What Yellowstone’s bison reveal about the power of the mob
Lucky hikers can observe bighorn sheep selectively graze – nibbling only tasty columbine blossoms growing alongside other grasses, sedges, and forbs. In another type of selective grazing, many ungulates follow spring green-up from lower elevation winter range to higher elevation summer habitat. This behavioral strategy described in herbivores from aquatic and terrestrial mammals to birds – green wave surfing – optimizes nutrient uptake as newly emerging plants provide higher quality forage.
A recent study of the last migratory herd of bison in North America showed that bison are not compelled to surf the green wave. Bison, which are aggregate grazers, create their own green-up through high intensity grazing resulting in faster, more intense green-up sustained over a longer time period. In mid- to late summer, when plant senescence typically results in a decline in forage quality, areas with high bison use showed increases in forage quality of 50-90% compared to nearby areas protected from bison grazing. Grazing intensity had a stronger impact on plant phenology than weather or environmental variables. This detectable change in green wave progression allowed the authors to conclude that migrating bison engineer the green wave.
Geremia and colleagues add scientific support from Yellowstone for what short duration, high intensity grazing of cattle attempts to achieve in agriculture – increased forage productivity.
"Migrating bison engineer the green wave" Chris Geremia, Jerod A. Merkle, Daniel R. Eacker, Rick L. Wallen, P. J. Whitea, Mark Hebblewhite, and Matthew J. Kauffman, PNAS www.pnas.org/cgi/doi/10.1073/pnas.1913783116