Diana F. Tomback (Ph.D.)
What is a trophic cascade?
The trophic cascade is an important concept in ecology: The loss of an apex (highest level) predator changes relationships among species lower in a food chain or food web (that is at lower trophic or ‘feeding’ levels), reducing the success of species at the very base—the plants, which are the primary producers. In other words, if predation is reduced by the loss of the apex carnivore, the number of herbivores increases and plant biomass declines; this is the first trophic cascade. The return of the apex predator results in a second trophic cascade, whereby historical ecological relationships are restored. However, the number of restored predators in a system must approach historical densities to generate these changes.
The first trophic cascade in Yellowstone National Park
During the late 19th and early 20th century, wildlife management was based on a value system in which certain animals were considered useful and others harmful. Wolves and other carnivores that competed with hunters for game or killed the occasional cow or sheep were targeted widely by professionals supported by the U.S. Bureau of Biology Survey and by ranchers and hunters rewarded with bounties. In national parks, park rangers carried rifles and were encouraged to shoot predators on sight—both wolves and mountain lions. After decades of this policy, Yellowstone’s wolf population was decimated; the last wolf was killed in in 1926. We now know that the extirpation of wolves had greater impact on vegetation health and biodiversity in general than did the reduction or loss of other apex carnivores in Yellowstone.
In the 1920’s, park naturalist Milton Skinner estimated the beaver population in Yellowstone to number at least 10,000; others actually documented an abundance of beavers and aspen in some areas of the park. In the 1930’s, National Park Service biologists first reported deterioration of the northern range, possibly from a combination of climate trends and overgrazing by a growing elk population. Elk not only graze the range but typically browse streamside vegetation and aspen. Over the ensuing decades, despite different approaches to elk management and fluctuations in elk numbers, willows and other wetland vegetation and aspen declined in some regions of the park. By mid-century, ecological changes were readily apparent in Yellowstone and especially in the northern range. These changes included loss of beavers and the wetland habitats they engineered, and reduction in the vegetation that depended on these habitats—willows, cottonwoods, streamside and upland aspen, as well as fruiting shrubs and forbs. These moisture-dependent vegetation communities are important for supporting breeding songbirds and as a source of berries for wildlife, and especially for bears. By 1952, few beavers were left, and many wetlands had converted to drier shrub and grassland communities.
The return of wolves to Yellowstone: the second trophic cascade
The Endangered Species Act, passed by Congress in 1973, paved the way for restoration of wolves in Yellowstone. That same year, the Rocky Mountain subspecies of wolf (Canis lupus irremotus) was listed as endangered, and within a few years a Wolf Recovery Plan was drafted for the northern Rocky Mountains. Between 1995 and 1997, 31 wolves from Canada and 10 wolf pups from northern Montana were released in the park. Ten years after reintroduction, the wolf population had grown to 169 animals in 16 packs. Twenty years after reintroduction, the number of wolves stabilized to just under 100, as habitat with a sufficient year-round prey became saturated. As of 2020, there are 94 wolves in 8 packs. Given that the protection of wolves within the park enabled predominantly natural interactions and processes, these numbers likely reflect historical population size and pack distribution.
After the first 10 years of wolf reintroduction, with wolves established throughout Yellowstone, researchers determined that willows were recovering, especially in the northern range, and beavers began recolonizing streams. Subsequent studies documented recovery in willows, aspen, cottonwood, and woody shrubs. Elk numbers declined, although factors other than direct wolf predation may have been in play, including altered habitat use by elk, more frequent movements by elk under the threat of predation, or elk competition with a growing bison population. Most researchers suggested “a wolf effect” was occurring in the park, leading to a second trophic cascade—the recovery phase.
This trophic structure recovery, however, has had a number of effects beyond the basic trophic cascade (Fig. 1): The resurgence of vegetation includes berry-producing shrubs, such as serviceberry and chokecherry, which are important food sources for both grizzly and black bears. The regrowth of streamside vegetation has led not only to the reappearance of beavers but also to a growing community of nesting migratory songbirds. Wolves outcompete coyotes, limiting their presence. Coyotes prey on pronghorn antelope calves and small mammals. Where wolves and coyotes interact, pronghorn calf survival improves, and small mammal populations increase. And finally, wolf kills are sometimes claimed by grizzly bears, improving their nutrition, but making wolves work harder to feed their pack members. Regardless, all leftovers now provision scavenging species, including ravens, magpies, golden and bald eagles, and even coyotes.
The extirpation of wolves had reverberating effects in some areas of Yellowstone, resulting in a decades-long decline in biodiversity and ecosystem health. With stunning prescience, Aldo Leopold, a father of conservation biology, observed and wrote about ecosystem decline in the Southwest following wolf extirpation in his seminal 1949 essay, “Thinking like a mountain.” Clearly, the first trophic cascade has been experienced in many places following wolf extirpation.
The restoration of wolves to Colorado could benefit rangelands and streamside areas that have experienced elk populations above local carrying capacity. Our natural wildlands in Colorado could be more complete in many ways with the reintroduction of wolves.
Fig. 1. The trophic cascade in Yellowstone, from Ripple et al. (2014).Page Break References Chase, A. 1987. Playing God in Yellowstone: the destruction of America’s first national park. Harcourt Brace Jovanovich, Publishers. Klaptosky, J. 2016. The plight of aspen: emerging as a beneficiary of wolf restoration in Yellowstone’s northern range. Yellowstone Science, vol. 24, pages 5-11. Leopold, A. 1949. Thinking like a mountain. In: Sand County Almanac. Oxford University Press, USA (1968). Ripple, W.J., and R.L. Beschta. 2012. Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction. Biological Conservation, vol. 145, pages 205-2013. Ripple, W.J., J.A. Estes, R.L. Beschta, C.C. Wilmers, E.G. Ritchie, M. Hebblewhite, J. Berger, B. Elmhagen, M. Letnic, M.P. Nelson, O.J. Schmitz, D.W. Smith,A.D. Wallach, and A.J. Wirsing. 2014. Status and ecological effects of the world’s largest carnivores. Science, vol. 343, online https://science.sciencemag.org/content/343/6167/1241484. Ripple, W.J., R.L. Beschta, J.K. Fortin, and C.T. Robbins. 2014. Trophic cascades from wolves to grizzly bears in Yellowstone. Journal of Animal Ecology, vol. 83, pages 223-233. Robinson, M. J. 2005. Predatory Bureaucracy: the extermination of wolves and the transformation of the West. University Press of Colorado. Smith, D.W. 2005. Ten years of Yellowstone wolves. Yellowstone Science, vol. 13, pages 7-33. Smith, D.W., D.R. Stahler, M.C. Metz, K.A. Cassidy, E.E. Stahler, E.S. Almberg, and R. McIntyre. 2016. Wolf restoration in Yellowstone: Reintroduction to recovery. Yellowstone Science, vol. 24, pages 5-11. Bio: Diana F. Tomback, Ph.D., is a professor in the department of Integrative Biology, University of Colorado Denver. Her fields of expertise are forest ecology and conservation biology.