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 The Ecological Impacts of Removing the Gray Wolf (Canis Lupus) from the Endangered Species List in the Yellowstone/ Idaho Area. Adam Meyer ENVS 190 13 May 2014 Canis lupus Credit: Gary Kramer / USFWS Meyer | 0 Table of Contents: Acronym list:……………………………………………………………………………………………………………2 Abstract:…………………………………………………………………………………………………………………..3 Introduction:…………………………………………………………………………………………………………….3 Methods:…………………………………………………………………………………………………………………..9 Analysis:…………………………………………………………………………………………………………………...10 Discussion:……………………………………………………………………………………………………………….17 References:………………………………………………………………………………………………………………19 Meyer | 1 Acronym list: ESA: Endangered Species Act NPS: National Park Service USFWS: United States Fish & Wildlife YNP: Yellowstone National Park Meyer | 2 Abstract: The goal of this project is to evaluate the ecological impacts of removing the gray wolf (Canis lupus) from the endangered species list in the Yellowstone/ Idaho area. Removing a keystone species has proven detrimental to ecosystems. In June 2013, the United States Fish & Wildlife [USFWS] has proposed to remove the gray wolf off the endangered species list. This decision is backed with scientific research that has proven that the gray wolf has grown outside their historic range and the reintroduction has proven successful. As of now, the gray wolf is preying on a lot of livestock which is causing issues with many livestock ranchers. Because of these reasons, the ranchers also want the gray wolves delisted. Introduction: Gray wolves were eradicated from the Yellowstone/ Idaho area in the 1940-­‐1970’s (http://www.nps.gov/yell/naturescience/wolfrest.htm). After the recognition of the wolf as a keystone species in the 1970’s, the United States added the gray wolf to the federal endangered species list. Following the listing of the gray wolf, Yellowstone National Park was in desperate need of a top predator to restore its ecosystem, and in the spring of 1995 the gray wolf was reintroduced to the park (http://www.nps.gov/yell/naturescience/wolfrest.htm). After close to 20 years of careful conservation practices, the gray wolf is now flourishing in the Yellowstone/ Idaho area. Currently, the gray wolf is still protected under the federal Endangered Species Act. This act protects the wolf from hunting and trapping; however, there are people that want the gray wolf delisted in North America. After several comprehensive studies, it has been determined that the gray wolf has outgrown its historic range and has proven the recovery Meyer | 3 methods effective. The USFWS has proposed to have the gray wolf delisted from the endangered species list because of these reasons. The gray wolf is a member of the dog family. Gray wolves are the ancient ancestors to our domesticated dogs and are related to the coyote (Canis latrans) and several other wild dog species. Like many other dog species the gray wolf is a pack hunter; however, it is not uncommon for an individual wolf to break from a pack and form its own pack (http://www.nps.gov/yell/naturescience/wolfinfo.htm). These individual wolves may remain alone from days to years before they can find a mate. Although wolves are pack hunters, an individual is still a highly efficient hunter. The gray wolf in most ecosystems is the top predator. There are several populations of gray wolf; however this study focuses on the Idaho/ Yellowstone population. The Yellowstone/ Idaho population of wolves is a larger than average wolf; males weigh between 100-­‐130 pounds and females weigh between 80-­‐110 pounds (http://www.nps.gov/yell/naturescience/wolves.htm). The average life span of these wolves is 5 years; however, they can live up to 12 years in the wild (http://www.nps.gov/yell/naturescience/wolves.htm). The average pack size in this area is 2-­‐11 individuals. The pack size varies under a number of conditions including food availability, disease, wolf mortality from other packs, and poaching (http://www.nps.gov/yell/naturescience/wolfinfo.htm). They mate in February and give birth to average of five pups in April. Each pack has its own unique structure, with some individuals being an alpha male or female and some members being subordinate (http://www.nps.gov/yell/naturescience/wolves.htm). Each member has its role in the pack. Packs mark their territory by urinating around the boundary and howling. A wolf Meyer | 4 caught crossing another wolf’s territory is most likely to be greeted with a