Research strongly supports the contributions of wolves in particular to the functioning and stability of the overall landscape (Ripple and Beschta 2012). Wolves prey primarily on large ungulates, hoofed mammals such as deer, elk and moose. By preying on the most vulnerable (diseased, young, old, weak or injured) individuals, wolves help keep prey populations healthier and more vigorous (Carbyn 1983). Predation by wolves also regulates ungulate distribution and group size, which impact overall native biodiversity (White et al. 2010, Ripple and Beschta 2012). When deer and elk become too abundant for their habitat, for example, they overgraze vegetation, leading to habitat degradation and potentially damaging effects on other native wildlife (Endress et al. 2012).
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In recent centuries, the removal of wolves has allowed smaller predators to dramatically increase in number and range, thereby increasing exploitation of their prey. In the southeastern United States, elimination of red wolves was followed by increased coyote and raccoon populations, which in turn caused a reduction in wild turkeys (Miller et al. 1997, Miller et al. 2012). Through competition, expanding coyote populations have suppressed the abundance of small predators such as foxes, which prey on smaller mammals that are responsible for infecting ticks with Lyme disease (Levi et al. 2012). The increase of these smaller mammals, in turn, is a likely cause of increases in the incidence and range of Lyme disease (Levi et al. 2012).
A cascade of positive impacts
In Yellowstone National Park, scientists have been carefully documenting the impacts of wolves on the ecosystem, noting many changes for the better since reintroduction. After an absence of about 70 years, wolves have triggered a “trophic cascade”— a series of direct and indirect effects on species diversity and abundance across various levels of the ecosystem (Berger et al. 2008; Beschta and Ripple 2010).
Predation creates these cascading effects by reducing prey numbers and density and altering prey behavior (White et al. 2012; Hairston et al. 1960; Beschta and Ripple 2009, 2010; Christianson and Creel 2008; Mao et al. 2005; Peterson 2001; Mech and Peterson 2002). Elk in the Yellowstone area have become wary of wolves (Fortin et al. 2005, Mao et al. 2005) and browse on different species of vegetation and at different intensity if wolves are present (Christianson and Creel 2008; White et al. 2012).
With less grazing pressure from elk, streambank vegetation such as willow and aspen is regenerating after decades of over-browsing (Kauffman et al. 2010; Beschta and Ripple 2008; Anderson 2007; Baril et al. 2011). This restored vegetation creates habitat for native birds, fish, beaver and other species. It also improves aquatic habitat by helping to stabilize channels and control erosion (Beschta and Ripple 2012).
Competition with wolves reduces coyote numbers and the impact they have on species such as pronghorn (Berger et al. 2008; Barnowe-Meyer et al. 2010). In some areas, wolves have contributed to reductions in Yellowstone’s coyote population by as much as 50 percent. As a result, pronghorn survival has increased from 20 percent to 70 percent (White et al. 2007). Populations of smaller predators, such as the red fox, have also increased (Crabtree and Sheldon 1999; Levi and Wilmers 2012).
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Scavenging species are also benefiting from the presence of wolves. Carrion feeders such as eagles, bears and magpies thrive on the remains left by wolves (Wilson and Wolkovich 2011; Wilmers et al. 2003; Wilmer and Getz 2005). Ravens are also known to follow wolves as a primary feeding strategy (Stahler et al. 2002). The remains of kills left by wolves help species survive food-stressed winters (Wilmers et al. 2003; Wilmers and Getz 2005).
Researchers expect the widespread benefits documented in the Northern Rockies to occur in other areas where wolves become reestablished. As the Mexican gray wolf population approaches an ecologically effective population density in the Southwest, for example, aspen recruitment, which is currently suppressed by elk browsing, could increase (Beschta and Ripple 2010b). Increased riparian vegetation from reduced ungulate browsing with wolves on the landscape (Kauffman et al. 2010; Beschta and Ripple 2008; Anderson 2007; Baril et al. 2011) may also aid the recovery of the threatened Apache trout, which benefits from the increased cover (Cantrell et al. 2005). The benefits to biodiversity and ecological health alone make a strong case for wolf restoration and recovery wherever feasible.
In the years since the wolf reintroduction, Yellowstone has become a premiere scientific laboratory for wilderness observation and ecosystem recovery. Scientists have come from around the world to watch the effect wild wolves have on the park. We have discovered that an ecological effect called the “trophic cascade” has taken over Yellowstone, with the wolves initiating a more natural ecosystem balance than has been seen in over 65 years.
In the past it was thought that an ecosystem was built from the bottom up... with plant life as the basis from which everything grew. Once healthy plants were established, insects, small rodents, birds, larger herbivores and finally the top predators fell into a balance with each other. Almost all conservation and reintroduction efforts were based on this idea. In a damaged area, biologists would first try to rebuild the plant life before doing anything else. However, some ecosystems could not be fixed before reintroducing an endangered top-level animal. In Yellowstone National Park, the US Fish and Wildlife Service was required by the Endangered Species Act to reintroduce wolves before balancing the plant base and herbivore populations.
In the years since the wolf reintroduction, Yellowstone has become a premiere scientific laboratory for wilderness observation and ecosystem recovery. Scientists have come from around the world to watch the effect wild wolves have on the park. We have discovered that an ecological effect called the “trophic cascade” has taken over Yellowstone, with the wolves initiating a more natural ecosystem balance than has been seen in over 65 years.
