The Keystone Role of Bison in North American Tallgrass Prairie - Bison increase habitat heterogeneity and alter a broad array of plant, community, and ecosystem processes

39 Throughout the history of Great Plains grasslands, North American bison (Bos bison, also known as Bison bison; Jones et al. 1992) and other large herbivores were abundant and conspicuous components of the biota (Wedel 1961, Stebbins 1981). Many of the earliest herbivores, particularly those that were primarily browsers, are now extinct, but their consumption of woody vegetation is thought to have played a critical role in the postPleistocene rise of the grassland biome and the subsequent increase in bison populations (Axelrod 1985, Hartnett et al. 1997). Indeed, the large herds of bison encountered by early Europeans on the Great Plains were likely the result of the rapid early-Holocene proliferation of this ungulate into a relatively young and expanding “treeless” grassland biome (Stebbins 1981, Axelrod 1985). In the most productive regions of the Great Plains, the eastern tallgrass prairies, abundant bison herds were noted by early explorers (Shaw and Lee 1997). Although herds were larger in the western shortgrass steppe, some have speculated that a greater density of bison could be supported in eastern tallgrass prairies than elsewhere in the plains (McHugh 1972). It is unfortunate, then, that despite the historic abundance of bison in tallgrass prairies, their ecological effects in these mesic grasslands are poorly understood. Knowledge of the bison’s role in tallgrass prairies is lacking because the extent of this grassland and the abundance of its dominant ungulate have declined dramatically and in tandem over the last 150 years. Although there is debate over the numbers of bison inhabiting the Great Plains before the 1800s (estimates range from 30 million to 60 million; McHugh 1972, Flores 1991), it is well documented that from 1830 to 1880 the slaughter of bison in the Great Plains reduced their numbers to an estimated low of a few thousand individuals. Widespread cultivation of the plains, which accompanied the near extirpation of the bison, reduced the once-vast tallgrass prairie (approximately 68 million hectares) to less than 5% of its presettlement range (Samson and Knopf 1994). The near-simultaneous reduction in herbivore abundance and grassland extent left little opportunity to assess bison–tallgrass prairie interactions. Today, thanks to conservation efforts (Berger and Cunningham 1994), bison numbers in the Great Plains have rebounded (to approximately 150,000), and significant public and private herds are maintained in several mixedand shortgrass prairie preserves and ranches. It is from these semi-arid grasslands, many of which escaped cultivation, that the most extensive knowledge of bison–grassland interactions has been generated (Peden et al. 1974, Coppock et al. 1983). By contrast, the current understanding of tallgrass prairie structure and function has been developed almost exclusively from studies of ungrazed tracts or from sites grazed by domestic cattle (Risser et al. 1981, Collins 1987, Howe 1994, Leach and Givnish 1996). Only recently have bison been reintroduced to tallgrass prairie sites that are large enough to assess both their influence on other biota and ecosystem processes, as well as their interactions with other important features of these grasslands, particularly fire (Collins et al. 1998, Coppedge and Shaw 1998, Knapp et al. 1998b). The Konza Prairie Research Natural Area in the Flint Hills of northeastern Kansas is the largest tract of unplowed tallgrass prairie (3500 ha) in North America dedicated to research (Knapp et al. 1998b). Konza Prairie was one of the original sites selected in 1981 for inclusion in the The Keystone Role of Bison in North American Tallgrass Prairie