Northern Yellowstone Research Articles

Aspen snag dynamics, cavity-nesting birds, and trophic cascades in Yellowstone's northern range

We examined current and possible future aspen stand dynamics and cavity-nesting bird abundances for Yellowstone's northern ungulate winter range. Our measurements suggest aspen stands located within Yellowstone National Park (YNP) are in a different condition than those located in the Gallatin National Forest (GNF), immediately outside the park. Stands inside YNP were composed of more snags and had fewer small-diameter live stems than stands in GNF. Six of the 11 cavity-nesting bird species we analyzed were more abundant inside YNP where aspen snags were more abundant. We discuss potential implications of current and future aspen stand dynamics in the context of wolf re-introduction and trophic cascades for cavity-nesting birds. Our results suggest aspen stands within YNP will experience a relatively high abundance of snags in the near future, followed by a period of low snag abundance that will influence the abundance of several cavity-nesting bird species. The relative composition of future aspen stands within YNP will likely differ from historical conditions, and stands outside YNP, for several decades.

Survey of Selected Pathogens and Blood Parameters of Northern Yellowstone Elk: Wolf Sanitation Effect Implications

ABSTRACT The restoration or conservation of predators could reduce seroprevalences of certain diseases in prey if predation selectively removes animals exhibiting clinical signs. We assessed disease seroprevalences and blood parameters of 115 adult female elk (Cervus elaphus) wintering on the northern range of Yellowstone National Park [YNP] during 2000–2005 and compared them to data collected prior to wolf (Canis lupus) restoration (WR) in 1995 and to two other herds in Montana to assess this prediction. Blood parameters were generally within two standard deviations of the means observed in other Montana herds (Gravelly-Snowcrest [GS] and Garnet Mountain [GM]), but Yellowstone elk had higher seroprevalences of parainfluenza-3 virus (95% CI YNP = 61.1–78.6, GS = 30.3–46.5) and bovine-virus-diarrhea virus type 1 (95% CI YNP = 15.9–31.9, GM = 0). In comparisons between pre-wolf restoration [pre-WR] (i.e., prior to 1995) seroprevalences with those post-wolf restoration [post-WR] in Yellowstone, we fo...

Yellowstone's destabilized ecosystem: elk effects, science, and policy conflict

Wagner, one of our most distinguished wildlife biologists, is a strong critic of ecological practices in the national parks. This book provides an assessment of the ecological history of Yellowstone's northern range, since before the park existed, showing the impact of US Park Service policies on the health of the areas they oversee. He demonstrates that elk had been historically rare throughout the region and that overgrazing by elk has seriously degraded the landscape and altered the structure of the area. This is a major contribution to reconstructing the ecology of this region over the course of the past 500 years. It is also a critique of US Park Service management policies and their stewardship of the nation's most cherished natural areas. Wagner's book will generate substantial attention and debate both in the scientific and policy/management communities.

WILLOW ON YELLOWSTONE'S NORTHERN RANGE: EVIDENCE FOR A TROPHIC CASCADE?

Reintroduction of wolves (Canis lupus) to Yellowstone National Park in 1995-1996 has been argued to promote a trophic cascade by altering elk (Cervus elaphus) density, habitat-selection patterns, and behavior that, in turn, could lead to changes within the plant communities used by elk. We sampled two species of willow (Salix boothii and S. geyeriana) on the northern winter range to determine whether (1) there was quantitative evidence of increased willow growth following wolf reintroduction, (2) browsing by elk affected willow growth, and (3) any increase in growth observed was greater than that expected by climatic and hydrological factors alone, thereby indicating a trophic cascade caused by wolves. Using stem sectioning techniques to quantify historical growth patterns we found an approximately twofold increase in stem growth-ring area following wolf reintroduction for both species of willow. This increase could not be explained by climate and hydrological factors alone; the presence of wolves on the landscape was a significant predictor of stem growth above and beyond these abiotic factors. Growth-ring area was positively correlated with the previous year's ring area and negatively correlated with the percentage of twigs browsed from the stem during the winter preceding growth, indicating that elk browse impeded stem growth. Our results are consistent with the hypothesis of a behaviorally mediated trophic cascade on Yellowstone's northern winter range following wolf reintroduction. We suggest that the community-altering effects of wolf restoration are an endorsement of ecological-process management in Yellowstone National Park.

