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Gray Wolf (Canis lupus) predation and scavenging of reintroduced American Bison (Bison bison) in southwestern Yukon.

With some particularly large bulls weighing [greater than or equal to] 1000 kg, American Bison (Bison bison, hereafter Bison) are the largest land mammal in North America. Given the potential quantity of biomass a Bison represents, preying or scavenging on Bison is seemingly profitable. Their physical size, social organization, and temperment, however, make Bison formidable prey (Fuller 1953; Smith and others 2000). Gray Wolves (Canis lupus; hereafter Wolves) and Grizzly Bears (Ursus arctos; hereafter Bears) are the only known predators of Bison, and they most often focus on calves and other young animals (Carbyn and Trottier 1988; Larter and others 1994; Smith and others 2000, 2001; MacNulty and others 2001; Varley and Gunther 2002; Wyman 2002).

In some areas, local Wolf packs are relatively adept at killing Bison (Carbyn and Trottier 1987; 1988; Larter and others 1994). This may be related, in part, to the length of time that Bison and Wolves have been sympatric, with Wolves longer accustomed to Bison on the landscape more readily viewing them as potential prey, and having sufficient time to learn how to hunt Bison through trial and error. In Europe, for instance, Wolves are known to prey and scavenge on European Bison (Bison bonasus) in the Bialowieza Forest, Poland, where both species have co-existed since the 1920s (Jedrzejewski and others 2002; Selva and others 2003). In North America, there are similar observations from long-established (>55 y) Bison populations in Wood Buffalo National Park and the Mackenzie Bison Sanctuary, Canada (Carbyn and Trottier 1987, 1988; Larter and others 2000), and Yellowstone National Park, USA (Smith and others 2000). However, there are no similar records from Bison populations that have been on the landscape for shorter periods of time (for example, <25 y).

After an absence of about 350 y, 170 Bison were reintroduced to southwestern Yukon, Canada in 1988 as part of a national recovery program to reestablish viable populations on their native range. The area hosts a full complement of native ungulates and large carnivores, including Bison, Moose (Alces americanus), Caribou (Rangifer tarandus), Dall's Sheep (Ovis dalli), Mule Deer (Odocoileus hemionus), Grizzly Bear, Black Bear (Ursus americanus), and Wolves. Unregulated by natural predators, the Bison population grew rapidly and is currently estimated at 1150 animals, based on aerial survey data (TS Jung, unpubl, data). As well, following a period of low densities of Caribou and Moose and non-lethal Wolf control (for example, sterilization; Hayes and others 2003), local knowledge indicates that the Wolf population has been recently increasing, with some observed packs containing [greater than or equal to] 18 Wolves (TS Jung, pers. obs.). In recent years, evidence of Wolverine (Gulo gulo) and Common Raven (Corvus corax) scavenging Bison gut piles left by human hunters has been observed (TS Jung, pers. obs.). There has been no evidence, however, of Wolves preying or scavenging on Bison, despite the recent increase in both local Bison and Wolf populations. According to the prey density hypothesis, kill rates by Wolves should increase as prey densities increase (Vucetich and others 2002). Here, I report the 1st observations of Wolves killing and scavenging Bison reintroduced to the Yukon.

On 18 November 2007, a local First Nation hunter reported 3 Wolves following and harassing a small group of Bison on a frozen pond near the north end of Aishihik Lake, about 105 km NE of Haines Junction, Yukon (UTM: Zone 08V, 363212E, 6737829N, WGS84; Fig. 1). The Wolves were observed to be paying particular attention to a small yearling. On 19 November 2007, the site was investigated and a yearling Bison was found dead. No other Bison or Wolves were observed in the area. Tracks in the snow indicated that the Wolves had chased and harassed the Bison for about 250 m along the shoreline of the frozen pond, with blood and Bison fur found scattered among the Bison and Wolf tracks in the snow (S Oakley, Yukon Department of Environment, pers. comm.). It appeared from the tracks that the Bison had lain down at least 3 times before coming to rest at the kill site. At the time the remains were found, less than one-third of the viscera had been consumed. Given the proximity in time and location, and the abundant evidence near the remains, it is believed that the yearling Bison was killed by Wolves and that it was the same Bison and Wolves that the hunter observed the day before.

