Migrating bison engineer the green wave

Proceedings of the National Academy of Sciences of the United States of America
By: , and 

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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.

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Publication type Article
Publication Subtype Journal Article
Title Migrating bison engineer the green wave
Series title Proceedings of the National Academy of Sciences of the United States of America
DOI 10.1073/pnas.1913783116
Volume 116
Issue 51
Year Published 2019
Language English
Publisher PNAS
Contributing office(s) Coop Res Unit Seattle
Description 7 p.
First page 25707
Last page 25713
Country United States
State California
Other Geospatial Yellowstone National Park
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