Bison limit ecosystem recovery in northern Yellowstone

Bison numbers increased more than fourfold between 2004 and 2015.

“Bison numbers increased more than fourfold between 2004 and 2015, and their ecosystem effects have similarly increased,”

Beschta et al. (2020) reported that 97% of mature lodgepole pine trees in their study area in Lamar Valley had been killed by bark removal.

“In the Lamar Valley of northern YNP, Beschta et al. (2020) reported that 97% of mature lodgepole pine trees in their study area had been killed by bark removal.”

Bison on the northern range increased from less than 1000 in the early 2000s to approximately 4000 animals in recent years despite annual culling.

“Bison on the northern range have increased from less than 1000 in the early 2000s to approximately 4000 animals in recent years (Figure 3) despite annual culling in winter as bison migrated outside the park boundary (Beschta et al., 2020; Geremia, 2022; Mosley & Mundinger, 2018; Plumb et al., 2009).”

Consumptive effects of bison can be significant where bison are present in high densities year after year, as in northern Yellowstone.

“These consumptive effects can be significant where bison are present in high densities year after year, as in northern Yellowstone (Beschta et al., 2020; Painter & Ripple, 2012; Rose & Cooper, 2016).”

Only recently, as elk have decreased and bison increased, have bison emerged as an important factor affecting the dynamics of these woody plants.

“Only recently, as elk have decreased and bison increased, have bison emerged as an important factor affecting the dynamics of these woody plants (Beschta et al., 2020; Painter & Ripple, 2012; Ripple et al., 2010; Rose & Cooper, 2016).”

Fallen trees provided temporary protection for young aspen from bison and elk.

“These fallen trees provided temporary protection for young aspen from bison and elk (Beschta et al., 2020; Ripple & Larsen, 2001), as elk densities decreased.”

Herbivory continued at high rates in some places, particularly where bison were in high concentrations and browsed on willow in the summer, as in Lamar Valley.

“Herbivory also continued at high rates in some places, particularly where bison were in high concentrations and browsed on willow in the summer, as in Lamar Valley (Painter andRipple 2012, Beschta et al. 2020).”

The temporary increase in cottonwood saplings in 2012 likely resulted from reduced ungulate herbivory in the early 2000s, when elk numbers were low and before bison numbers increased dramatically.

“This increase likely resulted from reduced rates of ungulate herbivory in the early 2000s, when elk numbers in the eastern sector had become low and before bison numbers had dramatically increased (Beschta et al. 2020).”

Since about 2007, the overall foraging pressure of Northern Range bison has increasingly exceeded that of elk, representing a profound shift in ungulate use within a relatively short period.

“Since about 2007, the overall foraging pressure of Northern Range bison has increasingly exceeded that of elk (Beschta et al. 2020), representing a profound shift in ungulate use within a relatively short period.”

In areas of high bison use, a lack of cottonwood recruitment and that of other woody species will continue the simplification of these potentially biologically diverse communities.

“Functionally impoverished riparian plant communities: In areas of high bison use, a lack of cottonwood recruitment and that of other woody species will continue the simplification of these potentially biologically diverse communities (Beschta et al. 2020; Kauffman et al. 2023).”

Bison most commonly forage along valley bottoms and other low-elevation areas of northern Yellowstone.

“bison most commonly forage along valley bottoms and other low-elevation areas of northern Yellowstone (Geremia and Hamilton 2019; Beschta et al. 2020; Yellowstone National Park 2024b)”

River sinuosity for the Lower Lamar decreased by ~30% over 1954-2015.

“That conclusion was supported by a subsequent analysis of river sinuosity for the Lower Lamar, where it was found to have decreased by ~30% over the relatively short period of only six decades (i.e., 1954–2015) (Beschta et al. 2020).”

In areas of high bison use such as the Lamar Valley, intensive herbivory, trampling, and bark damage are suppressing recruitment and degrading many riparian plant communities.

“In areas of high bison use such as the Lamar Valley, intensive herbivory, trampling, and bark damage are suppressing recruitment and degrading many riparian plant communities (Beschta et al. 2020; Kauffman et al. 2023; Painter et al. 2023).”

Bison horning and rubbing appear to be a relatively new disturbance process within Yellowstone National Park, capable of affecting the composition and structural diversity of cottonwoods, willows, aspen, and conifers.

“The physical effects of bison horning and rubbing appear to represent a relatively “new” disturbance process within the Park, capable of affecting the composition and structural diversity of cottonwoods and willows in riparian areas, aspen in uplands and riparian areas, and conifers along rivers and grassland edges (White et al. 2015; Beschta et al. 2020; Painter et al. 2012, 2023).”

In 2018, 97% of 91 mature lodgepole pines in the Lower Lamar study reach were dead, with cambium absent from 60-80% of their circumferences due to bison horning and rubbing.

“in 2018 Beschta et al. (2020) evaluated a stand of 91 mature lodgepole pines (dbh range = 32–49 cm) growing immediately adjacent to the river in the Lower Lamar study reach. Ninety-seven percent of them were dead, with cambium absent from 60–80% of their circumferences due to bison horning and rubbing.”

The general absence of functional riparian plant communities along the Lamar River will allow for accelerated riverbank erosion, high rates of channel migration, greater sediment loads, and increasingly wide active channels.

“Decreased streambank stability and altered channel morphology: The general absence of functional riparian plant communities along the Lamar River will allow for accelerated riverbank erosion, high rates of channel migration, greater sediment loads, and increasingly wide active channels (Rosgen 1993; Beschta et al. 2020).”

Intensive herbivory and trampling by bison can have major effects on wetlands and tributary streams, as well as their associated communities.

“Modified wetlands and tributary streams: Intensive herbivory and trampling by bison can have major effects on wetlands (e.g., springs, seeps, and areas adjacent to ponds or lakes) and tributary streams, as well as their associated communities (Beschta et al. 2020; Kauffman et al. 2023).”