by Douglas W. Smith, Daniel R. Stahler, Matthew C. Metz, Kira A. Cassidy, Erin E. Stahler, Emily S. Almberg, & Rick McIntyre
Anthony R.E. Sinclair, long-time researcher in the Serengeti of Africa, suggests that to understand an ecosystem, one also must know its human history. For the Serengeti, he refers to the 1889 outbreak of rinderpest that killed 95% of Africa's cattle and many wild ungulates, and the 19th century ivory trade, both of which drastically altered the plant-animal associations of the 20th century. When first studied in the 1960s, no one was aware of this history, which impeded an in-depth understanding of the ecosystem (Sinclair 2012). Yellowstone, too, has had human interventions that have affected its short and, sometimes, long-term ecological relationships; but compared to the Serengeti, our human history is better documented. Historic interventions into Yellowstone include the fur trade, market hunting, predator control, fire suppression, elk and bison reductions, rewilding of black and grizzly bears, and now wolf reintroduction. It is this last one which is the focus of this issue. Although wolf reintroduction lasted only three years and recovery has been a relatively recent historical event, this human intervention is likely to have impacts lasting well into the future.
Since the first Yellowstone Science special issue on wolves in 2005 (10th anniversary of reintroduction), a lot has happened and our understanding has improved. Wolves are no longer in the "colonization" phase of recovery, which dominated the story in the 2005 issue. Glimpses of a new Yellowstone are taking shape. Gone for most of the 20th century, wolves and other carnivores have made a comeback; but wolves are arguably the most notable as they are considered the dominant North American carnivore (based on distribution and abundance; Mech 1970). Bears were never eliminated, but were reduced; and cougars have now recolonized as well. With these increased carnivore densities, and including other factors, elk have declined and bison have increased, ushering in what now has to be considered a new era in Yellowstone (White et al. 2013). This new time may be the most "natural" in all of Yellowstone's long history.
"Natural" is what early park managers and outside scientists struggled to define (Pritchard 1999). This is ironic because at the time, most large carnivores were gone. Surely part of the definition of "natural" would include them, but public attitudes were strongly anti-carnivore; and this had a significant influence on policy (Pritchard 1999). Given this cultural backdrop, the human intervention of wolf reintroduction in the 1990s may be the most deliberate and high profile among these recent management actions. What follows is an update since the 2005 special issue on wolves, 20 years in. Likely the story will change each decade, but at least we can hope all of these carnivores will be around for some time, helping to keep Yellowstone as natural as it can be.
Human Attitudes and Wolf Recovery
In the first special wolf issue of Yellowstone Science, a change in human attitudes was highlighted as the most important factor in making wolf recovery possible. Humans are still the most important factor in wolf management, both inside and especially outside of the park. It is worth a brief review of the intertwined policy and people who pulled off this effort.
An early voice arguing for change before it was popular, Aldo Leopold mentioned Yellowstone as a place to restore wolves, "Yellowstone and its adjacent national forests…some of considerable size in which…[wolves] may be allowed to continue their existence without molestation" (Leopold 1944). He also added, "Are we really better off without wolves in the wilder parts of our forests and ranges?" (Leopold 1991). Ultimately, Leopold's vision for Yellowstone was realized and recognized when the first wolf pack to naturally form in Yellowstone in over 70 years was named the Leopold pack.
Another significant step was in 1975-1977, when John Weaver conducted a formal survey to look for wolves—he found none—and once again restoration was recommended through a reintroduction (Weaver 1978). Then Douglas Houston, in his landmark book The Northern Yellowstone Elk, did the same, calling Yellowstone "ideal" for wolves and that their absence was "the single greatest departure from the objective of maintaining natural ecosystems" (Houston 1982), harking back to the early park managers who tried to define the meaning of "natural," yet without wolves (or other carnivores). By the time Houston was recommending restoration, the U.S. Fish and Wildlife Service had already produced a Recovery Plan which was revised in 1987 (USFWS 1980, 1987). Both documents helped clear the way for more planning that culminated in approval—bipartisan approval—from Congress to restore wolves to Idaho, Montana, and Wyoming.
In short, the strategy was to nurture a new population that had immigrated from Canada into northwest Montana and to reintroduce wolves to central Idaho and Yellowstone. The goal was 30 breeding pairs across the region and approved management plans from the three states. In 1995 and 1996, and only in Yellowstone in 1997, 76 wolves from Alberta, British Columbia, and northwest Montana were released into central Idaho and Yellowstone: 41 in Yellowstone (14 from Alberta, 17 from British Columbia, 10 from northwest Montana), and 35 in central Idaho (Bangs and Fritts 1996). The West's new wolf era was underway. Some said this was the most significant wildlife conservation event of the 20th century for the United States. Changing human attitudes were revising the mystique of the old west–it would not go quietly or completely, nor should it. And, it still can be the "wild west," perhaps even wilder with the carnivores.