On 13 December 2007, wildlife enforcement officers found the carcass of a female Bison calf that appeared to have been killed by Wolves. The remains were found approximately 100 m from the shore of the Nordenskiold River, about 90 km NW of Whitehorse, Yukon (UTM: Zone 08V, 497140E, 6731566N, WGS84; Fig. 1). Wildlife conservation officers were alerted to the area by Raven activity and found the remains of a Bison calf in a patch of wet meadow vegetated with tall willows (Salix spp.; D Bakica and L Bill, Yukon Department of Environment, pers. comm.). Wolf tracks, along with patches of blood and Bison fur on the willows and snow within 150 m of the calf, indicated that the calf was killed by Wolves. No Wolves were seen in the area, but there were abundant Wolf tracks around the remains of the calf and bite marks were observed on the neck, back, flank, and hind legs. Only about 50% of the viscera and 5% of the body had been consumed and it was believed that the calf had been killed within the previous 24 h because it was not yet eaten by the Wolves. The calf was found about 700 m from where an adult cow Bison had been shot by human hunters. There was no evidence of feeding by Wolves on the remains of the adult. It is believed that the calf killed was that of the adult cow killed by a human hunter, and that the calf remained near the remains of the mother. Although this cannot be confirmed, studies suggest a strong mother-calf relationship in Bison (for example, Green 1992).

[FIGURE 1 OMITTED]

On 24 February 2009, an adult cow Bison was found dead by a local First Nation resident along the shore of Duck Lake, a small frozen lake 90 km NE of Haines Junction (Fig. 1). Again, the presence of Ravens drew attention to the carcass. The Bison had been fatally wounded by a human hunter a day earlier, but lost and reported to wildlife enforcement officers (R Obourne, Yukon Department of Environment, pers. comm.). On 28 February 2009, wildlife enforcement officers investigated the carcass and no feeding by Wolves was observed. On the evening of 1 March 2009, it appeared that Wolves had found and begun to feed on the carcass. The next day, 7 Wolves were observed feeding on the Bison (R Obourne, Yukon Department of Environment, pers. comm.). Wolves were heard howling nearby for the next 3 nights by a school group camping about 350 m away from the carcass. Sporadic observations over the next 7 days suggested that the Wolves fed on and stayed near the carcass. By 5 March 2009, about 60% of the carcass had been consumed, and by 9 March the Bison was completely consumed; only the rumen remained and the Wolves had left the area.

While no Wolves were directly observed preying on Bison, the evidence is compelling that the yearling and calf had been killed by Wolves and the adult was scavenged. Thus, these are the 1st observations of Wolves preying and scavenging on reintroduced Bison in southwestern Yukon. It is possible that some Wolf predation and scavenging had occurred before these observations, but if so it likely was rare. For example, since 1998 there have been 55 Bison monitored with radio-collars and none were observed killed or scavenged by Wolves or Bears (TS Jung, unpubl, data). In addition, >200 estimated days of aerial surveys to monitor or capture wildlife had been flown in the area since the reintroduction of Bison. None of these surveys yielded observations of Bison killed or scavenged by Wolves or Bears, although there were occasional observations of Moose and Caribou killed or scavenged (Yukon Department of Environment, unpubl, data). Finally, numerous area residents, outdoor recreationalists (including Bison hunters), and wildlife conservation officers traveling through the area (likely >2000 estimated person-days each year) also had not reported any observations of predation or scavenging of Bison. Thus, there had been reasonable opportunity for such observations, yet none had been reported.

The fact that the 2 Bison preyed upon by Wolves were young animals is consistent with most other observations (for example, Carbyn and Trottier 1988; Smith and others 2000). Clearly, calves and yearlings are safer and easier to kill than adults because they are smaller than adults, and presumably less dangerous and less experienced with predators.