A 2650-year-long record of environmental change from northern Yellowstone National Park based on a comparison of multiple proxy data

Geochemical, stable-isotope, pollen, charcoal, and diatom records were analyzed at high-resolution in cores obtained from Crevice Lake, a varved-sediment lake in northern Yellowstone National Park. The objective was to reconstruct the ecohydrologic, vegetation, and fire history of the watershed for the last 2650 years to better understand past climate variations at the forest-steppe transition. The data suggest a period of limited bottom-water anoxia, relatively wet winters, and cool springs and summers from 2650 to 2100 cal yr BP (700–150 BC). Dry warm conditions occurred between 2100 and 850–800 cal yr BP (150 BC and AD 1100–1150), when the lake was anoxic, winter precipitation was low, and summer stratification was protracted. The data are consistent with overall warmer/drier conditions during the Medieval Climate Anomaly, although they suggest a shift towards wetter winters within that period. The period from 850 to 800 cal yr BP (AD 1100–1150) to 250 cal yr BP (AD 1700) was characterized by greater water-column mixing and cooler spring/summer conditions than before. In addition, fire activity shifted towards infrequent large events and pollen production was low. From 250 to 150 cal yr BP (AD 1700–1800), winter precipitation was moderate compared to previous conditions, and the lake was again stratified, suggesting warm summers. Between 150 and 42 cal yr BP (AD 1800–1908), winter precipitation increased and spring and summer conditions became moderate. Metal pollution, probably from regional mining operations, is evident in the 1870s. Large fires occurred between ca. 1800–1880, but in general the forests were more closed than before. The Crevice Lake record suggests that the last 150 years of Yellowstone's environmental history were characterized by intermediate conditions when compared with the previous 2500 years.

Aspen patch and migratory bird relationships in the northern Yellowstone ecosystem

We evaluated the effects of aspen patch area and orientation (relative to North and an elevational gradient) on the early breeding season abundance and species richness of migratory and resident birds in the northern ungulate winter range of the Yellowstone ecosystem, USA. Using an information-theoretic model selection approach, we found patch area and basal area of aspen to be the most important covariates for long distance migrants, and patch orientation relative to elevational gradient the most important covariate for residents/short-distance migrants. Basal area of live aspen and aspen snags was marginally important for both migratory strategies, likely because aspen snags are an important habitat for most cavity-nesting species. Landscape ecological theory postulates passive interception of dispersing or migrating organisms by patches of suitable habitat. Our results suggest that residents/short-distance migrants are intercepted by patches that are oriented perpendicular to the elevational gradient of our study region resulting in greater abundances and species richness in those patches. However, long-distance migrants appear to use aspen patches without regard to orientation, but rather to patch area.

A Simple Solar Radiation Index for Wildlife Habitat Studies

Abstract: Solar radiation is a potentially important covariate in many wildlife habitat studies, but it is typically addressed only indirectly, using problematic surrogates like aspect or hillshade. We devised a simple solar radiation index (SRI) that combines readily available information about aspect, slope, and latitude. Our SRI is proportional to the amount of extraterrestrial solar radiation theoretically striking an arbitrarily oriented surface during the hour surrounding solar noon on the equinox. Because it derives from first geometric principles and is linearly distributed, SRI offers clear advantages over aspect‐based surrogates. The SRI also is superior to hillshade, which we found to be sometimes imprecise and ill‐behaved. To illustrate application of our SRI, we assessed niche separation among 3 ungulate species along a single environmental axis, solar radiation, on the northern Yellowstone winter range. We detected no difference between the niches occupied by bighorn sheep (Ovis canadensis) and elk (Cervus elaphus; P = 0.104), but found that mule deer (Odocoileus hemionus) tended to use areas receiving more solar radiation than either of the other species (P < 0.001). Overall, our SRI provides a useful metric that can reduce noise, improve interpretability, and increase parsimony in wildlife habitat models containing a solar radiation component.