fight (http://www.nps.gov/yell/naturescience/wolves.htm). The reason why the gray wolf is considered a keystone species is because of its role as a top predator. In most ecosystems, a top predator is needed for the ecosystem to function properly. Although the Yellowstone/ Idaho area does contain grizzly and black bears, these bears cannot kill large prey such as elk or deer on a regular basis. The gray wolf primarily feeds on ungulates where as a bear’s diet might contain berries and other plants. By feeding on these ungulates, the gray wolf effectively controls these species from over grazing an area. After making a kill, the wolves also contribute to several other species that prey on the carcass (http://www.nps.gov/yell/naturescience/wolfinfo.htm). Unlike mountain lions and grizzly bears, wolves abandon their prey after feeding on the carcass. Abandoning the carcass leaves much needed meat for scavengers, which include the coyote, bald eagle, golden eagle, grizzly bear, black bear, raven, magpie, and red fox (http://www.nps.gov/yell/naturescience/wolfinfo.htm). This extra food source for scavengers is essential in hard winters when other food sources are unavailable. During the early 1800’s, the native people worshipped and idolized the wolf; man and wolf lived harmoniously. That story would change however, westward expansion brought settlers from the east. Many settlers staked out a claim in what is now Idaho, Montana, and Wyoming (http://www.nps.gov/yell/naturescience/wolfrest.htm). These settlers made a living however they could, and many of them chose livestock ranching as a means to get ahead. Their livestock overgrazed many areas causing local wildlife to have a shortage on food. This shortage would cause a ripple effect, local wildlife would soon starve Meyer | 5 and populations would drop. The wolves that would prey on the local wildlife would also need to find a way to eat. It is believed that because the wolf is such an efficient hunter that it invokes fear in us. This meant preying on the livestock that replaced the local wildlife. Losing livestock to a predator was not something taken lightly, and men would poison, trap, and shoot any wolf that was viewed as a threat (Zmyj 1996). “Between 1914 and 1926, at least 136 wolves were killed in the park; by "In those days we had never heard
of passing up a chance to kill a
wolf. In a second we were
pumping lead into the pack, but
with more excitement than
accuracy…" ~Aldo Leopold
the 1940s, wolf packs were rarely reported. By the mid-­‐1900s, wolves had been almost entirely eliminated from the 48 states (http://www.nps.gov/yell/naturescience/wolfrest.htm)”. It was not only the wolf being poisoned and trapped, any predator viewed as a threat to livestock or “more desirable” wildlife like elk or deer were also hunted. This meant the death of many bears, coyotes, foxes, and cougars (Wilmers 2003). The 1950’s & 1960’s marked a harsh time for the wolves; more accurate rifles, Figure 1: (Zmyj 1996) Meyer | 6 stronger poisons, and more lethal traps were available to hunters and ranchers9 (Zmyj 1996). [Figure 1, shows some of inhumane methods some of the ranchers used to capture/ eradicate the wolves]. During this time, white men set out on a mission to eradicate the wolf and create a predator free ecosystem. The U.S. government even implemented a nationwide wolf control policy. This wolf control policy was highly effective; an intensive survey conducted in the 1970s found no evidence of a wolf population in Yellowstone (http://www.nps.gov/yell/naturescience/wolfrest.htm). This hunting free-­‐ for-­‐ all would soon come to a halt in 1973 when the Endangered Species Act [ESA] was passed (Perry 2012). The ESA would protect any plant or animal facing extinction. Under the Act a “threated” or “endangered” species will have a significant amount of its habitat protected in order for the species to fully recover. Because of the gray wolves low numbers, they were listed as an endangered species, and any person caught poisoning, trapping, or shooting the wolves for no reason would have been penalized heavily. Two decades after the passing of the ESA marked a controversial time in U.S. history, and on March 21, 1995, gray wolves were reintroduced back into Yellowstone National Park (http://www.nps.gov/yell/naturescience/wolfrest.htm). This decision would prove to be a heavily debated one through the years. Today, many farmers and Meyer | 7 ranchers still do not agree with the reintroduction of the wolves.