Given the distances involved (Fig. 1), it is likely that the Wolves involved in the observations made on 18 November 2007 and 24 February 2009, involved the same Wolf pack. The Wolves involved in the observation made on 13 December 2007, however, were most likely from a different pack. Subsequently, on 30 March 2010, I and others observed where possibly yet a different pack from the previous observations had appeared to attack an adult female Bison (based on tracks and blood in the snow), but left the area leaving the individual wounded but alive (TS Jung, pers. obs.; Fig. 1). Similarly, on 3 April 2010, 5 Wolves from presumably the same pack as the observations made on 18 November 2007 and 28 February 2009 were viewed from a helicopter closely following and apparently hunting a group of 4 Bison (K Egli, Yukon Department of Environment, pers. comm.; Fig. 1). Thus, at least 2 or 3 different packs of Wolves appear to be hunting reintroduced Bison in southwestern Yukon.

Whether Wolf predation and scavenging of Bison is a functional response to increased Bison density, possible increases in Wolf densities, or changes in the ratio of predator and prey (sensu Vucetich and others 2002), is unknown. Alternatively, predator or prey densities may have little to do with the onset of Wolf predation of Bison. Perhaps after 20 y of being sympatric with Bison, Wolves have had sufficient time to recognize Bison as potential prey and learn how to hunt them. This may be a density-independent process. Moreover, since 1998, human hunters have harvested close to 1000 Bison (Yukon Department of Environment, unpubl. data). The role that human hunting may have had in facilitating Wolf predation and scavenging on Bison, vis-a-vis provisioning of Bison gut piles and wounded animals that are either weakened or die later, is an important consideration. In particular, wounded Bison that die later may provide substantial quantities of food to Wolves prepared to scavenge carcasses. Forbes and Theberge (1992) provide data that suggests that scavenging by Wolves occurs more frequently than is recognized.

Regardless of the mechanism or trigger, Wolf predation on Bison may have substantial implications for the population dynamics of both species, as well as alternate prey species such as Moose and Caribou. For example, Wolves may respond numerically as a result of increasing Bison populations (Joly and Messier 2000). If kill rates increase, Wolves may limit Bison population growth rates (sensu Gasaway and others 1992; Messier 1994). Increased vigilance and the alteration of movement patterns, habitat use, and other behaviors may manifest as Bison learn to fear Wolves (Laundre and others 2001). In addition, Wolf predation of Bison may facilitate increased Wolf densities and subsequent declines of alternate prey such as Moose (Larter and others 1994) and Caribou.

While the apparent onset of Wolf predation and scavenging of reintroduced Bison may, in time, have substantial impact on the population dynamics of Wolves, Bison, and other local ungulate populations, predation and scavenging are important ecological processes. The goal of reintroducing Bison to their native range should have ecological restoration as the ultimate goal, not simply numerical recovery (Sanderson and others 2008; Jung and others 2010). Predation and scavenging of Bison is an indication that this reintroduced population is fulfilling some of the ecological functions that Bison likely once did prior to local extirpation.

Key words: American Bison, Bison bison, Canis lupus, ecological restoration, Gray Wolf, predation, reintroduction, scavenging, Yukon

Acknowledgements.--I thank wildlife conservation officers S Oakley, D Bakica, L Bill, and R Obourne for promptly alerting me of these observations and sharing their detailed records and deep knowledge of Yukon wildlife. K Egli, S Czterwertynski, and S Stotyn also kindly contributed observations from aerial surveys. I thank RD Hayes for early discussions on Bison-Wolf interactions. Funding for the patrols and aerial surveys that led to these observations was provided by the Yukon Department of Environment. LN Carbyn and an anonymous reviewer kindly provided thoughtful comments on an earlier draft.

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Yukon Department of Environment, PO Box 2703, Whitehorse, YK, Y1A 2C6; thomas.jung@gov.yk.ca (TSJ). Submitted 23 February 2010, accepted 4 October 2010. Corresponding Editor: Clayton Apps.
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Title Annotation:GENERAL NOTES
Author:Jung, Thomas S.
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Date:Sep 22, 2011
Words:2761
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