Coming Up in Yellowstone Science—The Grasslands & Grazing Issue
by P. J. White, Roy Renkin, Chris Geremia, & Stefanie D. Wacker
The grassland and sagebrush-steppe habitats in and near Yellowstone National Park (YNP) have been referred to as America’s Serengeti because they support abundant and diverse ungulates and their predators. Thousands of bison and elk, and hundreds of bighorn sheep, deer, and pronghorn migrate seasonally across the landscape where they interact with black bears, coyotes, grizzly bears, and wolves, thereby providing one of the premier places in the world to observe and photograph or film wildlife. However, these habitats also have been a source of controversy since the 1920s due to concerns about too many ungulates removing too much vegetation, compacting soils, and reducing the diversity of plants, especially in the northern region of the park known as the “northern range.”
Numbers of bison, elk, and pronghorn in the park were controlled during the 1930s through the late 1960s by shooting and removals, as well as public hunting in surrounding states. Park rangers stopped culling ungulates after 1968 and let numbers fluctuate in response to competition, forage availability, harvests, predation, and weather. Thereafter the abundance of elk increased rapidly, with almost 19,000 counted in northern Yellowstone and nearby areas of Montana during 1988. As their numbers increased, more elk began migrating to lower-elevation valleys outside the park during winter, leading some people to conclude the park was overgrazed with insufficient food for the existing numbers of animals. These changes led to contentious debates and independent assessments by the National Academy of Sciences and other groups about whether elk and other ungulates were overpopulated and irreversibly damaging the landscape through excessive grazing, soil compaction, and related effects.
The recovery of grizzly bears and wolves in the park by the mid-2000s contributed to a substantial decrease in counts of northern Yellowstone elk and, as a result, the debate about overgrazing waned. However, bison counts in northern Yellowstone tripled over the subsequent decade and intense grazing in some areas such as the Lamar Valley rekindled the debate about grazing effects on grasslands. This transition from an elk-dominated system to one with a more equal biomass of elk and bison was unprecedented and, as a result, the effects on grassland and sage-steppe communities were uncertain. Unlike elk and other ungulates, bison are constrained by surrounding states from migrating or dispersing much beyond the boundaries of YNP due to concerns about brucellosis transmission to livestock, competition with cattle for grass, human safety, and property damage. Thus, increasing bison densities and associated increases in the duration grasslands were grazed in the park led to concerns about high grazing intensities on some summer ranges that may not be sustainable over time.
Since 2012, biologists have conducted several monitoring and research efforts to document above-ground grass production, percent consumption by the grazing community, soil nutrient availability, soil organic matter, plant composition, bare ground, and litter at several sites in high-use bison areas. In addition, vegetation ecologists have been quantifying sage-steppe communities in the park to spatially describe the variability in plant community composition and be positioned to detect community changes in abundance, bare soil and litter, percent cover, and other metrics over time. The next issue of Yellowstone Science will include a series of feature and short articles describing the results of these efforts, and discussing the historic and current effects of grazing and other factors on grassland production and ecosystem stability.