Figure 2: http://www.fws.gov/mountain-­‐prairie/species/mammals/wolf/annualrpt13/figures/FINAL_Fig7a_Num-­‐BP-­‐
State_2013.pdf [Figure 2, shows the population trend of the gray wolf from 1982-­‐2013. This graph emphasizes the dramatic population growth following the reintroduction of the wolves.] Yellowstone National Park [YNP] was created as the first national park in 1872. YNP expands over 3,472 square miles. The park resides 96% in Figure 3: Map of Yellowstone. Lonely Planet. (2009) Meyer | 8 Wyoming, 3% in Montana, and 1% in Idaho (http://www.nps.gov/yell/planyourvisit/factsheet.htm). Yellowstone is home to many species including threatened Canada lynx and grizzly bear, and the endangered gray wolf (http://www.nps.gov/yell/planyourvisit/factsheet.htm). The history of the park states that the gray wolves were in the park when it was created (http://www.nps.gov/yell/naturescience/wolfrest.htm). United States created a Yellowstone National Park Act of 1872. It states the Secretary of the Interior "shall provide against the wanton destruction of the fish and game found within said Park (http://www.nps.gov/yell/naturescience/wolfrest.htm)" Although the wolves resided in the park, people viewed them as a “wanton destruction of the fish and game”, and the eradication methods followed. These eradication methods would continue for nearly 200 years until the passing of ESA in 1973. During 1995 to 1997, Yellowstone was designated as one of three recovery sites for the gray wolves and 41 wild wolves from Canada and northwest Montana were released in YNP (http://www.nps.gov/yell/naturescience/wolfrest.htm). Methods: The gray wolf is one of the most researched species in the world. I focused on the research pertaining to the gray wolf as an important species to ecosystems. Many of these studies compare what the Yellowstone/ Idaho area was like before the reintroduction of the wolf to what the ecosystem is now, and how the gray wolf has enhanced the area. I will be utilizing a meta-­‐approach style to conduct this study. I will review the current studies and literature on this topic. Meyer | 9 Analysis: There have been several studies pertaining to the listing of the gray wolf as an endangered species, one of which is an analysis conducted by Chris Woolston (2013). He reviewed the decision of USFWS to delist the gray wolf from the endangered species list. The USFWS argues that the gray wolf has outgrown its historic range and that the agency needs to focus on the recovery of the Mexican gray wolf now (Woolston 2013). However, many scientists and organizations believe that USFWS did not use proper researching methods for their studies. Without proper research methods, these studies would be incorrect. This was of particular interest because it shows the misconnection between scientists and general public. The general public believes that all studies conducted by a federal branch i.e. USFWS should be truthful and correct. Still many scientists speculate on the correct population of Canis lupus. Another study conducted by Creel and Rotella (2010) reviews the scientific justification of the 2009 delisting of the Rocky mountain population of the gray wolf. The researchers believe that USFWS based their justification on many social attitudes toward the wolves (Bruskotter et. al 2010). “In general, stakeholders calling for reductions in wolf numbers are concerned about three issues: livestock losses, effects on ungulates (particularly elk) and human safety. In 2008 and 2009, Northern Rocky Mountain wolves were responsible for an average of 203 confirmed kills of cattle (from a population of approximately 5.9 million cattle) and 538 confirmed kills of sheep (Creel and Rotella 2010)”. These stakeholders have a huge influence in the law making in the endangered species list. Creel and Rotella (2010) conclude that the USFWS should reevaluate their decision and use accurate population surveys to determine if the gray wolf Meyer | 10 is in need of delisting. This study is of particular interest because it solidifies the importance of “good science” when determining the need for a delisting of a species. Not only is “good science” a necessity when it comes to delisting an endangered species, it is severely important to be accurate in population estimates when the species that is in consideration for delisting is also one of the keystone species of the ecosystem. Another study conducted in 2009-­‐2010 researchers (Ausband, Rich, and Glenn) used surveys from hunters, howling and sign surveys to help predict wolf population in Idaho. The authors emphasize the importance of using more than one survey method to improve the accuracy of the study. The difficulties in estimating a wolf population is highlighted in this study, “Methods for estimating the size of large carnivore populations are financially and logistically challenging (Ausband etal. 2014)”. These difficulties could lead to inaccuracy which could misconstrue population estimates (Ausband et al. 2014). Since the gray wolf population is being evaluated for delisting, it is Figure 4: Radio-­‐collaring a wolf. http://www.nps.gov/yell/naturescience/upload/Wolf_AR_2011.
