Impacts of Reintroduced Gray Wolves on Ungulates and Scavengers within Yellowstone National Park

Aaron van der Groen

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Writer's comment: This review article was the second of four scientific papers written for English 104E (Scientific Writing) on the ecological impacts of gray wolves in Yellowstone National Park. While I have never visited the park, I became interested in Yellowstone ecology after reading several very interesting articles about the impact of wolves on park vegetation. Though I was not able to fit these topics into my review article, the original studies motivated me to investigate how prey species and scavengers might be affected by the wolves’ presence in the park. I would like to thank Aliki Dragona for her unending patience, support, and wit and the Shields Library librarians for their help with my adventures in interlibrary loans.

—Aaron van der Groen

Instructor's comment: Aaron's literature review on the ecological impacts of the reintroduction of gray wolves in Yellowstone National Park was the second of four very interesting papers on the subject. During the quarter in our ENL 104E (Science Writing) class, I saw Aaron take a rich, challenging topic, research it thoroughly, decide what to use out of the wealth of information he dug up, and turn it into four fascinating essays that addressed different audiences and varied purposes. His literature review published here is evidence of Aaron's ability to synthesize multiple sources of information without ever forgetting the needs of his readers and without compromising the integrity of his sources—and he manages it all in elegant, sophisticated prose that maintains his audience's interest throughout the paper.

—Aliki Dragona, University Writing Program

The Greater Yellowstone Ecosystem is a large public wildland area (58,026 km²) spread over the states of Idaho, Montana, and Wyoming, USA. In the center of this ecosystem lies Yel-lowstone National Park (8,991 km²), a protected home for a diverse array of flora and fauna as well as the site of many in-teresting topographical and thermal features. The 1995 and 1996 reintroductions of Canadian gray wolves (Canis lupus) into Yellowstone National Park signaled a change in the bio-logical landscape of the park, and many of the ultimate con-sequences of the reintroduction remain unknown (Smith et al. 2004). Newly formed predator-prey relationships are of par-ticular interest, and researchers are currently trying to under-stand the wolves’ impacts on prey species as well as on spe-cies impacted by the resulting trophic cascades. Under-standing the impacts of wolves on ungulate prey species and on closely linked scavenger species will enable officials to more accurately and effectively manage and preserve the park’s natural treasures.

Impacts on ungulate prey
Impacts on bison
Prior to their release in 1995 and 1996, the reintroduced wolves of Yellowstone National Park had not encountered bison (Bison bison). Even so, they adapted quickly to killing this novel prey item within the first 25 months of their reintroduction. During this time, the Yellowstone gray wolves killed bison where no other prey items were available (4/14 observed kills), where bison were the most prevalent prey item (9/14 kills), and during the late winter months when bison were especially vulnerable (3/14 kills, data not mutually exclusive) (Smith et al. 2000). In all, bison currently account for 4% of the Yellowstone wolves’ diet (Smith et al. 2004). Among the social classes of bison, wolves killed primarily bison calves, cows, and older adults in poor physical condition. Healthy male bison, on the contrary, tended not to flee when approached by the wolves and were often left alone (Smith et al. 2000).
      While researchers agree that male bison and female bison without calves have low vigilance levels in all areas of the park, both with and without wolves (Laundré et al. 2001; Pyare and Berger 2003), the vigilance levels of females with calves is still in question. While Pyare and Berger assumed no change in bison vigilance among any social class, in areas with and without wolves (2003), Laundré et al. observed that females with calves in areas occupied by wolves increased their vigil-ance time budget to 18.9 ± 2.5%. Adjusting accordingly, the increase in vigilance among female bison with calves equated to a decrease in foraging efforts (2001).

Impacts on moose
As with bison, moose (Alces alces) comprise 4% of the Yel-lowstone gray wolves’ diets (Smith et al. 2004). While few studies have been done on the impacts of reintroduced wolves on moose populations, one major study has focused on the Yellowstone moose’s anti-predator reactions. In general, moose in Yellowstone National Park lack basic anti-predator responses. Although the moose are confronted with a changing environment after the introduction of predators, they are not readily adapting. In particular, the frequency of rudi-mentary anti-predator behavior has remained low, and even female moose that have lost young show no overt response when in olfactory range of wolves. Vigilance levels in all so-cial classes and in areas with and without wolves have re-mained unchanged since the reintroduction of wolves into the park (Pyare and Berger 2003).

