Trends in the Distribution and Abundance of Bison in Wood Buffalo National Park

Ludwig N. Carbyn, Nicholas J. Lunn, and Kevin Timoney
Address for Ludwig N. Carbyn: Canadian Wildlife Service, 5320-122 Street, Edmonton, AB T6H 355, Canada.
Address for Nicholas J. Lunn: Canadian Wildlife Service, 5320-122 Street, Edmonton, AB T6H 355, Canada.
Address for Kevin Timoney: Treeline Ecological Research, 21551 Twp Rd. 520, Sherwood Park, AB TBE 1E3, Canada.
Reprinted from Wildlife Society Bulletin, volume 26, number 3, Fall 1998


We summarized changes in the distribution and abundance of bison (Bison bison) in Wood Buffalo National Park from 1971 to 1998. Based on annual aerial counts, a significant decline in bison abundance has occurred from approximately 11,000 animals in 1971 to 2,300 animals in 1998. The decline occurred south of the Peace River, which includes the Peace-Athabasca Delta. Cow and calf counts south of the Peace River from 1989 to 1996 indicated low calf production and very low yearling survival. It is not clear what factors are responsible for the decline. We suggest that long-term, multi-disciplinary research is required to further understand the changing dynamics within the Wood Buffalo National Park ecosystem.

Wood Buffalo National Park (WBNP) is situated in the northeast corner of Alberta, Canada, straddling the border with the Northwest Territories, and is North America's largest national park (44,800 km2). Wood Buffalo National Park was established in 1922 (the original area was 26,800 km2) to protect declining herds of bison (Bison bison). Potential and realized human impacts on the park's bison population include: protection of bison through legislation; introduction of diseased bison; construction of a hyroelectric dam on the Peace River and 1,100 km upstream of the Peace-Athabasca Delta; fire suppression; agricultural, pulp, and paper industrial expansion along the Peace and Athabasca Rivers (resulting in influxes of weed propagules and pollutants into the delta); construction of fixed-height weirs to stabilize summer water levels; disruption of deltaic evolution through prevention of a major evulsion of the Athabasca River in 1972; poisoning, hunting, and trapping wolves (Canis lupus); haying to supplement bison feed; herding; vaccination against anthrax; and slaughter for meat production (Peace-Athabasca Delta Implementation Committee [PADIC]. Peace-Athabasca Delta water management works evaluation, unpubl. rep., PADIC, Alta., Can., 1987; Wein et al. 1992; Carbyn et al. 1993; Northern River Basins Study, Annual unpubl. rep., Northern River Basins Study, Edmonton, Alta., Can., 1996; K. Timoney. Vegetation monitoring program, Task E-2-vegetation monitoring, unpubl. rep., Peace-Athabasca Delta Techn. Stud., Alta., Can., 1996; Timoney et al. 1997).

The bison population has undergone wide fluctuations in size within historical times. Widespread human hunting, without protection in the 19th century, caused populations to decline to critically low levels. A population of about 300 bison existed around the turn of the century (Graham 1923), and, after legal protection against hunting had been granted in 1893, the population increased to about 1,500 animals when WBNP was established. It appeared that conditions were favorable for natural increases in bison to occur.

From 1925 to 1928, 6,673 plains bison were introduced into WBNP; this resulted in significant long-term consequences for the existing bison herds. Many of the introduced bison were infected with brucellosis and tuberculosis. Despite harboring these bovine diseases, the introduced and resident population continued to increase. By 1934 the population in WBNP likely numbered between 10,000 and 12,000 animals (Fuller 1950). By the late 1940s and early 1950s, bison may have reached 12,500-15,000 animals (Fuller 1950, 1966). Various factors (e.g., slaughters, cessation of wolf poisoning, roundups for disease control, floods, diseases, predation, and habitat changes) may have contributed to or precipitated major declines in the number of bison in WBNP. Through a long-term monitoring program of aerial counts, Parks Canada documented that the numbers of bison decreased from 12,000 to 15,000 in the early 1950s, to 11,000 by 1971, and to approximately 2,300 by 1998 (Parks Can., unpubl. data).