pdf essential that population estimate surveys are accurate. [Figure 4, shows a team of researchers taking measurements and fitting a wolf for a radio collar. A lot of data regarding the species is gathered this way (Smith et. al 2010)] . Meyer | 11 Many other studies have been conducted regarding the wolf’s impacts on other species. Researches Ripple, Beschta, and Fortin (2014) analyzed the secondary effects of reintroducing wolves back into YNP. The researchers theorized that the threatened grizzly bear would benefit from a lower elk population. Lower elk populations would decrease forging from these ungulates and in turn have more berry-­‐producing shrubs for the grizzly bear to eat (Ripple et. al 2014). Their study on the impacts of ungulates on the threated grizzly bear population was the first of its kind. Their research and finding were significant to the importance of the gray wolf in the ecosystem. “We suggest researchers and policy makers consider wolves, trophic interactions and competition from wild and domestic ungulates when addressing research and management of grizzly bears (Ripple et. al 2014)”. This study signifies the importance of the gray wolf on the threatened grizzly bear population in the YNP. The grizzly bear and the elk are not the only species that the wolf has had a major impact on. The coyote, one of the parks other canine species had a huge population change after the reintroduction of the wolf. Before the wolf was reintroduced, the coyote was largest canine species in the area (Merkle et. al 2009). The study conducted by Merkle, Stahler, and Smith (2009) investigated the environmental impacts of the wolf on the coyote. Before the reintroduction of the wolves, the coyotes were over populated. Coyotes in that area preyed heavily on small mammals and birds (Merkle et. al 2009). This created an imbalance on the ecosystem. The wolves were documented to kill 25 coyotes and in 275 interactions wolves chased the coyotes (Merkle et. al 2009). This study suggests the importance of the wolf to moderate the coyote’s population. By moderating the coyote’s population, the wolves partially restored the ecosystem back to a natural state. Once again, this study highlights the importance of the wolf as a keystone species. Not only are coyotes Meyer | 12 and grizzly bears affected by the wolf’s presence, but the endangered Canadian lynx as well. Ripple, Wirsing, and Beschta (2011) studied the effects of the reintroduction of the wolves on the endangered Canadian lynx population. The results of their study theorize that the reintroduction of wolves will do two things, 1) bring the coyote population back to a healthy population, which will in turn increase the snowshoe hare population. The snowshoe hare is a primary food source for the lynx. And 2) bring the elk population back to a healthy population, which will in turn increase the forging sites for the snowshoe hare (Ripple et. al 2011). They emphasize the importance of little-­‐considered trophic and competitive interactions when attempting to recover an endangered carnivore such as the lynx (Ripple et. al 2011). This means if more wolves equals less coyotes, then less coyotes equals less competitive interactions, which in turn would lead to a rebounding lynx population. This is another study affirming the importance of the wolf to the Yellowstone/ Idaho area. The data from this study helps solidify the need for the wolf in this ecosystem. Not only do wolves create more prey for lynxes by reducing the number of coyotes, they also create more carrion for other scavengers. Researchers Wilmers, C.C., and Crabtree (2003) create an experiment looking at the carrion available to scavengers after the wolves have made a kill. They conclude that in the presence of the wolf, more large animals are killed and thus providing more carrion for scavengers. “By partially consuming their prey, wolves subsidize scavengers with a high calorie resource that may be essential for metabolic maintenance, growth and/or reproductive success (Wilmers et. al 2003)”.This carrion is particularly important during winter months when food is scarce for scavengers, thus creating a need for wolves. For example “Female grizzly bears with reliable high-­‐
energy foods, for instance, have been shown to attain larger body size and have bigger litter Meyer | 13 sizes than their counterparts with less reliable and/or lower-­‐calorie foods (Wilmers et. al 2003)”. As indicated above, the wolves have provided an essential resource for the threated grizzly bear; however, not only the grizzly bear benefits from this resource, “increasing the time over which carcasses are available during the winter, this carrion subsidy may contribute significantly to the biodiversity of the region (Wilmers et. al 2003)”. Many studies provided thus far have shown only the importance of the wolf to other fauna. There have been significant amounts of studies conducted pertaining to the wolf and its effects on plants. Scientists Beschta, R.L. and Ripple (2012) conducted