Impacts on elk
The most numerous and easiest to capture prey item for wolves in Yellowstone National Park is the elk or wapiti (Cer-vus elaphus) (Smith et al. 2000). Overall, elk account for 92% of the Yellowstone gray wolf’s diet and nearly 100% of the diet in early winter (X2 = 0.001, df = 1, P = 0.997). Of the elk kills, one major study found that around 43% were calves, 28% were cows, 21% were bulls, and 9% were of unknown sex or age, showing that Yellowstone wolves strongly selected for elk calves, tended to select against cows, and selected for bulls only in proportion to their relative availability. In general, more calves are killed in early winter, and more bulls are killed in late winter when bulls are the most vulnerable. This distribution of elk predation also tended to vary between sections of the park. In the Northern Range, more cows, fewer bulls, and about an equal number of calves were killed in comparison to the rest of the park. Also, Northern Range wolves killed a higher proportion of elk during late winter than did wolves in the rest of the park (Smith et al. 2004).
      Yellowstone elk have responded to increased predation rates from the reintroduction of wolves with an increase in vi-gilance. When compared to other ungulate species, elk have higher vigilance rates than bison and moose, presumably be-cause elk have a higher rate of predator lethality. When compared to elk outside of immediate wolf pack territories, the elk within Yellowstone gray wolf home ranges are overall more vigilant (Pyare and Berger 2000). Within the elk social classes, the clearest pattern for vigilance is seen in male elk. On average, bulls spend 12.8% of their daily time budget being vigilant. Vigilance levels did not differ between bulls in local areas containing wolves and in areas without wolves; nor did vigilance levels change from year to year. In elk cows, an increase in vigilance was observed. While vigilance levels in cows without calves did not statistically change for the first four years in areas without wolves (11.5–16.7%), a significant in-crease in vigilance occurred in the fifth year after wolf reintro-duction (30.5 ± 2.8%). In areas with wolves, vigilance in-creased significantly the first two years after wolf reintroduction (max 35.2 ± 3.6%) and stabilized during the third year. Fe-males with calves were the group most responsive to the rein-troduction of wolves into Yellowstone National Park, and sig-nificantly higher vigilance levels were seen the first year after the reintroduction in areas with and without wolf presence. In addition, vigilance levels stabilized during the second year, supporting the hypothesis that vigilance levels in elk would in-crease in response to the reintroduction of wolves and would eventually stabilize. Overall, the introduction of wolves into Yellowstone has caused elk to sacrifice as much as 20% of their foraging effort to increase vigilance efforts and avoid predation (Laundré et al. 2001).
      In spite of the predation pressure from the reintroduced wolves, the elk population has stabilized; gray wolf prey-switching behavior may, in fact, be partly responsible for the maintenance of elk populations. At the same time, re-searchers agree that this stability may be temporary. While the elk populations may be currently supported by recent mild winters and a population nearing carrying capacity, in the fu-ture, more severe winters may alter the predator-prey rela-tionship between the Yellowstone elk and wolves, and some researchers expect to see an increased wolf impact on elk population levels (Smith et al. 2004). Furthermore, the im-pacts of elk hunting in the vicinity of the park, combined with wolf impacts and inclement winter weather, may cause elk herd populations to drop dramatically (Eberhardt et al. 2003). Therefore, ungulate populations should be closely monitored, and proactive ungulate management is needed to prepare for increased predation effects from the reintroduced Yellowstone wolves (Eberhardt et al. 2003; Smith et al. 2004).