Looking back, the process to restore wolves to Yellowstone went surprisingly smoothly. Early U.S. Fish and Wildlife Service work, combined with the vision of National Park Service (NPS) Director William Penn Mott, the quiet leadership of Yellowstone Superintendent Robert Barbee, the detailed planning of the Recovery Team, support from the Clinton administration and Secretary of the Interior Bruce Babbitt, and final implementation by Edward Bangs and Steven Fritts of the U.S. Fish and Wildlife Service, with big assists from Yellowstone planners John Varley and Wayne Brewster, were the reasons for the success. Michael Finley was park superintendent when reintroduction occurred. Public support was significant, truly a ground swell of grassroots efforts from a variety of sources (The Wolf Fund and Defenders of Wildlife both at the forefront, plus many others). It is hard to imagine how such a controversial program achieved the success it did. Behind the scenes, there were many others, most notably Norman Bishop, an NPS employee who worked on his own time to educate the public about restoration and why to support it. Later the Yellowstone team led by Michael Phillips, and then Douglas Smith, carried the program to successful completion. Of course, most credit is due to the wolves; they only needed a little help.
Colonization to Saturation
Now, 20 years forth, some perspective on what happened can be achieved. Just by eyeballing the graph of annual park wolf counts (figure 1), we can characterize two phases over the last 20 years: Phase 1 is up to about 2008, where population growth was mostly positive; Phase 2 is where growth was mostly flat or even negative. This first period we refer to as the "colonization" phase (wolf numbers reached 174 in as many as 16 packs parkwide), and the second period the "saturation" phase (wolf numbers during this time hovered around 100 in 10 packs). Since 1997, after releases were completed, average population growth was about 10% per year, but year-to-year variation was greater, ranging from +62% to -43%. This characterization helps us understand much of what is happening ecologically and behaviorally. Will there be a Phase 3? Interestingly the northern Yellowstone elk herd, after precipitously declining, also appears to be stable since about 2010 (figure 2). Has some kind of equilibrium been achieved between wolves and other carnivores, elk, and bison?
With fewer elk, we suspect there are fewer vulnerable elk. Wolves make their living from vulnerable prey, and not just available prey, because prey are also dangerous to wolves. We conclude this because elk probably now exist below carrying capacity, unlike when there were more than 20,000 elk ("The Challenge of Understanding Northern Yellowstone Elk Dynamics after Wolf Reintroduction," this issue, for more details on the wolf-elk relationship). Elk below carrying capacity generally means there are plenty of resources for all; whereas, at carrying capacity, comparatively more elk will not have enough resources, be in poorer condition, and therefore be more vulnerable to wolf attacks. Anecdotally, our capture crews support the characterization of elk being in good nutritional condition in recent years. The contract capture crew, who catch and collar elk, tell us that Yellowstone elk are the leanest, meanest elk in all of western North America. Why? Probably because they are predator-tested and below carrying capacity. How would you like to be a wolf faced with killing one of these elk, an animal five to seven times your size and you have nothing to use but your teeth and pack mates?
Another factor impacting wolf numbers is disease. In fact, disease could be considered the defining feature of Phase 1, or the outcome of growth and high wolf density. During Phase 1 when population growth was mostly positive, there were three outbreaks (1999, 2005, and 2008) of canine distemper virus (CDV) that caused the population to decline (figure 1; "Infectious Diseases of Wolves in Yellowstone," this issue). After the first two declines, the wolves immediately increased the next year; we call this compensatory reproduction, and it likely occurred because there was abundant food in the form of vulnerable elk. After the third CDV outbreak in 2008, the wolves did not increase the next year; 2009 happened to be the year an epidemic of sarcoptic mange peaked within the park. We also learned the spread of both CDV and sarcoptic mange were somewhat dependent on wolf density; packs in areas of lower density had lower exposure rates to both CDV and mange (Almberg et al. 2012, 2015). Mange is a chronic infection and may be here to stay, but now with lower wolf densities will there be another CDV outbreak? We know that CDV does not impact only wolves, but probably all carnivores in Yellowstone. So how will this dynamic affect wolves and other wildlife in the future? These disease outbreaks may be even more complicated. Interestingly, during CDV outbreaks, black wolves appear to survive better than gray, or at least wolves that carry the black coat gene ("Yellowstone Wolves at the Frontiers of Genetic Research," this issue). This interaction between coat color and disease resistance is far from worked out but is an area of intense research.