several studies on the berry-­‐producing shrubs within YNP’s northern ungulate winter range. These studies were conducted 15 years after the gray wolves reintroduction into the park, which gave the plants ample time to regrow. The studies have provided evidence that the gray wolves predation on the ungulates has helped to restore berry-­‐producing shrubs stands to recover (Beschta et. al 2012). The berry-­‐producing shrubs are not only beneficial to ungulates but other species as well, Figure 5: (Beschta et. al 2012) Meyer | 14 and a fully recovered ecosystem is beneficial to all species. The importance and role of the gray wolf in this ecosystem is highlighted in this study, and in figure 5 [the images show the differences between the absence of ungulates in area (a) and the presence of ungulates in an area (b). “Contrasting aspen sites in the northern range of Yellowstone (September 2010): (a) A riparian site that has experienced reduced ungulate browsing pressure in recent years with resultant increases in the heights of serviceberry and chokecherry (foreground) as well as increased aspen and mountain alder (Alnus incana) recruitment (i.e., growth above browse level of elk; background) and (b) an upland site experiencing continued high levels of ungulate browsing, no increased heights of berry-­‐producing shrubs, and no aspen recruitment. Note the two-­‐tiered aspen stand structure comprising an overstory of mature trees and an understory of young aspen; the missing intermediate size/age classes occurred over multiple decades when elk herbivory, in the absence of wolves, prevented aspen recruitment (Beschta et. al 2012)”]. Another study conducted by Eisenber, Seager, and Hibbs (2013) evaluates the gray wolf and its importance to forest ecology. This study and Beschta’s 2012 study show the importance of the wolf to forest ecology. “Trophic cascades driven by apex predators via top-­‐down effects have been identified in ecosystems worldwide (Eisenberg et. al 2013)”. In this study it shows the effects of the gray wolf on aspen communities. They conclude that the wolves have helped the aspen groves back to a more natural state. They suggest “applying the precautionary principle to create healthier, more resilient aspen forests would suggest conserving apex predators, as feasible, where their ranges coincide with aspen (Eisenberg et. al 2013)”. In conclusion, the wolves have helped the ecosystem rebound to a more natural state, thus proving the importance of the wolf as a keystone species. Meyer | 15 If the gray wolf were to be delisted nationwide, many states would open a hunting season for the species. Many states that already have a recovered population have already chosen to use a hunting season as a means of population control. There have already been some studies on the effects of hunting on wolf populations. One of which, is the study conducted by Creel and Rotella (2010). Their study shows some of the possible effects of the delisting of the species. Creel and Rotella (2010) evaluate the impacts of an immediate hunting season following the delisting of a population of gray wolves in 2009. Idaho and Montana had a set 20% quota on population elimination, after the season a population estimate determined that 37% of the population was killed during the year of the delisting. This evidence is useful when determining the delisting of other wolf populations. If the entire gray wolf population were to decrease by 37% that could prove detrimental to the population. If wolves are to be kept on the endangered species list, alternative methods are available for nuisance control of the wolf. Researches Hawley, Rossler, and Gehring (2013) studied the likelihood of using electronic shock collars as a management tool for reducing livestock losses caused by gray wolves. Electronic shock collars could help reduce negativity towards wolves as well as livestock loss. This device would be an excellent non-­‐
lethal management tool. These shock collars are expensive, but these collars would help ranchers keep wolves off their property (Hawley et. al 2013). A lot of money and effort has gone into the research of the gray wolves (Way and Bruskotter 2012). USFWS now wants to shift their efforts to the recovery of the Mexican wolf population in the Southwest. (http://www.fws.gov/home/wolfrecovery/). “The Mexican wolf is the rarest subspecies of gray wolf in North America. Once common throughout portions of the southwestern United States, the Mexican wolf was all but Meyer | 16 eliminated from the wild by the 1970s (http://www.fws.gov/southwest/es/mexicanwolf/)”. A mere 83 Mexican wolves roam the wild today. This number is drastically less than the historic population number. This subspecies is also an essential species to an ecosystem, and it has shared a similar story to the gray wolf’s tragic history. A higher Mexican wolf population would be needed for the ecosystem to regain its natural state. [Figure 6, shows the home range of the Mexican gray wolf]. Discussion: Figure 6: http://www.azgfd.gov/w_c/es/mexican_wolf.shtml As a top predator and a keystone species, the gray wolf is an important asset to an area. The results of this discussion should assess the implications of removing the gray wolf from the endangered species list. I am hoping this research will help students become familiar with the current issues associated with removing gray wolves from the endangered species list. Aldo Leopold was an author and leading environmentalist. His work has inspired many conservationists. Many of his ideas were formed from his work with the wolf. Aldo was a ranger for USFWS and his job was to increase the deer population for the hunters. USFWS believed that the best way to increase the population was to eliminate the wolf and make a predator-­‐free ecosystem. "I was young then, and full of Meyer | 17 trigger-­‐itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise." This belief held true to many scientists and hunters at the time. This belief has carried on to many hunters and ranchers today. After his realization of his impacts of removal of the wolf from the mountains he worked, he changed his beliefs and redirected his career to obtain a more natural ecosystem. "I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer." Just as Aldo once believed that a predator-­‐free landscape was the solution to a flourishing game population, we too have to change our misconceptions and negativity towards the wolf. I believe the wolf is an essential species to the Yellowstone/ Idaho ecosystem because of its impact in helping restore the ecosystem back its natural state. This quality makes the gray wolf a keystone species to this area. Although there have been many studies indicating the benefits of the wolf to this ecosystem, there have also been many biased studies that have proven the wolf’s population higher than what it actually is. This higher population estimate would lead to a higher hunting quota. An incorrect population estimate could be detrimental to the wolf’s population (Ausband et al. 2014). I hope USFWS will reevaluate their studies and methods they used to conduct their population estimates. Meyer | 18 References: 1) Ausband, D.E., Rich, L.N., Glenn, E.M., Mitchell, M.S., Zager, P., Miller, D.A.W., Waits, L.P., Ackerman, B.B., and Mack, C.M. (2014). Monitoring gray wolf populations using multiple survey methods. Journal of Wildlife Management 78, 335-­‐346. 2) Beschta, R.L., and Ripple, W.J. (2012). Berry-­‐producing shrub characteristics following wolf reintroduction in Yellowstone National Park. For. Ecol. Manage. 276, 132-­‐138. 3) Bruskotter, J.T., Toman, E., Enzler, S.A., and Schmidt, R.H. (2010). Are Gray Wolves Endangered in the Northern Rocky Mountains? A Role for Social Science in Listing Determinations. BioScience 60, 941-­‐948. 4) Creel, S., and Rotella, J.J. (2010). Meta-­‐Analysis of Relationships between Human Offtake, Total Mortality and Population Dynamics of Gray Wolves (Canis lupus). Volume 5. pp. 1-­‐7. 5) Eisenberg, C., Seager, S.T., and Hibbs, D.E. (2013). Wolf, elk, and aspen food web relationships: Context and complexity. Forest Ecology & Management 299, 70-­‐80. 6) Hawley, J.E., Rossler, S.T., Gehring, T.M., Schultz, R.N., Callahan, P.A., Clark, R., Cade, J., and Wydeven, A.P. (2013). Developing a New Shock-­‐Collar Design for Safe and Efficient Use on Wild Wolves. Wildlife Society Bulletin 37, 416-­‐422. 7) Merkle, J.A., Stahler, D.R., and Smith, D.W. (2009). Interference competition between gray wolves and coyotes in Yellowstone National Park. Canadian Journal of Zoology 87, 56-­‐63. 8) Perry, S. (2012). The Gray Wolf Delisting Rider and State Management Under the Endangered Species Act. Ecology Law Quarterly 39, 439-­‐473. 9) Ripple, W.J., Beschta, R.L., Fortin, J.K., and Robbins, C.T. (2014). Trophic cascades from wolves to grizzly bears in Yellowstone. Journal of Animal Ecology 83, 223-­‐233. Meyer | 19 10)
Ripple, W.J., Wirsing, A.J., Beschta, R.L., and Buskirk, S.W. (2011). Can restoring wolves aid in lynx recovery? , Volume 35. pp. 514-­‐518. 11)
Smith, D.W., Bangs, E.E., Oakleaf, J.K., Mack, C., Fontaine, J., Boyd, D., Jimenez, M., Pletscher, D.H., Niemeyer, C.C., Meier, T.J., et al. (2010). Survival of Colonizing Wolves in the Northern Rocky Mountains of the United States, 1982-­‐2004. Journal of Wildlife Management 74, 620-­‐634. 12)
Way, J.G., and Bruskotter, J.T. (2012). Additional considerations for gray wolf management after their removal from Endangered Species Act protections. Journal of Wildlife Management 76, 457-­‐461. 13)
Wilmers, C.C., Crabtree, R.L., Smith, D.W., Murphy, K.M., and Getz, W.M. (2003). Trophic facilitation by introduced top predators: Grey wolf subsidies to scavengers in Yellowstone National Park. Journal of Animal Ecology 72, 909-­‐916. 14)
Woolston, C. (2013). Grey wolves left out in the cold. Nature 501, 143-­‐144. 15)
Zmyj, P.M. (1996). A FIGHT TO THE FINISH": THE EXTERMINATION OF THE GRAY WOLF IN WYOMING, 1890-­‐1930. Montana: The Magazine of Western History 46, 14-­‐25. Meyer | 20