Impacts on scavengers
Impacts on coyotes
Coyotes (Canis latrans) have been significantly affected by the reintroduction of wolves into Yellowstone. In particular, gray wolves have increased the amount of carrion biomass availa-ble to coyotes from the months of February through March. Rather than being a pulsed resource at the end of particularly severe winters (as was the case prior to wolf reintroduction), wolf-killed carcasses are more consistently available to sca-vengers throughout the winter months (Wilmers et al. 2003). In addition, coyotes have been generally observed to feed more often when in wolf territories (9% vs. 4% of time budget).
      While the consumption of wolf-killed ungulates is a quick source of high-quality food for coyotes, coyotes must also face the tradeoff of increased wolf confrontations (Switalski 2003). Since wolf reintroduction into the park, fatal interactions be-tween coyotes and wolves have led to an estimated 25–33% reduction in Northern Range coyote populations each winter. In response, coyotes locally adjusted their behavior to the presence of wolves, as shown by an increase in the proportion of vigilance (17% vs. 10%) and reduced resting rates in the daily time budget (36% vs. 54%). In assessing the long-term impact of wolves on coyotes, researchers speculate that the increased vigilance required for anti-predator response, com-bined with a decrease in the number of den helpers (from an increase in fatal interactions) may lead to a reduced survival of coyote pups (Switalski 2003).

Impacts on grizzly bears
Wolves and grizzly bears (Ursus arctos) interact often in Yel-lowstone National park, most often when defending offspring or competing for prey items. Although these two introduced species can, at times, be tolerant of each other’s presence when attacking prey, competition for the prey carcass is usually inevitable. Since the Yellowstone gray wolves are no match for the larger grizzly bears, wolves often lose their kills to a form of kleptoparasitism (MacNulty et al. 2001). The consequences of grizzly bear kleptoparasitism on wolf-killed prey are still unclear. MacNulty et al. provide the only source of information on modern wolf-bear interaction in Yellowstone National Park. These researchers suspect that wolf-killed ungulates may benefit the grizzly bears by supplying them with a new high-quality food resource. As a result, grizzly bear population density, adult female body mass, and litter size are expected to increase. However, if wolf-killed ungulates are unavailable to female grizzly bears, or if carrion is unavailable in the fall months preceding hibernation, the re-captured wolf-killed prey may fail to incur a net population benefit to Yellowstone grizzly bears (2001).

Since the reintroduction of gray wolves into Yellowstone Na-tional Park, many ecological changes have occurred. While the presence of a top-predator may currently have only a li-mited impact on some species (e.g., moose and bison), many other species (e.g., elk and coyote) have been permanently affected. By understanding these predator-prey relationships and the resulting trophic cascades that form, resource manag-ers will be better equipped to create proactive policies to en-sure the diverse habitats and dynamic wildlife interactions within Yellowstone National Park.


Works Cited

Eberhardt, L.L., Garrott, R.A., Smith, D.W., White P.J., and Peterson, R.O. 2003.                 Assessing the impact of wolves on ungulate prey. Ecological Applications 13(3):              776–783.
Laundré, J.W., Hernández, L., and Altendorf, K.B. 2001. Wolves, elk, and bison:                   Reestablishing the "landscape of fear" in Yel-lowstone National Park, U.S.A.               Canadian Journal of Zoology 79(8): 1401–1409.
MacNulty, D.R., Varley, N., and Smith, D.W. 2001. Grizzly bear, Ursus arctos, usurps       bison calf, Bison bison, captured by wolves, Canis lupus, in Yellowstone National       Park, Wyoming. Canadian Field–Naturalist 115(3): 495–498.
Pyare, S., and Berger, J. 2003. Beyond demography and delist-ing: Ecological recovery       for Yellowstone's grizzly bears and wolves. Biological Conservation 113(1): 63–73.
Smith, D.W., Drummer, T.D., Murphy, K.M., Guernsey, D.S., and Evans, S.B. 2004. Winter prey selection and estimation of wolf kill rates in Yellowstone National Park,             1995–2000. Journal of Wildlife Management 68(1): 153–166.
Smith, D.W., Mech, D.L., Meagher, M., Clark, W.E., Jaffe, R., Phil-lips, M.K., and              Mack, J.A. 2000. Wolf-bison interactions in Yellowstone National Park. Journal of        Mammalogy 81(4): 1128–1135.
Switalski, T.A. 2003. Coyote foraging ecology and vigilance in response to gray wolf            reintroduction in Yellowstone National Park. Canadian Journal of Zoology 81(6):             985–993.
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(6): 909–916.