Mamm Res (2015) 60:385–391 DOI 10.1007/s13364-015-0240-2
ORIGINAL PAPER
Winter diets of reintroduced bison (Bison bison) in northwestern Canada Thomas S. Jung 1
Received: 28 December 2014 / Accepted: 5 August 2015 / Published online: 18 August 2015 # Mammal Research Institute, Polish Academy of Sciences, Białowieża, Poland 2015
Abstract I examined the winter diets of a reintroduced population of bison (Bison bison) in a mountainous region of northwestern Canada. The impetus of this study was to examine the winter diets of bison in this ecological region and to test for sexual and seasonal variation in the winter diet. Microhistological analyses of fecal samples from animals of known sex- and age-classes were examined to determine winter diets. I evaluated the hypothesis that sexual segregation in bison may be due to differences in diet. In addition, diet composition between early and late winter was examined to test for seasonal differences. Overall, bison winter diets were diverse, including at least 31 genera of vascular plants, but dominated by sedges (Carex). Nine species of Carex were identified in the winter diet. Diets of adult males in early winter differed in percentages of 7 forage classes used from those in late winter. The proportion of shrubs increased in latewinter diets, while that of sedges decreased. No evidence was found to support hypotheses of sexual segregation in bison based on differences in diet, at least for this population during late winter. This study shows that bison diets in winter are more diverse than previously thought based on studies in lowland boreal environments, where dietary niche breadth was comparatively narrow. Dietary flexibility is important within the context of choosing new sites for reintroducing bison, or predicting colonization of vacant habitats as existing populations grow.
Communicated by: Dries Kuijper * Thomas S. Jung
[email protected];
[email protected] 1
Yukon Department of Environment, P.O. Box 2703, Whitehorse, Yukon Y1A 2C6, Canada
Keywords Bison . Bison bison . Diet . Microhistological analyses . Sexual segregation
Introduction American bison (Bison bison) are bulk feeders that can subsist on a low-quality diet composed of low protein and high fiber, due to high digestibility coefficients relative to other ungulates (Hawley et al. 1981). Their diet, however, can vary seasonally in response to changes in food availability (Larter and Gates 1991), and there can be differences in diet choices between the sexes within the same season (Post et al. 2001). Understanding variation in diet of bison is an important precursor to understanding seasonal habitat requirements (Larter and Gates 1991), sexual segregation (Post et al. 2001), or risksensitive foraging (Hernández and Laundré 2005) and may be useful in assessing the impact of population density on individual condition (Nicholson et al. 2006). From a conservation standpoint, understanding the dietary needs of bison across their historical range provides important information on habitat needs that can assist management programs aimed at restoring populations. Wood bison (B. b. athabascae) are a threatened subspecies of American bison that historically ranged in northwestern Canada and Alaska but are currently only found in 10 discrete, free-ranging populations. While some information is available on seasonal diets of wood bison from parts of their northern range, there is little information on their food habits from other ecological regions. For example, two studies reported on seasonal diets of wood bison in lowland boreal regions (Reynolds et al. 1978; Larter and Gates 1991). Both of these studies reported that wood bison diets were diverse, but dominated by graminoids, particularly sedges, rushes, and grasses, with a particular reliance on sedges in winter. Campbell and Hinkes
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(1983) reported that plains bison (B. b. bison) introduced to an area of interior Alaska where wet sedge meadows were also dominant had a diet similar to that of wood bison from other lowland boreal regions. Few data are available on diets of wood bison from more topographically diverse regions, which likely contain different types and proportions of vegetation communities and forage availability across the landscape. In 1988, wood bison were reintroduced to southwestern Yukon, Canada, as part of a national recovery program (Government of Yukon 2012; Jung et al. 2015). Their range is topographically diverse, with a complex mix of vegetation communities, but contains relatively few wet sedge meadows, a preferred foraging habitat for bison in lowland boreal regions (Reynolds et al. 1978; Campbell and Hinkes 1983; Larter and Gates 1991). Fischer and Gates (2005) examined the relative proportion of 7 forage classes in the winter diets of wood bison from this population. Their study was based on composite samples of fecal material collected at foraging sites during one winter. They reported winter diets similar to those found in lowland boreal regions, but with less reliance on sedges and rushes and more use of grasses. In this study, microhistological analyses of fecal samples were used to examine the variation in winter diet composition of wood bison in a mountainous region of northwestern Canada. The aims of this work were to extend the work of Fischer and Gates (2005) on the winter diets of wood bison reintroduced to northern montane regions and to test for differences between seasons and sexes. Differences between early-winter and late-winter diets in wood bison were investigated. Larter and Gates (1991) showed seasonal changes in wood bison diets, presumably in response to seasonal differences