Original Research
Are Bison Intermediate Feeders? Unveiling Summer Diet Selection at the Northern Fringe of Historical Distribution

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Abstract

Bison (Bison bison) were historically distributed throughout North America with the northern edge of the distribution occurring in north-central Manitoba and surrounding provinces. Despite bison occupying the boreal zone of North America, little is known of their forage selection patterns of herbaceous plant material when occupying pastures within a densely forested aspen ecosystem. In 2015 we initiated a study to examine forage selection patterns for bison among and within summer months (June–August). We hypothesized that vegetative composition of bison diets would be consistent with availability, would shift with forage availability, and would predominately consist of grass and sedge species. We opportunistically collected adult female bison fecal samples (N = 99) and identified forage composition using the DNA barcoding method. We estimated availability of forage to the lowest taxonomical level possible using a modified Daubenmire frame. Overall, bison diets were composed of 44.3% grass, 37.7% forb, 16.3% browse, and < 2% sedge and rush. Forage availability comprised 51.2% grass, 28.3% forb, 11.0% sedge, and 7.6% rush. All analyses indicated that use and availability for grass, forb, sedge, and rush differed (P ≤ 0.05) throughout the summer. Grass and forbs were important dietary components for bison, comprising > 80% of bison diets. However, bison selected for these two dietary components independently as the summer progressed. Our results indicate that these bison consume a large portion (~ 54.0%) of low-cellulose, high cell-soluble forages to meet their dietary needs. This suggests that bison may be or become intermediate foragers and are more like elk (Cervus elapus) than domestic cattle or sheep when inhabiting forested systems at the northern edge of their historical distribution. Herd managers and biologists should be cognizant of the importance of eudicots for bison and adopt a management plan that promotes a spatially heterogenous vegetative schematic.

Introduction

Historically, bison (Bison bison) inhabited most of North America, ranging as far north as Alaska and the Northwest Territories, to Mexico in the south, and spanning coast to coast, from New Jersey to California (Truett, 1996, Lammers et al., 2013). Bison currently occupy most of these same regions in North America (for the most part, with herds ≤ 300 individuals restricted to small, fenced areas), but information on bison forage selection in the Boreal Plains Ecozone is lacking. On the basis of forage selection and ruminant physiology, Hofmann (1989) describes three overlapping morphophysiological ruminant feeding types: concentrate selectors; intermediate-opportunistic mixed feeders (hereafter, intermediate feeder); and grass-roughage feeders. Concentrate selectors, like moose (Alces alces) or white-tailed deer (Odocoileus virginianus), have evolved to digest nutritious, high-soluble plant material, such as forbs and browse (e.g., any part of a woody plant; Hofmann, 1989). Intermediate feeders, like elk (Cervus elapus), opportunistically forage between both extremes, consuming a mixed diet while displaying short-term or seasonal dietary shifts in response to forage quality (Hofmann, 1989). Plains bison (Bison bison bison; portrayed by domestic cattle in Hofmann’s, 1989 Fig. 2.) are typically classified as grass-roughage feeders, almost exclusively foraging on graminoids (Peden et al., 1974, Larter and Gates, 1991), such as grasses (Poaceae) and sedges (Cyperaceae). However, recent research in mixed-grass prairies of the Midwest contradicts this classification, with bison diets comprising high concentrations of eudicots, primarily forbs (Bergmann et al., 2015, Craine et al., 2015).

