The Green Wave Hypothesis (GWH) is the idea that the emergence of springtime vegetation, starting at lower latitudes and progressing to higher altitudes, influences the migration of foraging birds and animals (Van de Graaf et al., 2006). New vegetation provides the best nutrient access for these grazers which influences how and where these animals forage. Researchers took a look at the foraging behavior of Yellowstone’s bison (Bison bison) to see how well their grazing behavior fits with the GWH. What the researchers found by analyzing data from satellite imagery, field experiments, and 64 migrating female bison with GPS, is that the collective grazing behavior of bison is creates conditions which manipulates the growth of new vegetation.
Herd animals like bison congregate in large groups to graze. This creates a situation scientists refer to as ‘grazing lawns’ in which the bison will intensely graze, leaving behind manure. The trampling from the bison along with the fertilizer stimulates new plant growth, essentially resetting the spring growth period. Just like a lawnmower keeps grass growing by clipping it, this process of grazing and depositing urine and dung creates a prolonged growth period. The influence of bison on vegetation growth is so pronounced, it can be seen on satellite imagery.
Researchers contend that the Green Wave Hypothesis needs to be “revised to include group-forming grazers that not only move to find forage, but create forage by how they move.”
Geremia, C., Merkle, J. A., Eacker, D. R., Wallen, R. L., White, P. J., Hebblewhite, M., & Kauffman, M. J. (2019). Migrating bison engineer the green wave. Proceedings of the National Academy of Sciences, 201913783. https://doi.org/10.1073/pnas.1913783116
Yellowstone’s migrating bison manipulate springtime green-up, USGS, December 2, 2019.
Van der Graaf, A. J., Stahl, J., Klimkowska, A., Bakker, J. P., & Drent, R. H. (2006). Surfing on a green wave-how plant growth drives spring migration in the Barnacle Goose Branta leucopsis. ARDEA-WAGENINGEN-, 94(3), 567.