in forage availability or energy or nutrient requirements. I predicted that diets in early winter would differ from those in late winter, when snow depths were greater and may constrain foraging efforts of bison (Telfer and Kelsall 1984), particularly for low-growing plants such as grasses. Bison are sexually dimorphic, with adult females weighing about 60 % that of adult males, and the sexes are often socially and spatially segregated for most of the year. Reasons for sexual segregation in ungulates are not clear, but several hypotheses have been put forth (reviewed by Main et al. 1996; Rucksthul and Neuhaus 2000, 2002). One of the leading hypotheses for sexual segregation in ungulates is the forage selection hypothesis (Rucksthul and Neuhaus 2000; referred to as the sexual-dimorphism-body-size hypothesis by Main et al. 1996). Rucksthul and Neuhaus (2000) noted that this hypothesis Bpredicts that the sexes segregate because sexual differences in body size lead to different energy requirements and hence food selection.^ The notion is that, for sexually dimorphic ungulates, males and females spatially segregate and select different forage species. With respect to bison, Post et al. (2001) showed that male and female plains bison (B. b. bison) consumed different diets and pointed to this as a reason
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why the sexes are spatially segregated during most of the year. Wood bison in southwestern Yukon are also spatially segregated during winter (T.S. Jung, unpublished data). I tested for differences in winter diets of wood bison males and females to see if differences in diet could provide support for the forage selection hypothesis of sexual segregation. I predicted that different sexes would not use different diets in winter because they appeared to use the same habitats during winter.
Methods Study area This study was conducted in the Boreal Cordillera Ecozone, near Haines Junction (60.8° N, 137.5° W), Yukon, Canada. Much of the area was above treeline (approximately 950 m above sea level [ASL]), with several mountain peaks >1500 m ASL and extensive alpine plateaus. Alpine areas are bisected by several large lakes, including Aishihik Lake and Taye Lake, and deeply incised river valleys. Lowland areas are largely open canopied forest, dominated by either white spruce (Picea glauca) or trembling aspen (Populus tremuloides). Small areas of wet sedge meadows and wet shrub meadows occurred in valleys at low elevations. Remnant boreal grasslands occur as small patches on southfacing slopes, also at low elevations. Climate was cold and semi-arid, with average temperatures in December and March being −21 and −9 °C, respectively. Snow cover persists during October to May, with the greatest accumulation occurring in March. Maximum annual snow depth was 27–70 cm (Yukon Department of Environment, unpublished data). An estimated 500 wood bison occurred in the study area during the time of the study (Government of Yukon 2012). Diet composition Fecal samples were collected from individual bison of known sex and age classes in March 1998 (late winter; n=7 adult females and eight adult males) and December 1998 (early winter; n=6 adult males). Fecal samples (10 %) for bison in boreal environments. Increased use of shrubs in late winter by male bison in southwestern Yukon may be due to limited availability of sedges and rushes later in the winter as snowfall accumulates and snow depths increase, making travel between sedge meadow patches more difficult. Alternatively, more use of shrubs may reflect shifts in habitats used in late-winter to sub-alpine areas or south-facing slopes where wind and increased solar isolation reduces snow depth and energetic costs associated with travel. In conclusion, while limited by small sample sizes and a single year of data, this study provides several key findings. First, there was no evidence that sexual segregation by wood bison in late winter is related to differences in diet; other mechanisms are likely responsible for this phenomenon. Rucksthul and Neuhaus (2000, 2002) and Bowyer (2004) provide various hypotheses for sexual segregation in ungulates that await formal testing and may provide the ecological rationale for why the sexes of bison are spatially segregated during winter. Second, the composition of bison diets change within winter, likely in response to forage availability. Finally, winter diets of bison in boreal environments may be more flexible than previously suggested by work in lowland
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habitats (Reynolds et al. 1978; Campbell and Hinkes 1983; Larter and Gates 1991). While wood bison do require wet sedge meadow habitats for forage, in this study, they made much less use of sedges and rushes than reported in lowland boreal regions. This is important because this data shows that in winter, when energetic demands are perhaps greatest, their diets are more diverse than previously thought based on studies in areas of extensive lowland habitats, where dietary niche breadth was comparatively narrow. Dietary flexibility is important within the context of choosing sites for reintroducing this threatened subspecies, or predicting colonization of vacant habitats as existing populations grow. Acknowledgments Microhistological analysis was done by Bruce Davitt at Washington State University. I thank Manfred Hoefs and Philip Merchant for the collection and management of fecal samples and associated data. Funding was provided by the Yukon Department of Environment. Two anonymous reviewers kindly provided comments on an earlier draft of the manuscript.
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