Its grazing and wallowing behaviour, as well as its size, are what make bison a keystone species of the Great Plains. Paine [32] was the first to define keystone species as those that have an extremely high impact on a given ecosystem relative to their small population. Numerous studies conclude that bison, both historically and presently, play a crucial role in the conservation of biodiversity in the Great Plains and can therefore be considered as a keystone species [13,31,33]. From this, it can be deduced that the bison can promote the biodiversity of both plants and the animals on the Great Plains to a greater extent than modern-day cattle farming [14].

Bison (and cattle) are considered generalist foragers and feed predominantly on roughage [34,35]. Bison feed almost exclusively on graminoids, i.e., grass-like plants [23], and generally avoid forbs and woody species [36], which make up less than 10% of their food spectrum [31]. Cattle also feed on graminoids [34], but forbs and woody vegetation often account for more than 15% of their food spectrum [37]. Based on these differences in eating habits, it can be concluded that cattle graze and browse a wider selection of plants than bison [38]. The more preferential grazing by bison of the prairie-dominant grass creates a significant change in the plant composition of a pasture [31] and promotes an increased occurrence of forbs [39]. Although bison grazing reduces the biomass of grass-like plants [16], the increased availability of water and nutrients as well as increased availability of light leads to an increase in recessive species [39]. Specifically, the increased grazing of the dominant matrix grasses of the prairie landscape, such as big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), and switchgrass (Panicum virgatum), thus leads to an increase in subordinate species with less competitive power, such as leadplant (Amorpha canescens), white aster (Symphyotrichum ericoides), and western ragweed (Ambrosia psilostachya) [23]. In particular, these herbs are vital for maintaining biodiversity on the prairie [31], as these plants stimulate pollinators and generally influence the micro-environment to a greater extent than grasses [40].

Bison behaviour, such as roaming, also enhances plant biodiversity. Cattle prefer to graze close to water bodies, while bison prefer areas further away [41]. Historical reports state bison can graze for several days over distances of up to a hundred kilometers between watering places [42]. The intensity of cattle grazing near water bodies can lead to poorer water quality through increased fecal concentration, as well as decreased plant growth, both of which subsequently reduce floristic diversity [43].

Through activities such as horning and rubbing against small bushes and trees such as cottonwood (Populus angustifolia) and aspen (Populus tremuloides), bison limit the occurrence of woody vegetation on the prairie [26]. In the vicinity of watercourses, this slows or prevents colonization by beavers and other species, which in turn has cascading effects on plant communities, river morphology, and biodiversity [44]. On the other hand, by rubbing against woody vegetation, bison prevent forest succession [33]. By reducing forest expansion and regeneration, bison have an advantageous effect on the development of herbaceous and grass-like plants, which serve as their and other livestock’s fodder [27].

The fur of bison has more primary hairs than that of any other member of the Bovidae family—10 times more than that of cattle [35]. Plant seeds are more easily caught in the thick and fine fur on a bison head [45]. Plants such as cockle burs (Xanthium italicum) and buffalo grass (Bouteloua dactyloides) benefit particularly from bison grazing because their seeds have barbs that easily catch in the fur of the animals during grazing [46]. Since bison migrate long distances, when able [47], and move about 50 to 90% more than cattle [41], plant seeds can spread over greater distances in bison grazing systems than with cattle [48].

An abundance of seeds is also spread through the feces of bison [45]. In addition, grazing ungulates, such as bison, consume relatively high lignin-containing plant biomass and return unstable forms of nitrogen (i.e., urine) to the soil, thus avoiding the otherwise slow mineralization of nitrogen in plant litter [31]. This highlights another advantage derived from bison grazing further from water bodies [38,40].

Through the disturbance to the landscape caused by their grazing, bison also affect the primary production, reproductive performance, and species structure of plants, as well as the humus content of the soil and soil quality [27]. As herds graze, they trample and wallow the landscape in varying degrees of strength and intensity, creating strong mosaic effects, which increase spatial and temporal heterogeneity [23]. Thus, the diversity of microhabitats increases compared to ungrazed areas and ultimately enhances the diversity of plant species [49]. Increased floristic diversity, derived from bison grazing, generally leads to greater ecological resiliency towards environmental extremes, such as droughts [23].

