged by wild ungulates and livestock. But wolves – once the most widely distributed animal on the continent – are no longer present on most of these lands.

Recent studies have focused on the effects of heavy browsing by native ungulates. But Beschta and Ripple (2006: 1536) point out:
[W]ithin the broader context of other studies that have evaluated grazing/browsing effects of domestic livestock the message is clear. Persistent, heavy grazing/browsing of streamside vegetation by either wild or domestic ungulates can lead to impoverished plant communities, channel instability and loss of hydrologic connectivity, all of which adversely affect the quality and extent of habitats for a wide range of aquatic/terrestrial biota.

Preliminary work on the Middle Fork of the John Day River in northeastern Oregon bears out this assessment. Recruitment of cottonwood and willow is reduced or no longer occurring in many areas. Cattle grazing is “potentially the most important factor” because it “has been the principal land use … since at least the late 1800s, whereas big game populations have only reached substantial numbers in recent decades.” Beschta and Ripple predict that the “long-term reduction in cottonwood recruitment [and] related deciduous woody riparian species will likely have major long-term ecological consequences for riparian and aquatic ecosystems associated” with the river and its floodplains. (2005: 154)

Even before the recent flurry of field studies documenting trophic cascades, scientists had identified the “crucial and irreplaceable regulatory role” of top predators and their “place in ecosystem management, restoration and conservation.” Their absence, warned Soulé and Terborgh (1999: 58), “appears to lead inexorably to ecosystem simplification accompanied by a rush of extinctions. Therefore, efforts to conserve North American biodiversity … will have to place a high priority on reestablishing top predators wherever they have been locally extirpated.”

Science offers few other tools for restoring these ecosystems. Human hunters can’t substitute for wolves or cougars in controlling elk or deer populations and behavior. Periodic culling or hunting seasons simply do not “replicate the persistent predation risk effects associated with wolves” – what scientists have called the “ecology of fear.” (Ripple and Beschta, 2005: 620; Brown et al. 1999).

The importance of behavioral effects is demonstrated by the ongoing deterioration of aspen and cottonwood in Yellowstone despite years of culling in the park and hunting just outside.

“For nearly three-quarters of a century, elk grew complacent after their main predator was removed from the scene.” Now, “elk must behave differently in the presence of wolves.” (Heidi Ridgley, Rocky Road Ahead for Wolves?, Defenders magazine, Spring 2008)

For these reasons, Ripple and Beschta recommend that “restoration goals should focus on the recovery of natural processes.” Active measures, including reintroducing lost keystone species, reestablishing historical ungulate migration routes, and ending domestic livestock grazing, can promote “passive restoration of other ecosystem processes and components.” (Ripple and Beschta, 2004: 765)

Other researchers concur. Mao and others (2005: 1704) concluded that restoring wolves to Yellowstone was “fundamental to meeting the ecological-process management function of national parks.” Pace and colleagues (1999: 487) commented more broadly that reestablishing cascades offers an opportunity “to manage food-web interactions more widely to serve societal needs.”

Wolf recovery is plainly controversial. But as Wyoming journalist Brodie Farquhar put it, “Every year the evidence becomes clearer that a restored ecosystem functions better than a managed one.” We need the help of top carnivores to heal these landscapes.

Much has been written about man’s best friend. Roger Caras muses, “Dogs have given us their absolute all. We are the center of their universe … They serve us in return for scraps. It is without a doubt the best deal man has ever made.”

Wolves would serve us for even less. We need only let them live.

To read the 2nd part of this story: click here!

Sources / Resources
Oregon State University, College of Forestry, Trophic Cascade Program, (with links to videos, publications, and a graphic illustration of the Zion National Park study)

Adaptation options for climate-sensitive ecosystems and resources. Synthesis and Assessment Project 4.4. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research (2008)

Morrell, Virginia, Aspen Return to Yellowstone with Help from Some Wolves. Science 317, 2007: 348-49.

Beschta, R.L. and W.J. Ripple. Rapid Assessment of Riparian Cottonwood Recruitment: Middle Fork John Day River, Northeastern Oregon. Ecological Restoration 23(3), 2005: 150-156.

Beschta, R.L. Reduced Cottonwood Recruitment Following Extirpation of Wolves in Yellowstone’s Northern Range. Ecology 86(2), 2005: 391-403.

Beschta, R.L. and Ripple, W.J. River channel dynamics following extirpation of wolves in northwestern Yellowstone National Park, USA. Earth Surface Processes and Landforms 31, 2006: 1525-1539.

Beschta, R.L. and Ripple, William J. Wolves, elk, and aspen in the winter range of Jasper National Park, Canada. Canadian Journal of Forest Research 37, 2007: 1873-1885.

Brown, J.S., Laundre, J.W., and Gurung, M. The ecology of fear: Optimal foraging, game theory, and trophic interactions. Journal of Mammalogy 80, 1999: 385–399.

Estes, J.A. et al., Trophic cascade, in vol. 4., Encyclopedia of Biodiversity, S. Levin, ed., 2001.

Halofsky, Joshua, and Ripple, William. Linkages between wolf presence and aspen recruitment in the Gallatin elk winter range of southwestern Montana, USA. Forestry 81, 2008: 195-207.

Larsen, E.J. and Ripple, W.J. Aspen Stand Conditions on Elk Winter Ranges in the Northern Yellowstone Ecosystem, USA. Natural Areas Journal 25, 2005: 326-338.

Lopez, Barry. Of Wolves and Men. New York: Simon and Schuster, 2004 (new edition; originally published 1978).

Leonard, Jennifer A. et al. Ancient DNA evidence for Old World origin f New World dogs, Science 298, 2002: 1613-16.

National Research Council. Ecological Dynamics on Yellowstone’s Northern Range. Washington, D.C.: National Academies Press, 2002.

Pace, Michael et al., Trophic cascades revealed in diverse ecosystems, TREE 14, 1999: T 483.

Ripple, W.J. and Beschta, R.L. Hardwood tree decline following large carnivore loss on the Great Plains, USA. Frontiers in Ecology and Environment 5, 2007: 241-246.

Ripple, W.J. and Beschta, R.L. Linking a cougar decline, trophic cascade, and catastrophic regime shift in Zion National Park. Biological Conservation 133, 2006: 397-408.

Ripple, W.J. and Beschta, R.L. Linking Wolves and Plants: Aldo Leopold on Trophic Cascades. BioScience 55, 2005: 613-621.
Ripple, W.J. and Beschta, R.L. Restoring Yellowstone’s aspen with wolves. Biological Conservation 138, June 2007: 514-519.

Ripple, W.J., and Bescheta, R.L. Trophic cascades involving cougar, mule deer, and black oaks in Yosemite National Park. Biological Conservation 141, 2008: 1249-1256.

Ripple, W.J. and Beschta, R.L., Willow Thickets Protect Young Aspen. Western North American Naturalist 65, 2005: 118–122.

Ripple, William J. and Beschta, Robert L. Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems? BioScience Vo.l 54 No. 8, August 2004: 755-766.

Ripple, W.J. and Beschta, R.L. Wolves, elk, willows, and trophic cascades in the upper Gallatin Range of Southwestern Montana, USA. Forest Ecology and Management 200, 2004: 161-181.

Soulé, Michael E. and Terborgh, John. Continental Conservation: Scientific Foundations of Regional Reserve Networks. Washington, D.C.: Island Press, 1999.

U.S. Fish and Wildlife Service, Notice of Interagency Cooperative Policy Regarding the Role of State Agencies in Endangered Species Act Activities, 59 Federal Register 34274, July 1, 1994.