The distribution of the Greater Yellowstone Ecosystem grizzly bear (Ursus arctos) population has expanded into areas unoccupied since the early 20th century. Up‐to‐date information on the area and extent of this distribution is crucial for federal, state, and tribal wildlife and land managers to make informed decisions regarding grizzly bear management. The most recent estimate of grizzly bear distribution (2004) utilized fixed‐kernel density estimators to describe distribution. This method was complex and computationally time consuming and excluded observations of unmarked bears. Our objective was to develop a technique to estimate grizzly bear distribution that would allow for the use of all verified grizzly bear location data, as well as provide the simplicity to be updated more frequently. We placed all verified grizzly bear locations from all sources from 1990 to 2004 and 1990 to 2010 onto a 3‐km × 3‐km grid and used zonal analysis and ordinary kriging to develop a predicted surface of grizzly bear distribution. We compared the area and extent of the 2004 kriging surface with the previous 2004 effort and evaluated changes in grizzly bear distribution from 2004 to 2010. The 2004 kriging surface was 2.4% smaller than the previous fixed‐kernel estimate, but more closely represented the data. Grizzly bear distribution increased 38.3% from 2004 to 2010, with most expansion in the northern and southern regions of the range. This technique can be used to provide a current estimate of grizzly bear distribution for management and conservation applications. © 2013 The Wildlife Society.
Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km 2 of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height. 2 m and diameter at breast height , 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems. 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P 5 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P 5 0.0001), conifer establishment (P 5 0.0001), and cattle (Bos spp.) grazing (P 5 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2-10 km 2 areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time. Resumen Se muestreó el reclutamiento y la densidad de tallos aéreos de Populus tremuloides Michx. de 315 clones en 1991 y 2006 en un área de 560 km 2 en la Invernada Norte de Yellowstone (NYWR). Un objetivo primario de este estudio fue observar si el estatus de P. tremuloides mejoró entre 1991 y 2006 evidenciando una cascada trófica causada por el lobo (Canis lupus). El reclutamiento de tallos nuevos (altura. 2 m y diámetro a la altura del pecho , 5 cm) representa crecimiento de brotes recientes por encima de la línea de ramoneo de los ciervos (Cervus elaphus), mientras que los tallos aéreos (todos los tallos. 2 m) representan la cohorte de tallos que asegurarán la sustentabilidad del clon. Las densidades de tallos aéreos decreció en un 12% (P 5 0.04) a la escala de paisaje según una comparación realizada con un prueba de ''t'' apareada. Los tallos aéreos decrecieron en un 58% de los clones individuales y en un 63% de las 24 micro-cuencas del área de estudio. El segundo objetivo de este estudio fue determinar cuáles factores influenciaron los cambios en la densidad de P. tremuloides. El ramoneo invernal de los ungulados (P 5 0.0001), el establecimiento de coníferas (P 5 0.0001), y el pastoreo bovino (Bos spp.; P 5 0.016) contribuyeron a la disminución en la densidad de tallos aéreos según un análisis de efectos mixtos utilizando una transformación logarítmica de las medianas. El 80% de los clones fueron clasificados en la categoría de nivel de uso medio a elevado...
Aspen (Populus tremuloides) on the northern Yellowstone winter range has declined over the last half-century. Beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991 in an attempt to reverse this trend. In 2005, we assessed the efficacy of this project by quantifying the long-term effects of beaver on aspen stands and the riparian area in this drainage. Between 1990 and 2005, the canopy cover of mature aspen decreased more than 62%, whereas immature aspen cover more than tripled, resulting in a total aspen canopy cover decrease (p < 0.05) from 43 to 25% (a loss of 7.25 ha). Willow canopy cover increased from 10 to 14% during the same period. The impacts of beaver on aspen stands were estimated by comparing vegetative changes among control sites with less than 10% beaver use (n = 5), active beaver sites (n = 6), sites inactive for 1-3 years (n = 7), sites inactive for 4-6 years (n = 4), and sites inactive for 7-11 years (n = 5). Aspen sprout and sapling densities were greater (p = 0.01) on sites which were active and inactive for 1-3 years compared to the other sites. Aspen ramets were not able to grow taller than 2 m on sites without beaver activity for 4-11 years due to ungulate herbivory. Although beaver stimulated the growth of aspen sprouts and saplings, ungulate herbivory prevented successful aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range 14 years after beaver reintroduction.
Past research indicates that whitebark pine seeds are a critical food source for Threatened grizzly bears (Ursus arctos) in the Greater Yellowstone Ecosystem (GYE). In recent decades, whitebark pine forests have declined markedly due to pine beetle infestation, invasive blister rust, and landscape-level fires. To date, no study has reliably estimated the contribution of whitebark pine seeds to the diets of grizzlies through time. We used stable isotope ratios (expressed as δ13C, δ15N, and δ34S values) measured in grizzly bear hair and their major food sources to estimate the diets of grizzlies sampled in Cooke City Basin, Montana. We found that stable isotope mixing models that included different combinations of stable isotope values for bears and their foods generated similar proportional dietary contributions. Estimates generated by our top model suggest that whitebark pine seeds (35±10%) and other plant foods (56±10%) were more important than meat (9±8%) to grizzly bears sampled in the study area. Stable isotope values measured in bear hair collected elsewhere in the GYE and North America support our conclusions about plant-based foraging. We recommend that researchers consider model selection when estimating the diets of animals using stable isotope mixing models. We also urge researchers to use the new statistical framework described here to estimate the dietary responses of grizzlies to declines in whitebark pine seeds and other important food sources through time in the GYE (e.g., cutthroat trout), as such information could be useful in predicting how the population will adapt to future environmental change.
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