Expert Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

Key Messages

  • All tree-sized aspen in the northern range are now more than 80 years old, and in the absence of recruitment their abundance will continue to decline. Species associated with aspen will likely decline along with tree-sized stems.
  • Although conifer forests are used by ungulates, there is no evidence that ungulates affect their species composition.
  • Based on the best available evidence, that no major ecosystem component is likely to be eliminated in the near or intermediate term.
  • Composition and productivity of grassland communities in the northern range have not changed much with increases in herbivory.
  • Currently, in the northern range, herbivory by elk on young aspen is intense and has probably prohibited recruitment since 1920. Although there have been fluctuations in climate since 1920, none has been large enough or persistent enough to account for the failure of aspen recruitment.
  • Factors whose influences are related to population density (called density dependent) interact with factors whose influences are not (called density independent) to regulate elk and bison populations in the northern range.
  • Given the complexities involved in managing Yellowstone's dynamic ecosystems, there is a continuing need for rigorous research and public education.
  • Human activities adjacent to YNP have large effects on the animals present at least seasonally within the park. The animals do not have free access to the adjacent areas that formerly were available to them as migration corridors and winter range.
  • Humans have caused some changes (e.g., the introduction of timothy grass and other exotics).
  • In addition to density-dependent factors, elk and bison populations also are affected by density-independent factors such as weather and because ungulates and their food do not always vary in a synchronous way.
  • Large ecosystems in general and YNP's northern range in particular are dynamic. Ecosystems change in unpredictable ways.
  • Recent restoration of wolves to YNP may allow evaluation of their role in aspen and willow recruitment and maintenance, but scientific information is lacking to understand the role of past development and hunting outside the park on elk behavior and migration patterns.
  • Some ungulate populations tend to fluctuate regardless of human management intervention. The pronghorn population has fluctuated widely during the past century and has been declining recently. Adverse factors include coyote predation and hunting on private land outside the park. Also, pronghorn may be affected by competition for food with elk, mule deer, and bison during severe winters.
  • The architecture, size, recruitment, and coverage of sagebrush have been changed by elk, pronghorn, bison, and mule deer. The effects are more significant at lower than at higher elevations in the northern range.
  • The changes in the northern range are the result of the number of ungulates in the area combined with biophysical factors such as climatic variability, but current methods do not allow us to separate the relative contributions of each of these effects.
  • The northern range is not on the verge of crossing some ecological threshold beyond which conditions might be irreversible. The same is true of the region's sagebrush ecosystems, despite reductions in the number and size of plants at some lower elevations.
  • There is a strong density-dependent signal in northern range elk and bison population dynamics, but their responses differ: bison tend to expand their range to areas outside YNP when their population exceeds roughly 2,500, whereas reproductive rates in elk decline when their populations exceed roughly 15,000.
  • There is little evidence for an ungulate effect on the summer range communities, with the exception of young aspen, which are heavily browsed.
  • Wolves also affect the population dynamics of ungulates as well as those of other predators in YNP.
  • YNP's practice of intervening as little as possible in the ecology of ungulates within YNP will likely allow the persistence of the northern range ecosystem and its major components as long as there is no large change in climate. If the NPS decided that it needed to intervene to protect species like aspen and the species that depend on tree-sized aspen stems, localized interventions would be prudent.