Whitebark Pine, Population Density, and Home-Range Size of Grizzly Bears in the Greater Yellowstone Ecosystem

Daniel D Bjornlie,Cecily M Costello,Mark A Haroldson,Michael R Ebinger,Frank T Van Manen,Daniel J Thompson,Gil Bohrer

doi:10.1371/journal.pone.0088160

Daniel D Bjornlie, Cecily M Costello + Show 5 more

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https://doi.org/10.1371/journal.pone.0088160

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Abstract

Changes in life history traits of species can be an important indicator of potential factors influencing populations. For grizzly bears (Ursus arctos) in the Greater Yellowstone Ecosystem (GYE), recent decline of whitebark pine (WBP; Pinus albicaulis), an important fall food resource, has been paired with a slowing of population growth following two decades of robust population increase. These observations have raised questions whether resource decline or density-dependent processes may be associated with changes in population growth. Distinguishing these effects based on changes in demographic rates can be difficult. However, unlike the parallel demographic responses expected from both decreasing food availability and increasing population density, we hypothesized opposing behavioral responses of grizzly bears with regard to changes in home-range size. We used the dynamic changes in food resources and population density of grizzly bears as a natural experiment to examine hypotheses regarding these potentially competing influences on grizzly bear home-range size. We found that home-range size did not increase during the period of whitebark pine decline and was not related to proportion of whitebark pine in home ranges. However, female home-range size was negatively associated with an index of population density. Our data indicate that home-range size of grizzly bears in the GYE is not associated with availability of WBP, and, for female grizzly bears, increasing population density may constrain home-range size.

Highlights

Following its listing as threatened under the Endangered Species Act in 1975, the grizzly bear (Ursus arctos) population in the Greater Yellowstone Ecosystem (GYE) grew from approximately 200–350 bears in the mid-1980s [1] to at least 600 in 2012 [2]
The random intercept term for individual bears ranged from s^dbear = 0.38 to s^dbear = 0.54 for the four dataset combinations based on sex and home-range metric (MCP and local convex hull (LCH)) for our most saturated model
Our findings suggest that for female grizzly bears in the GYE, home-range size is more strongly associated with population density than with availability of whitebark pine (WBP)

Summary

Introduction

Following its listing as threatened under the Endangered Species Act in 1975, the grizzly bear (Ursus arctos) population in the Greater Yellowstone Ecosystem (GYE) grew from approximately 200–350 bears in the mid-1980s [1] to at least 600 in 2012 [2]. Substantial decline of an important, high-elevation food source, whitebark pine (WBP; Pinus albicaulis) seeds, began in the early 2000s. This decline has been attributed to warming temperatures in alpine and montane systems of western North America, facilitating an irruption of native mountain pine beetles (Dendroctonus ponderosae) that has killed vast stands from New Mexico to Canada [6]. Regardless of cause, on monitoring transects established in the GYE, 27% of marked trees .1.4 m tall (all age classes) died during 2008–2013, 37% of tagged trees .10 cm and #30 cm in diameter, and 72% of trees $30 cm in diameter [7]

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