Migrating bison engineer the green wave

Abstract

Newly emerging plants provide the best forage for herbivores. To exploit this fleeting resource, migrating herbivores align their movements to surf the wave of spring green-up. With new technology to track migrating animals, the Green Wave Hypothesis has steadily gained empirical support across a diversity of migratory taxa. This hypothesis assumes the green wave is controlled by variation in climate, weather, and topography, and its progression dictates the timing, pace, and extent of migrations. However, aggregate grazers that are also capable of engineering grassland ecosystems make some of the world's most impressive migrations, and it is unclear how the green wave determines their movements. Here we show that Yellowstone's bison (Bison bison) do not choreograph their migratory movements to the wave of spring green-up. Instead, bison modify the green wave as they migrate and graze. While most bison surfed during early spring, they eventually slowed and let the green wave pass them by. However, small-scale experiments indicated that feedback from grazing sustained forage quality. Most importantly, a 6-fold decadal shift in bison density revealed that intense grazing caused grasslands to green up faster, more intensely, and for a longer duration. Our finding broadens our understanding of the ways in which animal movements underpin the foraging benefit of migration. The widely accepted Green Wave Hypothesis needs to be revised to include large aggregate grazers that not only move to find forage, but also engineer plant phenology through grazing, thereby shaping their own migratory movements.

Keywords: bison; grazing lawn; green wave; migration; surfing.

Fig. 1.

The relationship between spring green-up, forage quality, and bison diet quality over the growing season. (A) The instantaneous rate of green-up (IRG) peaks when the rate of green-up, as indexed from the NDVI curve, is most rapid. (B) The daily locations of bison in 2014 (for illustrative purposes) indicate that after peak green-up, most bison fall behind the wave of green-up. (C) Despite foraging in habitat patches past peak green-up, bison maintain high-quality diets throughout the growing season. Diets were measured as the ratio of crude protein (CP) to digestible organic matter (DOM) in fecal samples collected in 2014 from migrating bison. (D) Plant forage quality measured as the ratio of nitrogen to carbon (N:C) in grasses at key foraging areas, is highest at peak IRG and declines as the green wave passes by.

Fig. 2.

Comparison of green-wave surfing by migrating bison and mule deer. Typical green-wave surfing by a representative bison (A) and mule deer (B) in the Greater Yellowstone Ecosystem (United States), 2014. (C) A perfect surfer would use each location along its migratory path on the date of peak IRG (black 1:1 line). Mule deer (n = 12) move with the wave of green-up that progresses from low to high elevations during spring, extending their exposure to peak green-up (E). (D) Green-wave surfing in bison (n = 12) is mixed, however, with individuals slowing down and letting the green wave pass them by, thereby missing out on the full availability of green-up (F). Green-up is indexed by IRG (Fig. 1A). Bars in E and F represent the range of dates that peak IRG is available (thin bars) and used (thick bars). All data were collected in 2014. Cartography by the University of Oregon InfoGraphics Lab.

Fig. 3.

The effect of bison grazing on plant-forage quality. (A) Small-scale exclosure experiments found that bison, particularly in areas of high grazing intensity, can consume more than 50% of available plant biomass. (B) High-intensity grazing increased plant-forage quality, measured as the ratio of nitrogen to carbon (N:C) in shoot tissue, with smaller effects in plots receiving moderate (C) or low-intensity (D) grazing. Asterisks depict significant differences among grazed and exclosed plots. Plant and grazing metrics were derived from a field experiment of n = 30 sites with grazed (treatment) and exclosed (control) plots maintained from 2012 to 2017.

Fig. 4.

Grazing effects on broad-scale patterns of the green wave. (A) Bison created massive grazing lawns, which are larger than most fenced bison preserves in North America, as they migrated across the (B) extensive Yellowstone grasslands. Greenness of 25- to 30-ha grasslands encompassing grazing experiment sites changed with decadal shifts in bison grazing intensity. (C) When bison grazing was more intense, vegetation greened up earlier, more intensely, and for a longer duration, as measured by NDVI and IRG. (D) More intense grazing also elevated net aboveground production. In C and D, blue and yellow lines show model predicted NDVI for the same grassland under high and low bison grazing. Gray lines show spline-fitted NDVI curves for grazing experiment sites during 2005 to 2015. (E) At small scale, vegetation conditions outside exclosures were kept in an early phenological stage even late in the growing season.