A Seventy‐Year History of Trends in Yellowstone's Northern Elk Herd

ABSTRACT We analyzed counts of northern Yellowstone elk (Cervus elaphus) in Yellowstone National Park, Wyoming, USA, over 70 years to evaluate the effects of changing management on population trends. Population reduction efforts and hunter harvests during 1932–1968 removed 71,330 elk and decreased estimated abundance from 16,000 to 6,000 elk. Abundance increased to approximately 17,000 elk (λ = 1.19) when removals ceased and harvests were very small during 1969–1975. Moderate to liberal hunter harvests of antlerless elk outside the Park during 1976–2004 removed a relatively consistent proportion (26 ± 0.1 [SD]%) of females that migrated outside the park, mostly from prime‐age (3–15 yr) classes with high reproductive value. Substantial winterkill was infrequent (1989, 1997), but it significantly reduced calf survival when it occurred. Wolves (Canis lupus) were reintroduced in 1995–1996 and rapidly increased in abundance (λ = 1.23) and distribution. Estimated wolf kill of elk now exceeds hunter harvest, but has a smaller effect on population dynamics because wolves concentrate on calves and older females (>14 yr) with low reproductive value. During 1995–2004, estimated abundance decreased from 23,000 to 12,000 elk. The recent ratio of wolves to elk is relatively low compared to the estimated equilibrium ratio, suggesting that the wolf population may yet increase in the future. Thus, reduction of harvests of prime‐aged female elk to decrease removals of animals with high reproductive value and increase adult female survival appears essential. We analyzed the relative impact of removals by hunters and by wolves using Fisher's (1930) reproductive value and found that the impact of hunters is far more important than that by wolves, a finding of broad significance.

Survival of Adult Female Elk in Yellowstone Following Wolf Restoration

Counts of northern Yellowstone elk (Cervus elaphus) in northwestern Wyoming and adjacent Montana, USA, have decreased at an average rate of 6–8% per year since wolves (Canis lupus) were reintroduced in 1995. Population growth rates of elk are typically sensitive to variations in adult female survival; populations that are stable or increasing exhibit high adult female survival. We used survival records for 85 radiocollared adult female elk 1–19 years old to estimate annual survival from March 2000 to February 2004. Weighted average annual survival rates were approximately 0.83 (95% CI = 0.77–0.89) for females 1–15 years old and 0.80 (95% CI = 0.73–0.86) for all females. Our estimates were much lower than the rate of 0.99 observed during 1969–1975 when fewer elk were harvested by hunters, wolves were not present, and other predators were less numerous. Of 33 documented deaths included in our analysis, we attributed 11 to hunter harvest, 14 to predation (10 wolf, 2 unknown, 1 cougar [Puma concolor], and 1 bear [Ursus sp.]), 6 to unknown causes, and 2 to winter-kill. Most deaths occurred from December through March. Estimates of cause-specific annual mortality rates were 0.09 (0.05–0.14) for all predators, 0.08 (0.04–0.13) for hunting, and 0.07 (0.03–0.11) for wolves specifically. Wolf-killed elk were typically older (median = 12 yr) than hunter-killed elk (median = 9 yr, P = 0.03). However, elk that winter outside the park where they were exposed to hunting were also younger (median = 7 yr) than elk that we did not observe outside the park (median = 9 yr, P < 0.01). Consequently, differences in ages of elk killed by wolves and hunters may reflect characteristics of elk exposed to various causes of mortality, as well as differences in susceptibility. Unless survival rates of adult females increase, elk numbers are likely to continue declining. Hunter harvest is the only cause of mortality that is amenable to management at the present time.

Selection of Northern Yellowstone Elk by Gray Wolves and Hunters

We compared selection of northern Yellowstone elk (Cervus elaphus) by hunters in the Gardiner Late Hunt and northern Yellowstone wolves (Canis lupus) with regard to sex, age, and impacts to recruitment. We compared harvest data from 1996–2001 with wolf-killed elk data from 1995–2001. We assessed the effects of hunting and wolf predation on reproductive female elk by constructing a life table and calculating reproductive values for females in the northern Yellowstone herd. We devised an index of total reproductive impact to measure impacts to calf production due to hunting and wolf predation. The age classes of female elk selected by wolves and hunters were significantly different. Hunters selected a large proportion of female elk with the greatest reproductive values, whereas wolves selected a large proportion of elk calves and older females with low reproductive values. The mean age of adult females killed by hunters throughout the study period was 6.5 years, whereas the mean age of adult females killed by wolves was 13.9 years. Hunting exerted a greater total reproductive impact on the herd than wolf predation. The combined effects of hunters killing prime-aged females (2–9 yr old), wolves killing calves, and predation by other predators has the potential to limit the elk population in the future. Yellowstone is unique in this regard because multiple predators that occur sympatrically, including hunters, wolves, grizzly bears (Ursus arctos), black bears (Ursus americanus), cougars (Felis concolor), and coyotes (Canis latrans), all prey on elk. Using an Adaptive Harvest Management process the known female elk harvest during the Gardiner Late Hunt has been reduced by 72% from 2,221 elk in 1997 to 620 elk in 2004. In the future, hunting harvest levels may be reduced further to partially offset elk losses to wolves, other predators, and environmental factors.