Multiple studies have come to the conclusion that plant biodiversity is higher in bison grazing than cattle grazing systems [29,39]. Thus, Freese et al. (p. 4, [14]) state that “Restoring the ecological role of bison is a prerequisite to large-scale and comprehensive restoration of biodiversity in the Great Plains and other grassland regions of North America that bison once inhabited.” However, studies often fail to address critical factors such as stocking density, grazing intensity, and herd structure, which also influence rates of biodiversity. Little literature is available on the comparison of bison and cattle grazing for similar animal and environmental factors [34,50], and multiple studies describe the difference in biodiversity from bison or cattle grazing as marginal [43,50]. Steuter & Hidinger [37] state that biodiversity and the biological resources of pastureland are not only determined by the single choice between allowing bison or cattle to graze but also by the overall stocking rate and aligning intensive grazers, i.e., cattle, with intensive agricultural systems and extensive grazers, i.e., bison, with vast rangelands. Light to moderate grazing is found to be preferable to intensive grazing in the context of biodiversity considerations, regardless of whether it is combined with bison or cattle [24].

Bison, along with birds, small insects, arthropods, and various pollinators from these strains and classes, are all indispensable for the conservation of grassland biodiversity [16] and have a significant influence on the structure and stability of the ecosystem [51].

Bison grazing creates a landscape mosaic, which in turn provides more (micro)habitats for small mammals, birds, insects and other animal species, in contrast to a homogeneously managed landscape [52]. For example, an increase in flowering herbaceous plants, compared to grasses, promotes pollinators [40]. Landscape mosaics are essential for most of the native bird species on the Great Plains [53]. The transformation of landscape mosaics into large uninterrupted areas, be it farmland or intensively used pastureland, generally reduces the range of these native birds [49]. The occurrence of grassland birds on the Great Plains has declined sharply in recent decades, mainly due to the intensive grazing practices [11]. Some bird species, such as upland sandpiper (Bartramia longicauda) [25], grasshopper sparrow (Ammodramus savannarum) [11], and bobwhite quail (Colinus virginianus) [49], benefit from the promotion and expansion of landscape mosaics through bison grazing. These species of birds reside, largely, on the ground and require a heterogenous landscape—large open spaces to find food in large quantities [11,54] as well as areas with high vegetation to shelter their breeding grounds [31]. However, not all bird species benefit from bison grazing and the resulting disturbance to the landscape. For instance, the henslow sparrow (Ammodramus henslowii) is disturbed by large ungulates (including cattle) grazing, and the bird is rarely found in these areas [55].

Other larger herbivores also benefit from landscape mosaics created through bison grazing. Especially browser species, such as the pronghorn antelope (Antilocapra americana) and the mule deer (Odocoileus hemionus), benefit from the subsequent diversity of forbs and shrubs promoted by bison grazing; these plants are the main food source for both browser species [49]. In addition, studies have shown that bison grazing increases the number of prairie dog colonies [56,57], which serve as a food source for some bird species, such as burrowing owl (Athene cunicularia) [58] and ferruginous hawk (Buteo regalis) [59]. Furthermore, prairie dog burrows provide shelter and protection for various small mammal species [49].

The significant decrease in grass-like plants and the increase in forbs with bison grazing results in sites with less biomass but increased nitrogen availability, which are generally preferred by herbivorous arthropods [28]. An increased amount of forbes also leads to a greater heterogeneity of habitats, in terms of plant productivity, species composition and structure, for insects [60].

The distinctive wallowing behaviour of bison [10,14] is important for coat and skin care, insect repellency, sun protection, and social interaction of bison [61]. Wallowing of the animals on the same areas leads to topsoil displacement and compaction, which results in micro-sites with high soil moisture and clay content [27]. The resulting bare areas often have a diameter of 3 to 5 m and a depth of 10 to 30 cm [31]. Eventually, the abandoned wallows lead to microhabitats with altered physical resources and a distinct biodiversity [29]. Due to the high soil compaction, rainwater accumulates in the left-over wallows in spring, which leads to habitat for wetland species [62]. The wallows are increasingly used as breeding grounds for amphibians [29]; for example, the Great Plains toad (Anaxyrus cognatus) and the Plains spadefoot toad (Spea bombifrons) often rely on these seasonal bodies of water for reproduction [63].

Although wallowing and other bison behaviours initially lead to reduced plant biomass and growth rate [49]. and bison grazing can negatively impact plant diversity and arthropod diversity in the short term; the long-term effects are more complex and lead to increased heterogeneity and species richness [64]. Therefore, bison grazing has a long-lasting effect on the environment with wide-ranging effects on spatial function and biodiversity of the ecosystem [65].