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Some bears ride the green wave

Two bears in tall grass.
Brown bears track all kinds of food sources, from salmon runs to berries, and green up when grasses are at their best. Based on the timing of different foods, bear locations and use of space can be better understood and predicted.

Spring in northern and mountainous areas leads to melting snow and the return of green vegetation. This transition does not happen all at once, but proceeds upwards and northwards as spring progresses, a phenomenon that has been dubbed the "green wave." The newly emergent vegetation is highly nutritious. It has been well documented that many herbivores follow or track the green wave. However, the question of whether or not omnivores, such as brown (grizzly) bears, track the green wave has remained. Researchers, including from the National Park Service, from across western North America set out to answer this question using data from 127 brown bears from across the species’ range on the continent. They found, depending on the population, that 22-85% of bears tracked the green wave. As new forage matures and becomes less nutritious, animals switch to different resources. The results of this study can help us better understand brown bear ecology and improve conservation planning for this iconic species.

A test of the green wave hypothesis in omnivorous brown bears across North America

Abstract

Herbivorous animals tend to seek out plants at intermediate phenological states to improve energy intake while minimizing consumption of fibrous material. In some ecosystems, the timing of green-up is heterogeneous and propagates across space in a wave-like pattern, known as the green wave. Tracking the green wave allows individuals to prolong access to higher-quality forage. While there is a plethora of empirical support for such behavior in herbivorous taxa, the green wave hypothesis (GWH) is nuanced based on factors such as body morphometrics and digestive capacity. Furthermore, little is known about whether other taxa, such as omnivores, track the green wave. Our objective was to assess whether the GWH can be extended to explain the movements of omnivores. Using GPS collar data from seven populations (n = 127 individuals) of brown bears Ursus arctos across their entire North American range, we first tested whether bears tracked the green wave. Using conditional resource selection functions (RSFs), we found that variation in proxies of vegetative forage quality better explained movement and habitat selection than proxies of forage biomass in over half of the bears in our study, providing evidence of green wave tracking. Second, we assess factors that explained variation in green wave tracking using linear mixed effects models. Green wave tracking in brown bears was explained by the variation in availability of green-up within spring home ranges, and how green-up transitioned across those home ranges. Our results demonstrate that the GWH can partially explain movement of a non-migratory omnivorous species, extending the generality of the GWH as a broad predictor of animal space use. The green wave is another resource wave brown bears track, and our findings help predict brown bear space use, which can be used to guide conservation and habitat restoration efforts.

Bowersock, N. R., L. M. Ciarniello, W. W. Deacy, D. C. Heard, K. Joly, C. T. Lamb, W. B. Leacock, B. N. McLellan, G. Mowat, M. S. Sorum, F. T. van Manen, and J. A. Merkle. 2023. A test of the green wave hypothesis in omnivorous brown bears across North America. Ecography :e06549.

Gates Of The Arctic National Park & Preserve, Yellowstone National Park

Last updated: August 22, 2023