Patrick J. Jackson’s research while affiliated with Nevada Department of Wildlife and other places

Following gray wolf (Canis lupus) reintroduction to Yellowstone National Park and central Idaho, USA, in the mid‐1990s, the species range has expanded into western Montana, eastern Oregon, and eastern Washington. By 2011, wolves reached northern California and formed multiple packs within a decade of their arrival in the state. Gray wolf observations have been sporadic, however, in the comparatively open and nonforested ecosystems such as the nearby northern Great Basin. During a broad‐scale, camera‐trapping study, we detected a gray wolf on an unbaited camera trap in northwest Nevada. This observation represents the 2nd confirmed sighting of a gray wolf in the state of Nevada since the 1920s and the first documented camera trap detection for the species in the state. We discuss this observation in the context of historical gray wolf presence in Nevada and the potential for the species to establish in the northern Great Basin.

Globally, animals that are range-restricted are frequently becoming species of conservation concern, in part due to competitive exclusion by phylogenetically and ecologically similar species that are more tolerant of human disturbance. However, climate and land use changes to natural landscapes can create pockets of refugia for range-restricted species. Western gray squirrels (Sciurus griseus) are native to the west coast of North America, principally California and western Oregon. Over the past several decades, Western Gray Squirrel populations have declined in human-dominated areas, with increased competition from introduced congeneric species native to eastern North America cited as a primary driver. Despite declines in their established range west of the Pacific Crest in western North America, western gray squirrels are extending their range into the Great Basin, where they were not historically found. Using a network of remote camera traps deployed across the Sierra Nevada–Great Basin ecotone in northwestern Nevada, we detected western gray squirrels across 16 of 100 camera-trapping sites. The majority of detections were located in piñon–juniper woodland, a land cover type not previously occupied by this species. Occupancy modeling revealed that western gray squirrels were equally likely to occur in piñon–juniper woodland compared to mature pine forest that they occupy elsewhere in their range. A species distribution model parameterized with historical gray squirrel observations (pre-1950), indicated increased climatic suitability for the species on the eastern side of the Sierra Nevada in recent decades, which may have facilitated this range expansion. Our findings reveal the potential for species declining in their historical range to colonize novel habitats that become increasingly suitable as a result of human-driven changes to ecosystems.

Investigating spatial patterns of animal occupancy and reproduction in peripheral populations can provide insight into factors that form species range boundaries. Following historical extirpation, American black bears ( Ursus americanus ) recolonized the western Great Basin in Nevada from the Sierra Nevada during the late 1900s. This range expansion, however, has not continued further into the Great Basin despite the presence of additional habitat. We aimed to quantify whether reduced reproduction toward the range edge contributes to this range boundary. We analyzed black bear detections from 100 camera traps deployed across black bear distribution in western Nevada using a multistate occupancy model that quantified the probability of occupancy and reproduction (i.e., female bears with cubs occupancy) in relation to changes in habitat type and habitat amount toward the range boundary. We detected a strong effect of habitat amount and habitat type on the probability of black bear occupancy and reproduction. At similar levels of landscape‐scale habitat amount (e.g., 50%), estimated probability of occupancy for adult bears in piñon‐juniper woodlands near the range boundary was 0.39, compared to ~1.0 in Sierra Nevada mixed‐conifer forest (i.e., core habitat). Furthermore, estimated probability of cub occupancy, conditional on adult bear occupancy, in landscapes with 50% habitat was 0.32 in Great Basin piñon‐juniper woodlands, compared to 0.92 in Sierra Nevada mixed‐conifer forest. Black bear range in the western Great Basin conforms to the center–periphery hypothesis, with piñon‐juniper woodland at the range edge supporting ecologically marginal habitat for the species compared to habitat in the Sierra Nevada. Further geographic expansion of black bears in the Great Basin may be limited by lower occupancy of reproducing females in piñon‐juniper woodland. Center–periphery range dynamics may be common in large carnivore species, as their dispersal ability allows them to colonize low‐quality habitat near range edges.

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 multi‐faceted 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 418 individuals (95% CI = 239–740). Declining density towards the range edge, coupled with a relatively stable range of black bears in Nevada observed since 2000, suggests that further species range expansion into the western Great Basin may be limited by habitat quality and availability.

Changes in topography, such as terrain elevation and slope, are an important source of landscape complexity influencing the ecology of animals, particularly in mountainous landscapes. In such landscapes animals navigate changes in elevation and slope in their daily movement. Despite the importance of topographic variation, studies of animal ecology in mountainous landscapes tend not to explicitly consider those effects on species detection. We deployed a broad‐extent, coarse resolution camera‐trapping system across a landscape with considerable complexity and quantified the influence of topographic variables on detection probability conditional on occurrence for multiple mammal species. Specifically, we examined the fine‐scale effects of terrain steepness and topographic position (i.e. ridges, mid‐slopes or valleys) on detection probability for 14 mammal species at camera‐traps. We found that detection probability increased on gently sloping terrain for six species and decreased on the steepest slopes sampled for three of these species and three additional species. Among four other mammal species, detection probability changed according to local topographic position though the directionality of these responses varied among these species. Several species, primarily meso‐carnivores as well as larger‐bodied species, like mule deer and black bears, were more detectable on gentle slopes than flat terrain. This pattern suggests that many species may use moderately steep terrain for the resources or heterogeneity they provide. Topographic position had comparatively less effect on species detection probabilities, suggesting that this variable does not have a strong effect on fine‐scale space use of animal species in mountainous regions. These relationships suggest that researchers should consider local terrain when siting camera traps in mountainous landscapes and analyzing survey data from such landscapes. Studies that compare the detection of mammal species at cameras deployed in close proximity will improve our understanding of fine‐scale topographic effects on mammal movement and detection.

Apex predators can shape communities via cascading top–down effects, but the degree to which such effects depend on predator life history traits is largely unknown. Within carnivore guilds, complex hierarchies of dominance facilitate coexistence, whereby subordinate species avoid dominant counterparts by partitioning space, time, or both. We investigated whether a major life history trait (hibernation) in an apex carnivore (black bears Ursus americanus) mediated its top–down effects on the spatio-temporal dynamics of three sympatric mesocarnivore species (coyotes Canis latrans, bobcats Lynx rufus, and gray foxes Urocyon cinereoargenteus) across a 15,000 km² landscape in the western USA. We compared top–down, bottom–up, and environmental effects on these mesocarnivores using an integrated modeling approach. Black bears exerted top–down effects that varied as a function of hibernation and were stronger than bottom–up or environmental impacts. High black bear activity in summer and fall appeared to buffer the most subordinate mesocarnivore (gray foxes) from competition with dominant mesocarnivores (coyotes and bobcats), which were in turn released by black bear hibernation in winter and early spring. The mesocarnivore responses occurred in space (i.e., altered occupancy and site visitation intensity) rather than time (i.e., diel activity patterns unaffected). These results suggest that the spatio-temporal dynamics of mesocarnivores in this system were principally shaped by a spatial predator cascade of interference competition mediated by black bear hibernation. Thus, certain life history traits of apex predators might facilitate coexistence among competing species over broad time scales, with complex implications for lower trophic levels.