Black bear density and habitat use variation at the Sierra Nevada‐Great Basin Desert transition

Abstract

In the first 2 decades of the twenty-first century, American black bear (Ursus americanus) populations rebounded with range expansions into areas where the species was previously extirpated. While there are a number of factors that limit range expansion, habitat quality and availability are among the most important. Such factors may be particularly important in western Nevada, USA, at the transition zone of the Sierra Nevada and the Great Basin Desert. We deployed a multifaceted data collection system including motion-sensitive cameras, noninvasive hair sampling and genotyping, and global positioning system (GPS) tracking. We analyzed data using spatial capture-recapture to estimate population density and dynamic occupancy models to estimate habitat use. Black bear habitat use and density were substantially higher in the Sierra Nevada than the Great Basin Desert and had strong positive relationships with the presence of conifer land cover in the transition zone. The average black bear density was >4 times higher in the mixed-conifer forests of the Sierra Nevada (12.4 bears/100 km 2 ) than in desert mountain ranges with piñon (Pinus monophylla)-juniper (Juniperus spp.) woodland (2.7 bears/100 km 2 ). The low-elevation shrub and grassland portions of the study area had even lower estimated black bear density (0.6 bears/100 km 2 ) and probability of use (0.03, 95% CI = 0.00-0.09). Across these spatially variable configurations in black bear density, we estimated the population size to be

Spatial patterns of reproduction suggest marginal habitat limits continued range expansion of black bears at a forest‐desert ecotone

“…For example, considering sites with 50% habitat within 5 km, estimates of conditional reproduction probability indicate only one third of such sites occupied by adults would also be occupied by cubs in the Great Basin (R i = 0.32); whereas nearly all such sites in the Sierra Nevada occupied by adults would also be occupied by cubs (R i = 0.92). Previous analyses focused on spatial patterns of population density in this population likewise demonstrated lower black bear density in piñon-juniper woodland even after taking into account the amount of habitat (Sultaire et al, 2023). Taken together, these results suggest that habitat on the edge of black bear range in the western Great Basin is ecologically marginal for the species compared to core habitats in the Sierra Nevada.…”

“…Consistent with patterns elsewhere in their range, black bears have expanded their range from the Sierra Nevada into the western Great Basin since the late 1900s (Lackey et al, 2013); however, the geographic extent of their range in Nevada has not changed appreciably since the early 2000s (Lackey, 2004;Sultaire et al, 2023).…”

“…Average annual precipitation varies from 1400 mm/year in the higher elevations of the Sierra Nevada to 180 mm in low elevation desert (PRISM Climate Group, 2021 ). The region is mountainous with elevations ranging from approximately 1100 to 3400 m. Black bears recolonized the semi‐arid mountain ranges in the Great Basin from refugial populations in the Sierra Nevada during the late 1900s (Lackey et al., 2013 ; Malaney et al., 2018 ); however, their range has been fairly static since the early 2000s (Sultaire et al., 2023 ). The greater Sierra Nevada population could be considered a peripheral extension of black bear range from the north where populations are more continuous.…”

“…During the summer months (June–August 2018–19, July–September 2020) sites were visited every 7–10 days to retrieve photos from cameras and apply a scent attractant within the viewshed of the camera. During this time, barbed‐wire hair snags were also deployed in camera viewsheds to collect hair for genetic identification of individuals (Sultaire et al., 2023 ). Because we could not distinguish adults and cubs from genetic data, we do not consider this data source in this manuscript.…”

“…To capture potential heterogeneity in detection probability driven by variation in black bear site use frequency, as measured at the temporal resolution of 30-day secondary periods, we included the effect of proportion habitat (conifer forest and piñon-juniper) within a 1-km buffer of sites on detection probability (Sultaire et al, 2023).…”