Modelling adaptive, spatially aware, and mobile agents: Elk migration in Yellowstone

The potential utility of agent‐based models of adaptive, spatially aware, and mobile entities in geographic and ecological research is considerable. Developing this potential, however, presents significant challenges to geographic information science. Modelling the spatio‐temporal behaviour of individuals requires new representational forms that capture how organisms store and use spatial information. New procedures must be developed that simulate how individuals produce bounded knowledge of geographical space through experiential learning, adapt this knowledge to continually changing environments, and apply it to spatial decision‐making processes. In this paper, we present a framework for the representation of adaptive, spatially aware, and mobile agents. To provide context to this research, a multiagent model is constructed to simulate the migratory behaviour of elk (Cervus elaphus) on Yellowstone's northern range. In this simulated environment, intelligent agents learn in ways that enable them to mimic real‐world behaviours and adapt to changing landscapes.

Linking wolves to willows via risk-sensitive foraging by ungulates in the northern Yellowstone ecosystem

After an absence of approximately 70 years, gray wolves ( Canis lupus) were re-introduced into Yellowstone National Park in the mid-1990s. We studied the potential influence of wolf/ungulate interactions upon willow ( Salix spp.) growth in the valleys of the Gallatin and Lamar Rivers, as well as Slough and Soda Butte Creeks, in the northern Yellowstone ecosystem. When we compared willow heights from photographs taken prior to 1998 (willows <2 m tall) with those taken in 2004, we found an increase in willow height for 22 of 42 sites within the study area. Based on comparisons of the chronosequence photos, since wolf introduction none of the 16 upland riparian sites showed an increase in willow height, while 22 of 26 of the valley-bottom riparian sites had willow height increases. In 2004, willow height exhibited a strong inverse relationship with the percentage of browsed stems ( r = −0.81, p < 0.01, n = 42). Results of regressions for valley-bottom sites indicated that view distance, impediment distance, and the number of bison ( Bison bison) flops were inversely related to willow height ( p ≤ 0.02). Increased willow heights were not significantly ( p = 0.18) related to patterns of moisture availability. Willow height increases documented in this study appear to have been at least partially due to behaviorally mediated trophic cascades involving wolves and ungulates, via a mechanism of predation risk. While willow release (i.e., increased height growth) within the study area is in a very early stage, results suggest potentially important indirect effects of a top carnivore in a terrestrial food chain that may aid in the restoration of riparian species and the preservation of biodiversity.

Molecular evidence for historical and recent population size reductions of tiger salamanders (Ambystoma tigrinum) in Yellowstone National Park

Population declines caused by natural and anthropogenic factors can quickly erode genetic diversity in natural populations. In this study, we examined genetic variation within 10 tiger salamander populations across northern Yellowstone National Park in Wyoming and Montana, USA using eight microsatellite loci. We tested for the genetic signature of population decline using heterozygosity excess, shifts in allele frequencies, and low ratios of allelic number to allelic size range (M-ratios). We found different results among the three tests. All 10 populations had low M-ratios, five had shifts in allele frequencies and only two had significant heterozygosity excesses. These results support theoretical expectations of different temporal signatures among bottleneck tests and suggest that both historical fish stocking, recent, sustained drought, and possibly an emerging amphibian disease have contributed to declines in effective population size.

