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.

Details of how these data were collected can be found in the Methods and Supplementary Materials of Geremia et al. 2019, Migrating bison engineer the green wave (Proceedings of the National Academy of Science). A brief summary of each dataset follows:

bisonsurfdata.csv - Data to replicate analysis of green-wave surfing in bison. See Methods and Text S1 in Supplementary text for details. Columns: id = animal id; year = year of day of animal location; jul = julian date of animal location; maxIRGdate = julian date of max IRG for the location. 

fecaldata.csv - Data to replicate analysis of bison diet quality over time. See Methods for details. Columns: year = year fecal sample was collected; julianday = julian day fecal sample was collected; CP = crude protien of sample; DOM = digestible organic matter of sample.

leafdata.csv - Data to replicate anlaysis of plant-forage quality as it relates to days from peak IRG. See Methods and Text S1 for details. Columns: year = year plant tissue sample was collected; julianday = julian day plant tissue sample was collected; leafN = N of sample; leafC = C of sample; NETDFPIRG = absolute value of the number of days between the julian date the sample was collected and the julian day of peak IRG for the pixel where the sample was collected.

functionalNDVIdata.csv - Data to replicate functional NDVI analysis. See Methods and Text S3, S4, S5, and S6 for details. Columns: site = name of grazing experiment site; year = year of NDVI data; bisonuseindex = grazing intensity index; swe = Snow Water Equivelant value; precip = precipitation value; temp = temperature value; slope = slope of site in degrees; aspect = aspect of site in degrees; elev = elevation of site; columns 10 through 42 = NDVI values for julian dates 57 through 313 at 8 day intervals.

grazingexperimentdata.csv - Data to replicate grazing experiment analysis. See Methods and Text S3 and S5 for details. Columns: siteyrid = site and year combined; site = name of grazing experiment site; year = year of data collection; julianday = julian day of data collection; plottype = type of plot, control (for plots within fenced exclosure) or experimental (plots outside in grazed areas); plotnumber = both control and experimental plots were replicated with up to 6 replicates each; shootbiomass = shoot biomass of sample; leafN = N of sample; leafC = C of sample; SiteAnnualgrazingintensity = grazing intensity index for the year. Note that site grazing intensity can be slightly less than 0 due to sampling variation. See Text S3 in regards to "adding positive and negative increments."

grazingintensitydata.csv - Data to to build the linear relationship between field measured grazing intensity and landscape modeled grazing intensity. See Text S3 and S5 for details. Columns: siteyrid = site and year combined; site = name of grazing experiment site; year = year; grazingintensity = is field measured grazing intensity using the plot data; bisonuseindex = the averaged value for bison use from scaled Brownian Bridge Movement Models for the larger areas around each grazing experiment site. 

fullspringbisonsurfdata.csv – Additional data to replicate analysis of green-wave surfing in bison described in Geremia et al. 2020 Response to Craine: Bison redefine what it means to move to find food. Columns are the same as described in bisonsurfdata.csv. Data file includes locations as described in the response letter.

Run analyses.R – Native R code to replicate analyses in Geremia et al. 2019 and Geremia et al. 2020. Run by downloading and unzipping files to a user defined working directory and specifying the working directory in the code header.

Details of how these data were collected can be found in the Methods and Supplementary Materials of Geremia et al. 2019, Migrating bison engineer the green wave (Proceedings of the National Academy of Science). Please contact the bison project at Yellowstone National Park if you plan to use these data for any purpose. Thank you.