Date Published: February 24, 2016
Publisher: Public Library of Science
Author(s): Robin Steenweg, Mark Hebblewhite, David Gummer, Brian Low, Bill Hunt, Govindhaswamy Umapathy.
Interest in bison (Bison bison, B. bonasus) conservation and restoration continues to grow globally. In Canada, plains bison (B. b. bison) are threatened, occupying less than 0.5% of their former range. The largest threat to their recovery is the lack of habitat in which they are considered compatible with current land uses. Fences and direct management make range expansion by most bison impossible. Reintroduction of bison into previously occupied areas that remain suitable, therefore, is critical for bison recovery in North America. Banff National Park is recognized as historical range of plains bison and has been identified as a potential site for reintroduction of a wild population. To evaluate habitat quality and assess if there is sufficient habitat for a breeding population, we developed a Habitat Suitability Index (HSI) model for the proposed reintroduction and surrounding areas in Banff National Park (Banff). We then synthesize previous studies on habitat relationships, forage availability, bison energetics and snowfall scenarios to estimate nutritional carrying capacity. Considering constraints on nutritional carrying capacity, the most realistic scenario that we evaluated resulted in an estimated maximum bison density of 0.48 bison/km2. This corresponds to sufficient habitat to support at least 600 to 1000 plains bison, which could be one of the largest 10 plains bison populations in North America. Within Banff, there is spatial variation in predicted bison habitat suitability and population size that suggests one potential reintroduction site as the most likely to be successful from a habitat perspective. The successful reintroduction of bison into Banff would represent a significant global step towards conserving this iconic species, and our approach provides a useful template for evaluating potential habitat for other endangered species reintroductions into their former range.
Few free-ranging wild bison (Bison bison, Bison bonasus) populations currently occur in North America (n = 27) and Eurasia (n = 36)[1,2], and there has been growing interest in restoring the species to other portions of historic range [3–6]. It has been mainly through reintroductions that conservation efforts have brought bison back from near extinction on both continents [1,7], but in North America, they occupy less than 1% of their historical range . Bison were extirpated from the wild across most of the species’ former range, although there are now an estimated half-a-million bison present in North America, mostly in captive herds . In Canada, wild plains bison (Bison bison bison) were completely extirpated in the late 19th century  and they are designated as “threatened” by the Committee On the Status of Endangered Wildlife In Canada (COSEWIC) . Despite the 2005 recommendations of COSEWIC, plains bison are not yet listed on Schedule 1 of the Species At Risk Act (SARA), primarily because of potential economic implications for the Canadian agricultural bison industry . Plains bison, therefore, are not currently protected under SARA and a national recovery strategy is not required, legally; however, the majority of wild, plains bison in Canada occur within National Parks where they are protected under the Canada National Parks Act .
Based on our habitat suitability model and estimates of nutritional carrying capacity, there appears to be sufficient habitat of high-enough quality to support a relatively large population of plains bison year-round in Banff that could significantly contribute to improving their global conservation status. Despite evidence for a reduction in habitat suitability during winter by approximately 50%, the estimated winter population appears sufficiently large to support a greater population size than most other extant populations in North America . Much of Banff’s subalpine and alpine regions, however, are not predicted to be functional bison habitat during winter, when bison habitat is limited to lower-elevation montane and subalpine areas in the Red Deer, Panther, Cascade and Bow valleys. This is because of the combination of favored landcover types, as well as snow depths that increase at higher elevation and bison responses to topography. Moreover, bison habitat suitability was strongly positively influenced by fire during both seasons, and especially in the winter when prescribed and natural fires in low elevation winter ranges contributed significantly to bison habitat suitability, as evidenced in empirical studies [44,49,56,57]. Given the paucity of summer RSF studies on bison and the demonstrated use of steep areas in Utah by the Henry mountains population , there remains uncertainty about bison use of steep terrain, especially in summer. Assuming that bison only use flat terrain in summer may result in an underestimation of bison distribution and thus movements in summer. Although water is not likely to be as important for bison in Banff compared to more arid areas such as Utah, snow is likely to have a strong effect on both bison habitat selection and carrying capacity. There is agreement among studies regarding the importance of snow for limiting bison habitat suitability (see methods). The striking difference between bison HSI models for summer and winter (compare Fig 4a and 4b) implies the importance of considering both seasons when predicting bison distribution and carrying capacity. Because of deep snow in many areas in Banff, bison are likely to use mainly valley bottoms in winter, and may only expand up to mid slopes during summer. In high snow-depth winters, higher-quality habitat is restricted to areas of low snow accumulation, such as the Red Deer and Bow valleys and far eastern slopes. There was also consistency between previous studies in selection for grasslands, shrub lands, and the strong avoidance of coniferous forests. Despite a few studies that showed minimal effects of burns on bison habitat selection, a growing number of studies clearly demonstrate that burns are important in improving bison habitat suitability [44,49,56,57]. Given that burns enhance green forage biomass and reduce standing dead biomass, especially in grasslands, quantifying bison use of burns post reintroduction will be important in both winter and summer.