Declines of such a high-profile resource, particularly in the presence of contagious bovine diseases, caused a major political debate on the future of the bison in the park (Gates et al. 1997). Current efforts to find solutions to deal with the issue of bovine disease and bison management began in 1989, when the federal government set up a panel under the Federal Environmental Assessment and Review Office (FEARO; Chisholm et al. 1998). In August 1990 the review panel recommended replacing the existing herds with disease-free animals from "pure" wood bison held in Elk Island National Park (central Alta.) and possibly elsewhere. The public response was quick and largely negative, and, thus, a second review process was put in place. The Northern Buffalo Management Board was formed in June 1991 and presented its report to the Minister of the Environment in March 1993. The Board recommended that studies be required before final decisions could be made.

In Apri11995 the Minister of Canadian Heritage announced a 5-year Bison Research and Containment Program (BRCP). After preliminary meetings, the Research Advisory Committee (RAC), which advises Parks Canada with respect to the planning, implementation, and evaluation of the research component of the Program, sent out requests for research proposals in July 1996. The objectives of the study selected were to assess the prevalence of brucellosis and tuberculosis in, and their impact on, the bison population of WBNP.

The long-term monitoring of bison was important because it provided background information for other studies (Carbyn et al. 1993, WBNP 1995). At the same time that the bison monitoring was being conducted, hydrological, topographic, vegetation, and other studies were being conducted under the Peace-Athabasca Delta Technical Studies. In addition, observations on calf production and survival to the yearling stage were conducted from 1989 to 1996 in a portion of the Peace-Athabasca Delta. Records were also kept on wolf and bison interactions and predation. We summarize the trend in the distribution and abundance of bison within WBNP from 1971 to 1998, document observations of cow-calf ratios, yearling survival, and wolf predation in 1 area of the park, and present the case for integrated research.

Study area

Wood Buffalo National Park encompasses a range of habitats and natural features that include boreal forests, extensive willow shrub carrs, gypsum-karst topography, remnant grasslands, riparian white spruce (Picea glauca) and balsam poplar (Populus balsamifera) forests, and 1 of the largest freshwater deltas in the world (Dirschl et al. 1974; Airphoto Analysis Assoc. Integrated Resource Survey, unpubl. rep., WBNP, Alta., Can., 1979; Carbyn et al. 1993; Timoney and Robinson 1996; Schwarz and Wein 1997).

We divided WBNP into areas north of the Peace River (Area I) and south of the Peace River (Area II) to analyze aerial counts. Five main bison herds occur within WBNP: 4 predominately within Area I and 1 within Area II. Sufficient interchange occurs, and the herds are not considered isolated subpopulations. However, considerable stability exists in years of no flooding within the delta (WBNP 1995). Approximately 80% of the bison in WBNP are associated with primary ranges that cover about 9,000 km2 (Area I includes about 4,000 km2, and Area II includes about 5,000 km2), leaving approximately 35,800 km2 as secondary ranges (Carbyn et al. 1993).

Area I is covered with boreal forest of white spruce (Picea glauca), black spruce (P. mariana), aspen (Populus tremuloides), balsam poplar, larch (Larix laricina), and paper birch (Betula papyrifera), interspersed with various wetlands (some of which are saline or sulphate-rich) and dominated by shrubs, bluejoint reed grass (Calamagrostis canadensis), and awned sedge (Carex atherodes). Areas suitable for bison in Area 1 are primarily associated with the Slave River and with meadows adjacent to several creeks. Area II contains areas of boreal forest and the Peace-Athabasca Delta, the latter of which is the most important primary winter and summer range for bison in WBNP (Carbyn et al. 1993). Currently, dominant nonaquatic cover types in the delta are willow savannah, consisting primarily of flat-leaved willow clones (Salix planifolia) with Bebb's willow (S.bebbiana), pussy willow (S. discolor), sandbar willow (S. exigua), and other species in a meadow matrix of willow carrs, bluejoint reed grass meadows, awned sedge wet meadows, and spangletop (Scolochloa festucacea) marshes (K. Timoney. Vegetation monitoring program, Task E-2-vegetation monitoring, unpubl. rep., PADIC, Alta., Can., 1996).

The Peace-Athabasca Delta is the world's largest boreal fresh water delta and occupies an area of about 3,200 km2 (K. Timoney. Vegetation monitoring program, Task E-2-vegetation monitoring, unpubl. rep., Alta., Can., 1996). The Peace-Athabasca Delta is a dynamic ecosystem, and it is shaped by ice-jam and open-water flooding, wetting-drawdown cycles, deposition of sediments and elevation of surfaces, channel wandering, headwater capture, seiches, and flow reversals of streams. Although the delta represents only a small portion of WBNP, it has contained the highest concentration of bison in recent years. In years of substantial flooding (e.g., 1974, 1996, and 1997), the numbers of bison found in the delta diminishes as bison move to higher ground. The recent floods will continue to influence the north-south dynamics of bison in the park. How the bison will respond to those floods and what impacts the floods will have on their range requires further study.