Another interesting aspect of the saturation phase is it appears wolves have occupied most of the suitable wolf habitat in Yellowstone (figure 3). Habitat requirements for wolves include protection from humans, year-round availability of ungulate prey (Mech and Boitani 2003), and enough space so pups are protected from other packs (Smith et al. 2015). For Yellowstone the first requirement is universally met. But with the park being such a harsh winter environment, many ungulates migrate out, making large portions of the park unsuitable for year-round wolf occupation. For example, due to the large population size and only partially migratory nature of northern Yellowstone elk, the northern range of Yellowstone is fully occupied (figure 3). No other area within the park has as many wolves or is contiguously occupied. Other areas of occupation are Pelican Valley, but to exist here wolves have to range widely or switch to eating bison in the winter because all the elk migrate out. Thorofare, Bechler, and Snake River regions are also occupied; but wolves living in these areas must range widely and often have to leave the park. The Madison-Firehole River area and Hayden Valley contain wolves that often migrate to the northern range in winter. These factors have caused wolf territory size to vary accordingly: northern range wolf pack territories are smaller, averaging 274 km2/106 mi2 (range = 58-1151 km2/22-444 mi2); whereas, interior territories are comparatively larger and average 620 km2/239 mi2 (range = 105-1675 km2/41-647 mi2).
What Protection Brings
Besides organizing across the landscape, wolves also organize themselves into social units called packs. In fact, this behavior is what makes wolves so unique from other wildlife. Referred to as cooperative breeding, few mammals live this way; and the resulting sociality drives much of wolf life history. Most wolf packs outside Yellowstone suffer high levels of human-caused mortality. In a study of wolf mortality in Idaho, Montana, and Wyoming, it was found that about 80% of wolves were killed by humans (Smith et al. 2010). This influences pack structure. Conversely, low human-caused mortality in Yellowstone allows for richer age structures and more complex social organization within wolf packs, including very different roles for old individuals within the group ("Territoriality and Inter-Pack Aggression in Gray Wolves," this issue). This protection from human hunting has also led to larger packs. Average pack size was about ten wolves through 2008; when the population declined, so did wolf pack size but not by much, to around nine wolves. The range of pack sizes was from 2 (considered the minimum size for a pack) to 37, which may be the largest pack ever recorded. This pack, the Druid Peak pack of 2001, was so large it was socially cumbersome and only rarely observed together. Ultimately, this pack split into four different packs (Druid Peak, Geode Creek, Agate Creek, and Buffalo Fork) over the course of fall and early winter.
These complex social groups may be a hallmark of wolf packs in Yellowstone and have been intriguing to study. Some of these packs stick around for a long time, with an average of about 12 years, but some are longer. Mollie's pack (originally Crystal Creek pack and renamed after the late Director of the U.S. Fish and Wildlife Service) and Yellowstone Delta pack (originally Soda Butte pack) are two examples, and notably both packs are from first year (1995) reintroductions. We're not sure why these two packs have persisted so long; but some possibilities are that they staked out a good territory before other wolves could (the benefit of being first), they don't live in a competitive environment like the northern reaches of the park, and there have been some long-term individuals in these packs that may have been the "social glue" ("Wolf Turf: A Glimpse at 20 Years of Wolf Spatial Ecology in Yellowstone," this issue, examines some of these ideas).
Socially complex packs usually have pups, but importantly, not as often as presumed. Some say 90% of wolf packs in any given year produce pups; but in the protected confines of Yellowstone, we find that the number of breeding packs each year is lower than that–about 70- 80% reproducing packs each year. Why this is so could be due to many factors: death of a breeder, limited food, disease, or competition between packs, collectively referred to as density dependence.
Beginning with low density and high food abundance early on, wolf reproduction was super-charged, particularly on the northern range. Ample food availability, coupled with increasing wolf density, led to more deviations from a monogamous, single-pair breeding structure within packs; about 25% of wolf packs in Yellowstone had more than one litter. Only recently has this started to drop. But average litter size is about 4.7 pups with about 3.1 pups surviving until December of that year ("Motherhood of the Wolf," this issue). Typically pup survival is about 70%; but some years, particularly the ones of CDV outbreaks, pup survival can be less than 20%.
The Value of Yellowstone
As a whole, Yellowstone wolves have added to our understanding and appreciation of wolves everywhere. Crowds of visitors continue to come to view them. This was made possible by early park managers who had a vision and fought a decades-long struggle that led to Yellowstone being more "natural," or at least more pristine than when it was established. This protection, or "natural baseline" as some early scientists called it, has led to greater insights into how nature works. Protection of the park has allowed for extensive research and insights into wolf ecology, from coat color disease immunity to discovering matrilineal pack organization, as well as the other topics covered in this issue. What a rarity in this modern-day sea of humanity. So despite all the historic human interventions and disturbances, each one and each time adding to our knowledge, Yellowstone has remained and thrived—largely because of the love so many have for it. Wolves are just one more thing making it slightly more natural and wild; and as Durward Allen said, "wildness needs wolves" (Allen 1979).