Ecologists accept the Forage Maturation Hypothesis (FMH) that young plants optimize food quality by balancing nutrient and biomass availability (1). The Green Wave Hypothesis (GWH) applies the FMH to migrating herbivores by linking the timing, pace, and extent of migrations to waves of new plant growth (2, 3). The GWH portrays animals as “slaves to the wave” that must move with green-up to eat the best foods. We show that bison are less constrained (4). Their intense grazing in large aggregations resets and stimulates new plant growth, which allows them to forage on high-quality foods while they let the wave pass them by.
We have evaluated the relationship between the date bison occupied locations and date of satellite-derived peak instantaneous rate of green-up (IRG) (2, 4–6). Craine (7) reevaluates our data and concludes we mischaracterized the bisons’ migrations, suggesting they start spring well ahead of the wave, wait for it to pass, and only try to catch it at the end.
We disagree with his reassessment, because he incorrectly pools our data across individuals and years (7). The timing of spring is different for each individual, beginning the first day and ending the last day that peak IRG is available across their home range. Craine’s analysis includes many locations when spring had not yet started or had already ended for individuals, which confounds his results.
When an animal trails the green wave, it is important to analyze surfing from the first day of spring until the animal arrives at the location of the last day of spring. To address Craine’s (7) concern, we identified this interval for each migration and further divided them into six equally spaced time periods. We fitted mixed linear models with a random effect term for each individual and year. These analyses support our initial conclusions that bison start spring moving near the crest of the wave and progressively fall behind. The fixed effect terms capturing population-level trends show a consistent slowing of movements from the first (β1 = 0.55, SE = 0.04) to the last (β6 = 0.22, SE = 0.02) period (Fig. 1 A−C). Despite moving slower than the wave, bison surf by staying near the crest of the wave for nearly half of spring, averaging 9 d ahead during the first period, within 1 d during the second period, and 10 d behind during the third period (Fig. 1D).
Fig. 1.
Bison fall behind the green wave as spring progresses. Spring is the time when peak IRG occurs and is different for each individual migration. (A) Bison surf best during the first time period of spring, (B and C) progressively fall behind, (D) but remain near the crest of the wave for the first half of spring. In A−C, shading corresponds to spring periods indicated in D and lines show the mean fixed effect between date of occupation and the date of peak IRG. If the population surfed perfectly, the model predicted lines would be identical to the 1:1 black line. Background gray points show the entire Global Positioning System dataset [analyzed by Craine (7)]. (E) Resource selection functions show bison strongly select for IRG in early spring, but select for areas of intense grazing as spring progresses. (F) Ratios of crude protein (quality) to digestible organic matter (quantity) in fecal samples (4) show that bison obtain optimal diets (8) when they forage at, and particularly after, the crest of the green wave.
To further explore Craine’s (7) concern, we used resource selection functions to show that bison initially strongly select for IRG. As spring progresses, bison, instead, select for areas of intense grazing, a variable measured in our initial paper (4) (Fig. 1E). Both selection strategies act together to optimize diet in accordance with the FMH. Bison surf and then fall behind the green wave to regraze areas, which prolongs the period of maximized food quality and quantity (Fig. 1E).
We appreciate Craine's (7) scrutiny. The analyses this discussion prompted reaffirm (4) that the GWH needs to be revised for ungulates that migrate and graze intensely in large aggregations.
Footnotes
The authors declare no competing interest.
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