Annual aerial surveys of bison, initiated in the early 1970s, were restricted, randomized counting procedures whereby each primary winter-range unit was systematically surveyed, and all bison were counted; secondary ranges were only partially and opportunistically surveyed (Stelfox and Kingsley 1975, Teillpany and Cooper 1975).

Surveys were flown in a Cessna 210 fixed-wing aircraft at an altitude of about 600 m and at a cruising speed of 180 km/hour. They were flown in late February and March, when snow and vegetation conditions afforded optimum sightability and herds concentrated on winter ranges. Survey transects were set 2.5 km apart and plotted on topographical maps

(1:250,000) prior to each flight. Surveys over secondary ranges were not pre-plotted but followed meadows and other suitable bison habitat. The survey crew consisted of a pilot, a navigator, and 2 observers.

Because of the potential errors inherent in total-count aerial surveys, Stelfox and Kingsley (1975) evaluated the methodology and statistical reliability. The surveys were flown 4 times during each winter of 1972-1975, and the means, standard errors, and coefficients of variation were subsequently calculated. Based on these data, Stelfox

and Kingsley (1975) concluded that this type of survey was a statistically reliable method for estimating the abundance of bison in the park. In subsequent years, each survey involved a single coverage of all the primary areas and portions of the secondary areas.

To obtain segregation counts, observations of wolf predation techniques, and anti-predator maneuvers by bison, herds were followed for 301 days between 1989 and 1996 by >2 observers travelling between the mouth of Lousy Creek and Sweetgrass Creek in Area II. Observers routinely approached bison at close range ( -15 m) by gradually moving closer to the herds or by watching herds move and placing themselves within the herd's travel path. Both classified counts (i.e., segregation counts) and observations of the physical condition of bison were possible when animals were within close range.

We observed wolf and bison interactions by following bison herds. Particular attention was paid to number of wolf packs and wolf population dynamics (e.g., No. of dens, No. of young produced, territoriality, movements, and hunting strategies) within Area II.



Fig. 1.  Estimates of the abundance of bison in Wood Buffalo National Park, north (Area 1) and south (Area II) of the Peace River, 1971 - 1998.

The distribution and abundance of bison changed from 1971 to 1998 (Fig. 1). Since 1971, there has been a significant decline in the number of bison counted in WBNP (Pearson product moment r = -0.895, 25 df, P < 0.001). Although a significant decline occurred in Area II (r = -0.897, 25 df, p 0.5). From 1971 to 1981, the mean number of bison counted in Area II was 5,748 ± 744 compared to 1,320 ± 178 from 1982 to 1992 (t-test, t = 5.507,17 df, p 0.375) in the mean number of bison counted in Area I (1971-1981, x = 1,357 ± 122; 1982-1992, x = 1,487 ± 75).


Table 1.

Estimates of the number of bison in Wood Buffalo National Park, north (Area I) and south (Area II) of the Peace River, 1971-1998 (surveys were not flown in 1982, 1986, or 1993),


Area I

Area II


Proportion in Area I













































































































































As a consequence of the decline of the number of bison in Area II, the proportion of the number of bison counted within WBNP that were found in Area I has increased since 1971 (Table 1). Only 12-19% of all the bison counted from 1971 to 1974 were in Area I, compared with 84% in 1998 (Table 1). An apparent anomaly in the counts occurred in 1976, when there was a documented southward movement of approximately 1,000 bison in January, followed by a northward movement of large herds of bison in April (parks Can., unpubl. data). Thus, during the March survey period, fewer animals would have been counted in Area I and a greater number counted in Area II than would have been typical in other years.


Table 2.

Changes in calf-cow ratios, yearling-cow ratios, and recorded wolf kills (numbers in parentheses refer to scavenged carcasses) in the Delta study area (Area II), 1989-1996, based on ground observations.