Adaptive management for reintroductions: Updating a wolf recovery model for Yellowstone National Park

Ecological models have their greatest potential for conservation in the context of adaptive management, but there are few examples where models have been updated as new data have become available. We update a predator–prey model built to anticipate the consequences of wolf ( Canis lupus) reintroduction into Yellowstone National Park, with new data accumulated since wolves were released in 1995. Observed response to wolf recovery allows us to evaluate our ability to predict system dynamics and thereby address concerns about future impacts of predation on elk ( Cervus canadensis) numbers and harvest by hunters. Structural assumptions of the model include a dynamic carrying capacity for elk, K elk( t), varying as a function of winter severity and summer forage production. During severe winters the aerial extent of Yellowstone's Northern Range gets smaller resulting in density-dependent migration of elk outside the park where they are subject to hunter harvest. The updated model predicts that hunter harvest of elk will cause a decline of mean herd size relative to that expected without harvest. Wolf predation results in a further 21% reduction in elk herd size with wolf consumption averaging approximately 1035 elk annually. Elk harvest is reduced by wolves yet annual hunter harvest was sustained at an average of 1089 elk. Predation and hunter harvest is density-dependent providing a stabilizing influence that reduces the risk of severe elk population decline. Using simulation models in adaptive management of the wolf–ungulate system in Yellowstone reinforces agency management policies, especially the density-dependent harvest quota for elk.

Influence of harvest, climate and wolf predation on Yellowstone elk, 1961‐2004

In the period following wolf (Canis lupus) reintroduction to Yellowstone National Park (1995–2004), the northern Yellowstone elk (Cervus elaphus) herd declined from ∼17 000 to ∼8000 elk (8.1% yr−1). The extent to which wolf predation contributed to this decline is not obvious because the influence of other factors (human harvest and lower than average annual rainfall) on elk dynamics has not been quantified. To assess the contribution of wolf predation to this elk decline, we built and assessed models based on elk‐related data prior to wolf reintroduction (1961 to 1995). We then used the best of these models to predict how elk dynamics might have been realized after wolf reintroduction (1995 to 2004) had wolves never been reintroduced. The best performing model predicted 64% of the variance in growth rate and included elk abundance, harvest rate, annual snowfall, and annual precipitation as predictor variables. The best performing models also suggest that harvest may be super‐additive. That is, for every one percent increase in harvest rate, elk population growth rate declines by more than one percent. Harvest rate also accounted for ∼47% of the observed variation in elk growth rate. According to the best‐performing model, which accounts for harvest rate and climate, the elk population would have been expected to decline by 7.9% per year, on average, between 1995 and 2004. Within the limits of uncertainty, which are not trivial, climate and harvest rate are justified explanations for most of the observed elk decline. To the extent that this is true, we suggest that between 1995 and 2004 wolf predation was primarily compensatory.

The politics of barstool biology: Environmental knowledge and power in greater Northern Yellowstone

Critical researchers of underdevelopment have established a well-known record celebrating the environmental knowledges of subsistence communities in contested wildlife conservation zones. Similar battles are being fought over science, uncertainty, and wild animals in the American west, however, with far less attention to local epistemologies. Often dismissed as “barstool biology”, the ecological knowledges of local hunters in the Northern Yellowstone ecosystem are rooted in environmental experience and situated politics. How does local hunter knowledge diverge or converge with that of state officials, environmentalists, ranchers, and other constituencies, and to what effect on wildlife management policy? This paper seeks to answer that question, reviewing recent research amongst local resource users, managers, and activists in Montana. By rendering empirical the question of local knowledge around America’s oldest national park, rather than trying to “read it off” political affiliation, education, or livelihood, a clearer picture of power, knowledge, and conservation emerges. The results suggest that emerging management policies have developed from the discursive alliance of landowners, outfitters, and environmentalists, shifting priorities towards enclosure and exclusion in wildlife at the expense of other silent constituencies.

YELLOWSTONE BISON FETAL DEVELOPMENT AND PHENOLOGY OF PARTURITION

Abstract Knowledge of Yellowstone bison (Bison bison) parturition patterns allows managers to refine risk assessments and manage to reduce the potential for transmission of brucellosis between bison and cattle. We used historical (1941) and contemporary (1989–2002) weights and morphometric measurements of Yellowstone bison fetuses to describe fetal growth and to predict timing and synchrony of parturition. Our method was supported by agreement between our predicted parturition pattern and observed birth dates for bison that were taken in to captivity while pregnant. The distribution of parturition dates in Yellowstone bison is generally right-skewed with a majority of births in April and May and few births in the following months. Predicted timing of parturition was consistently earlier for bison of Yellowstone's northern herd than central herd. The predicted median parturition date for northern herd bison in the historical period was 3 to 12 days earlier than for 2 years in the contemporary period, respe...

