Effect of Wolves on the Ecosystem

            Recently, I have been researching the negative effects of wolves on today’s livestock production industry. In this post, I want to explore more in depth the effect that the presence or absence of wolves has on the ecosystem. More specifically, I would like to answer the question, “Are the benefits to the ecosystem worth the cost to the livestock industry to reintroduce wolves in the United States?”

            To learn more about wolves’ contributions to the ecosystem, I read an article published in 2004 entitled “Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems?”¹ Although somewhat dated, the science presented in this article is nonetheless accurate and applicable to understanding the effect on the local ecosystem of the extirpation and subsequent reintroduction of wolves in the US. 

            The article begins by summarizing how presence or absence of wolves can magnify to affect large-scale changes on an ecosystem. This occurs through the process of trophic cascades, which are described in the article as the indirect effects of changes at one level of the food chain on organisms at other levels. The presence of wolves has two types of effects on the wolves’ ungulate prey species: lethal and nonlethal. An example of a cascading lethal effect would be the direct killing of ungulates by wolves, resulting in lower ungulate populations. A nonlethal effect would be behavioral changes of the prey species in order to avoid interactions with the predator. 

            Both lethal and nonlethal effects of wolves on herbivores subsequently affects the growth of woody plants, such as willow, aspen, and cottonwood. In the absence of wolves, foraging behaviors and browsing pressure of herbivores limits growth and recruitment of these woody plant species. In the presence of wolves, lower densities and predator-avoidance behavior of herbivores lessens browsing pressure on woody plants. More specifically, herbivores spend less time inside wolf territories which creates areas of refuge where these plants can grow unimpeded by browsing of herbivores. Growth and recruitment of these plants is important for ecosystem diversity because they provide habitat for smaller mammals and birds.

            In addition to affecting herbivore species and vegetation, wolves contribute to the ecosystem by providing food for scavenger species. They also keep mesocarnivore populations (such as coyotes) low, which protects birds and small vertebrates from overexploitation.

            The second portion of the article discusses the changes seen in the Yellowstone northern winter range since the reintroduction of wolves in 1995. Since the article was published in 2004, the data in this area is not up to date. However, at the time of the article, much change had already been affected in this ecosystem by the presence of wolves. The data presented focuses on elk and woody plants.

            In separate studies on aspen and cottonwood, both species were seen to have reduced recruitment (i.e. survival of juvenile plants to join the population) in the 1920s and 1930s, coinciding with the extirpation of wolves from the area. This reduced recruitment was found to be independent of climate or other events such as fire or floods. The National Parks Service attempted to reduce the numbers of the elk herds in order to relieve some of the browsing pressure on these plants. As a result, elk herds were reduced from between 8,000 and 11,000 in the early 1950s to between 4,000 and 8,000 in the 1960s. When these efforts ceased in 1969, elk numbers increased to between 12,000 and 18,000 from the 1970s to the 1990s. Regardless of this decrease and increase in elk numbers, woody plants were not relieved from browsing pressure until the reintroduction of wolves in the mid-1990s. Since then, increased recruitment of aspen, willow, and other woody plants has been observed, especially in areas of high predation risk for herbivores.

                                  Willow along Blacktail Creek in spring 1996 (left) and 

                                  summer 2002 (right).¹

           
            After reading this article, it seems apparent to me that wolves are an important element of their native ecosystems and that removal of wolves from these ecosystems has had a negative effect. The overall benefit of wolves to the ecosystem is to promote biodiversity by affecting prey behavior, thereby reducing browsing pressure on woody plants and allowing formation of habitats for smaller mammals and birds. In my opinion, the pros of wolf reintroduction for the environment outweigh the cons of wolf reintroduction for livestock producers. However, I maintain that the conservation community should recognize the cost of living with wolves that is paid by livestock producers, and make every effort to communicate and compromise with these producers.

1.Ripple, W.J., & Beschta, R.L. (2004). Wolves and the ecology of fear: can predation risk structure ecosystems? Bioscience, 54(8), 755-766.