No. Bison

Calves/100 cows

Yearlings/100 cows

Recorded wolf kills






























































Data on calf-cow and yearling-cow ratios, with recorded kills by wolves, were collected from 1989 through 1996 (Table 2). For years in which calf-cow ratios were available for the spring and fall (1989-1996, n = 6), the mean number of calves per 100 cows was not significantly different (t = 1.148, 10 df, p > 0.275) between spring (x = 32.2 ± 2.9) and fall (x = 25.5 ± 5.0), which suggests that there was no significant calf mortality between spring and fall. Inspection of the data indicated that calf losses may have been higher from 1989 to 1991 than from 1994 to 1996. Partitioning the data into these 2-year groups (i.e., 1989-1991 and 1994-1996), the difference in the mean number of calves per 100 cows between spring (x = 27.3 ± 2.9) and fall (x = 16.3 ± 2.9) approached significance in 1989-1991 (t = 2.677, 4 df, p 0.70) in the mean number of calves per 100 cows between spring (x = 37.0 ± 3.2) and fall (x = 34.7 ± 5.8) in 1994-1996. These results must be interpreted with caution because of the small sample sizes.

In 1996 there were more calves per 100 cows in the fall compared to in spring, perhaps because severe flooding resulted in shifts in the distribution of bison herds. The result may have been an increased number of calves counted in the fall from different portions of the range and likely removed from wolf denning areas, where the calves would have been more susceptible to predation.

There was a significant difference (t = 2.874, 8 df, p < 0.025) between the observed mean calves per 100 cows (x = 22.4 ± 4.9) and the observed mean yearlings per 100 cows (x = 7.6 ± 1.7) from years in which we had calf-cow ratios in the fall of year X and yearling-cow ratios in the spring of year X + 1 (n = 5).

We recorded wolf predation in 25 calves and 2 adult cows during 301 days of field work between 1989 and 1996 (Table 2). In addition, wolves scavenged 6 adult bull carcasses. There were no records of summer wolf predation on yearlings.

Discussion and management implications

A decline has occurred in the number of bison counted annually in WBNP. Initially, it was thought that diseases were directly responsible for the overall decline (Environ. Assessment Panel 1990). However, it is unlikely that disease could be the sole cause of the decline. Although Areas I and II contain herds with infected bison, the numbers have declined only in Area II, which also includes the Peace-Athabasca Delta. Peterson (1991) also questioned whether there was sufficient evidence to implicate disease in the decline to warrant management action without first conducting additional research.

Other alternative hypotheses to explain the decline have been proposed, including differential vulnerability to wolf predation, coupled with reduced reproductive potential caused by disease (Carbyn et al. 1993). But, again, disease occurs in both areas: food limitation as a function of scaling population size to body size (Nudds 1993), and a "disease-predation " hypothesis proposed by Gates (1993). The current lack of identification of the most probable cause(s) of the decline, combined with the fact that diseases pose a potential risk to cattle and bison, has caused major socio-ecological-economic debates (Gates et al. 1997). One of the outcomes of the debates and public hearings has been the identification of the need for effective and coordinated research efforts to adequately address the study of bison population dynamics, bovine diseases, habitat changes, and wolf predation.

Since 1971 a variety of research initiatives have been undertaken to examine the hydrology of the Peace-Athabasca Delta; changes in vegetation; responses to variable water regimes; movement and migration of bison within the park, with particular emphasis on proximity of cattle herds (problems of disease transmission); and the incidence of disease within bison herds (Dirschl et al. 1974, Cordes 1975, Carbyn et al.

1993, WBNP 1995, Timoney et al. 1997). Despite the research value of such studies, they were never integrated into a multi-disciplinary , broad-fronted approach, wherein a greater understanding of links between various changes in the ecosystem might have been related to the numerical status of bison herds, reproduction, survival, and movements of bison. For example, studies on bison movements were not linked to food availability (vegetation studies), snow conditions, hydrology, or vegetation within the Peace-Athabasca delta.

Furthermore, why bison in WBNP appear to have lower reproduction than bison elsewhere is an issue that needs to be addressed (Reynolds et al. 1982, Larter et al. 1993, Aune et al. 1998). Fuller (1966) estimated a conception rate of 0.67 for cows in WBNP using data collected from animals killed at the annual slaughters. The spring calf-cow ratios reported here (x = 30.3 ± 2.9 calves per 100 cows, n = 8), although not directly comparable to pregnancy rates, indicate low reproduction in WBNP. Disease, an aging population (due to low recruitment), wolf predation, and variable habitat conditions may be causative factors. Some factors are in place that, currently and in the recent past, have resulted in lower calf production than what may be possible under optimal conditions. For example, under conditions of adequate forage, the calving rates of free-grazing, captive bison >2 years of age has been >95%, and 100% is not uncommon (B. D. Rutley, Cent. for Agric. Diversification, pers. commun.).