Northern Yellowstone elk after wolf restoration

We analyzed counts, vital rates, and limiting factors for northern Yellowstone elk (Cervus elaphus) before and after wolf (Canis lupus) restoration in 1995–1996 to evaluate predictions that elk numbers would move to a lower equilibrium point with corresponding density-related changes in vital rates. Elk counts decreased from approximately 17,000 in 1995 to 8,335 in 2004. Pregnancy rates for prime-age females (3–15 years) during 2000–2003 were high (0.90) and similar to those during 1950–1967 when elk density was 30% lower (5–9 elk/km2). The survival rate for prime-aged females was 0.85 (95% Cl = 0.81–0.87) compared to 0.99 when harvests were low and wolves absent. The proportions of elk harvested each year increased as elk numbers decreased during 1990–2002 but departed from this anti-regulatory trend as permit levels were reduced in 2003–2004. Snow pack strongly influenced elk vulnerability to hunting by increasing migration to lower elevations. Thus, harvests removed a relatively constant proportion (27±5%) of animals that migrated out of the park each year, primarily prime-aged females with high reproductive value. Conservative estimates of wolf off-take (>1,000 elk) exceeded harvests by 2003, with wolves primarily selecting calves and older elk with lower reproductive value. Recruitment decreased as the ratio of wolves to elk increased, and wolves maintained high kill rates and rapid population growth despite a 50% decrease in elk counts. Elk numbers likely will continue to decrease until 1) levels of harvest and predation decrease sufficiently, 2) there is sufficient time for recruitment of calves to prime breeding age, and 3) there is a numerical response of wolves to fewer elk. We recommend that managers quickly adjust antlerless permit quotas to population size in a density-dependent manner so that harvests do not accelerate the decrease in elk numbers.

Incisor wear and age in Yellowstone bison

Abstract Biologists commonly use tooth eruption and wear patterns or cementum annuli techniques to estimate age of ungulates. However, in some situations the accuracy or sampling procedures of either approach are undesirable. We investigated the progression of several quantitative measures of wear with age, using permanent first incisors from Yellowstone bison (Bison bison), and tested for differences between sexes and herds. We further investigated the relationship of wear and age to explore an age-estimation method. Labial-lingual width (LLW) correlated best with assigned age (r2=0.66, males; r2=0.76 females). Labial-lingual width differed between sexes, with females showing ∼0.2 mm more wear than males. Additionally, differences in rate of wear existed between bison of the northern and central Yellowstone herds (1.2 and 0.9 mm/year, respectively). We developed a regression formula to test the power of LLW as an estimator of Yellowstone bison age. Our method provided estimated ages within 1 year of the ...

REDUCED COTTONWOOD RECRUITMENT FOLLOWING EXTIRPATION OF WOLVES IN YELLOWSTONE'S NORTHERN RANGE

Cottonwoods (Populus spp.) represent an important tree component of riparian plant communities within the winter range of the northern Yellowstone elk (Cervus elaphus) herd, yet young cottonwoods are highly palatable to ungulates. To assess potential herbivory impacts associated with wild ungulates following the removal of gray wolves (Canis lupus), long-term cottonwood recruitment and stand dynamics were studied. While results indicated that the establishment of young cottonwoods is an ongoing process in Yellowstone's northern range, their relatively small stature and hedged appearance were indicative of high browsing pressure by wild ungulates, principally elk. At five study sites, the diameter at breast height (dbh) of all cottonwood trees ≥5 cm was determined; increment cores were also obtained for developing tree age vs. dbh relationships. At the La Duke Spring and Devils Slide Sites, which represent refugia sites since elk have limited access (due to local terrain conditions and cultural developments), cottonwood recruitment has been occurring during the last half century, indicating that climatic factors have not prevented cottonwood recruitment during that time. However, over this same period cottonwood recruitment no longer occurred at sites within the Soda Butte and Lamar Valleys. The overall decline in cottonwood recruitment at these locations occurred following the extirpation of wolves from Yellowstone National Park in the 1920s. At the Buffalo Ranch Site, where ranching of buffalo [bison] (Bison bison) and activities associated with culling of elk were centered, cottonwood recruitment continued after the loss of wolves but stopped following the curtailment of ungulate management activities at these facilities in the late 1960s. Taken together, data from the five sites reported herein along with the results of other studies provide compelling evidence that the absence of wolves in the northern Yellowstone has had a major impact on the recruitment of cottonwood and other woody browse species.