Diet selection of herbivores is typically determined through comparison of vegetative composition of use and forage cover-abundance (hereafter, availability; Larter and Gates, 1991). Selection of a food item can be assumed if use is greater than forage availability (Johnson, 1980). In contrast, avoidance of a food item can be assumed if use is less than availability (Klein, 1970, Johnson, 1980). Typically, herbivore diets are quantified using the microhistological analysis method, first described by Baumgartner and Martin (1939) and later verified in numerous studies (e.g., Denham, 1965, Sparks and Malechek, 1968). Microhistological analysis is popular and has been used to quantify ungulate diets worldwide (Jenks et al., 1996, Schuette et al., 1998, Gibbs et al., 2004, Beck and Peek, 2005, Forsyth and Davis, 2011). DNA barcoding is a relatively new technique currently in use for analysis of herbivore diets (Valentini et al., 2009) and is gaining popularity (Czernik et al., 2013, Bergmann et al., 2015, Craine et al., 2015, Kartzinel et al., 2015). The technique has proven more accurate for quantifying composition of complex plant mixtures; 75% of the plant DNA extracted from fecal samples was identifiable to the genus level versus 20% using microhistological analysis (Soininen et al., 2009).

Plains and wood bison (B. bison athabascae) diets have been analyzed across North America using microhistological analysis and observational forage bouts (Peden et al., 1974, Peden, 1976, Reynolds et al., 1978, Larter and Gates, 1991, Plumb and Dodd, 1993, Knapp et al., 1999). To our knowledge, only two studies have analyzed bison diets from feces using DNA barcoding (Bergmann et al., 2015; Craine et al., 2015); however, neither study analyzed forage availability to assess diet selection. Therefore, the objectives of our study were to determine forage selection patterns of herbaceous (nonwoody stem) plant material for bison in pastures among and within summer months (June–August) in central Manitoba. We hypothesized that vegetative composition of bison diets would be equal to availability and that bison diets would vary as the availability of forage shifted between pastures. Additionally, we hypothesized that bison would select for grass and sedge species as would be expected for a grass-roughage feeder (Hofmann, 1989).

Section snippets

Study Area

Our study was conducted June–August 2015 within the East, East-Center, South Sclater, and North Sclater pastures on the Pine River Ranch (lat 51o47′N, long 100o30′W), which is part of Olson’s Conservation Bison Ranches located within the Rural Municipality of Mountain (south), Manitoba, Canada. The privately operated ranch encompasses 12 500 ha of 9 cross-fenced pastures (range: 291  3 316 ha) and manages approximately 350 mature (≥ 3 yr old) female bison, which are rotationally grazed during

Results

Overall, bison diets were composed (mean ± SEM) of 44.3 ± 3.5% grass, 37.7 ± 2.6% forb, 16.3 ± 2.3% browse, 1.1 ± 2.4% sedge, and 0.6 ± 1.3% rush, while forage availability was 51.2 ± 1.9% grass, 28.3 ± 1.5% forb, 11.0 ± 1.3% sedge, and 7.6 ± 0.7% rush (Table 1). Bison use and availability differed (P ≤ 0.05) for each taxonomical group between months throughout the summer. Grass and forbs comprised > 80% of bison diets (see Table 1). However, bison selected grass during June (0.19; t24 = 2.22; P

Discussion

Change in dietary composition throughout the summer was not surprising as North American bison are temperate ruminants that take advantage of available vegetation throughout the summer (Schuler et al., 2006, Fortin and Fortin, 2009). However, bison diets did not shift according to availability as predicted. Grass became more abundant as summer progressed, but use declined, whereas use of forbs increased as availability decreased (see Fig. 1). This may be explained by plant phenology (Bergmann

Implications

We recognize that bison reintroductions across North America are becoming more popular (Alaska Department of Fish and Game, 2015, Banff National Park, 2015, Steenweg et al., 2016), thus increasing the need for better understanding diet selection across North America. To date, managers and biologist have used management plans and estimated stocking rates for bison that focus primarily on providing sufficient forage availability of monocots (e.g., grasses, sedges, rushes). However, the high

Acknowledgments

Research was funded in part by Olson’s Conservation Bison Ranches LTD and South Dakota State University’s Department of Natural Resource Management. We thank R. Metcalfe for field and logistical support. Thanks also to J. M. Craine and A. J. Beckley, Jonah Ventures, Boulder, Colorado, for analysis of bison fecal samples. N. Martorelli provided statistical assistance. Thanks to W. M. Inselman, W. M. Giuliano, and two anonymous reviewers for insightful comments that improved our manuscript

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