The management history of bison in WBNP is complex. It is not possible at present to account for the 20th century population fluctuations. Changes to the delta's bison population may be within the range of natural variation of an ungulate population within a large, dynamic ecosystem. Alternatively, the mid-century bison decline centered on the Peace-Athabasca Delta may simply be an inevitable adjustment to an unsustainably high number of bison related to human introduction of thousands of bison, predator control, haying, and vaccination. Furthermore, significant changes in the frequency of floods caused by ice-jams have occurred in the delta in the past 170 years, and these changes have been correlated with synoptic-level climatic variation driven by centennial-scale variations in solar activity (Timoney etal.1997). The period 1915-1950 was a time of frequent floods in the delta, coincident with the major population rise in the delta's bison population.

Parts of the bison range (e.g., Linkstand and Lousy Creek and Sweetgrass areas) show signs of degraded condition (e.g., high cover of non-native thistle [Cirsium arvense], silverweed [Potentilla anserina], and foxtail barley [Hordeum jubatum], and compacted soils; K. Timoney. Vegetation monitoring program, Task E-2-vegetation. monitoring, unpubl. rep., Peace-Athabasca Delta Technical Studies, Alta., Can., 1996). The degraded range may be a relict of former bison populations (recent past) rather than a consequence of current use. Weed cover has declined in these areas in the wake of recent major floods (K. Titloney, pers. obs.). Areas where bison concentrate in he delta are often those with saline soils as evidenced by salt crusts and halophytes (e.g., alkali grasses, [Pucnellia spp.]; K. Timoney, pers. obs.). Elsewhere, much of the delta supports meadows and savannahs in good to prime condition, yet they appear to be under-used or unused (e.g., south of Mamawi Lake, and southeast of Lake Claire). In the Mackenzie Bison Sanctuary, awned sedge meadows are the preferred winter range of wood bison, and willow savannahs are preferred in summer (Larter 1988). Many thousands of hectares of these habitats exist in the delta; thus, food limitation may not be a factor .

Wildlife agencies and conservation organizations have embarked on programs to re-establish free-ranging herds of bison in northern Canada. Although the Mackenzie Sanctuary population is closest to the diseased herds in WBNP , it is not clear whether enough scientific information exists to answer fundamental questions regarding the ecology and status of the WBNP herds and, thus, warrant these animals being declared as "surplus" to current conservation needs (Geist 1991).

Due to the highly politicized nature of the debates on the bison of WBNP, the recommendations for the need for research did not result in quick action. After 18 months of deliberations, the Northern Buffalo Management Board recommended that, because significant gaps still exist in our overall understanding of the epidemiology and ecological role of brucellosis and tuberculosis, predation, forage availability in relation to flooding, and other fundamental aspects of the WBNP ecosystem, current projects to study the effects of diseases on bison need to be integrated with studies of the other components of the ecosystem.

What is the range of natural variation of the delta ecosystem and its bison population? What are the mean and temporal variability of the delta's carrying capacity and primary productivity? How do flooding, drying, fire, climatic change, and succession relate to the bison population? Why is so much of the delta seemingly unused by bison? The relationships between mineral-rich soils, bison nutrition, and delta bison distribution require study. These and other questions remain largely unanswered.

Key information gaps exist. However, the value of any studies of delta bison will likely be limited unless research efforts are integrated. Fresh insights are required to gain a better understanding of the dynamics of bison ecology in this ecosystem. These are most likely to be achieved when the methodologies applied are diverse and multi-disciplinary, including traditional and naturalistic observations, radiotracking, habitat analysis, nutritional studies, disease presence and transmission, winter ecology , predation and studies of long-term variation in the delta ecosystem. In-depth analyses of existing data are also needed. Too many changes have taken place in the delta to permit facile explanations or to justify narrowly focused research.


Aerial bison surveys in Wood Buffalo National Park have been conducted by the warden staff since 1971; these data form much of the basis for this paper. Their dedicated involvement is gratefully acknowledged. J.Chisholm provided unpublished data from Park files. We thank the residents in Fort Chipewyan who, over the years, assisted in field transportation within the Peace-Athabasca Delta. S. Barry assisted in the statistical analysis of the data and V. Jespersen typed the manuscript. B. Rutley provided unpublished data on bison calf production in captive herds. C. Blyth and L. Comin reviewed an earlier draft.

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