Implications of the variable availability of seasonal foods on the home ranges of black bears, Ursus americanus, in the Sierra Nevada of California

Abstract

Background: Sugar pine (Pinus lambertiana) seeds and oak (Quercus spp.) acorns are both important fall food sources for a variety of wildlife in the Sierra Nevada, but both have variable mast production and are in decline. Sugar pines are in decline due to white pine blister rust (Cronartium ribicola) infection and oaks are in decline due to fire exclusion and mule deer (Odocoileus hemionus) predation. To examine how a change in availability of seed and acorn crops from these trees could have a cascading effect on associated wildlife, we studied their relationship to black bear (Ursus americanus) fall ranges in Sequoia National Park, California. The distribution of seed-bearing sugar pines overlaps with the bears' summer range, whereas acorn-bearing oaks occur at lower elevations. We used GPS collars and field observations to collect location data on ten wild, adult, female bears during the summer and fall of both 2005 and 2006, and then compared these data on habitat use with the Park's vegetation map of available habitat. Results: Our results indicate that the inter-annual variability in the availability of these natural foods is closely related to the seasonal ranges of black bears. In the fall of 2005, when blue and black oak acorns were scarce but other acorns were abundant, bears remained within their summer ranges to feed on sugar pine seeds. In the fall of 2006, when blue oak acorns were abundant, bears shifted out of their summer ranges to feed on acorns and forego sugar pine seeds, even though the seeds were more abundant than in 2005. Incidents between humans and black bears were highest in 2006 while bears were moving between their summer ranges and the oak belt. Conclusion: In the fall, black bears make heavy use of both sugar pines and oaks. Although they prefer acorns to sugar pine seeds, the loss of either food source would lead to an increased dependence on the other. When both are unavailable due to continued decline or simultaneously low mast crops, an increase in human-bear incidents is likely.Keywords: Acorn production, Black bear, Blister rust, Home range, Preference, Sudden oak death, Sugar pine, Ursus americanus Background In California's Sierra Nevada, white pine blister rust (Cronartium ribicola) threatens the continued existence of sugar pines (Pinus lambertiana). This fungal pathogen appeared in the Southern Sierra in the 1960s, infecting many of the region's sugar pines, which are now in decline [1,2]. Oaks are declining due to fire exclusion and mule deer (Odocoileus hemionus) predation [3,4]. The pathogen that causes sudden oak death (Phytophthora ramorum) has not yet arrived in the Sierra, but if it does, it will also threaten the region's black oaks (Quercus kelloggii) [5]. A loss of sugar pines will likely have a cascading effect on the associated wildlife, especially if it coincides with a reduction in oaks, but with little existing research on these associations, the impacts are difficult to predict [6]. To begin addressing this gap in information, we studied the impo...

Measuring the realized niches of animals using stable isotopes: from rats to bears

“…Results from our analysis (Fig. 6) support the general notion that plants and acorns are major contributors to black bear nutrition in the Sierra Nevada of California (Graber & White 1983;Greenleaf et al 2009;Mazur, Klimley & Folger 2013).…”

“…) support the general notion that plants and acorns are major contributors to black bear nutrition in the Sierra Nevada of California (Graber & White ; Greenleaf et al . ; Mazur, Klimley & Folger ). In spring, plants are higher in protein content and lower in fibre than later in the year (Mealey ) and herbage is virtually unlimited to black bears, providing the protein necessary for their growth in spring (Noyce & Garshelis ).…”

“…). Unfortunately for black bears, however, both oaks (acorns) and sugar pines (pine nuts) are hard mast species with unpredictable availability in the Sierra Nevada and are threatened by white pine blister rust ( Cronartium ribicola ) and both mule deer foraging and fire exclusion, respectively (Mazur, Klimley & Folger ); in addition, sudden oak death ( Phytophthora ramorum ) could threaten oaks if this disease moves in from coastal California. Female bears move great distances (Koike et al .…”

“…Female bears move great distances (Koike et al . ) and have high activity levels (Garshelis & Pelton ), especially during low mast years (Mazur, Klimley & Folger ). High levels of activity by lactating black bears suggest that increased foraging efforts are required to meet the additional energy and protein requirements for lactation and cub growth (Eagle & Pelton ).…”

Animal Biotelemetry: First anniversary

“…We have published original research articles on the physiology, behavior, and ecology of diverse species ranging from Chinook salmon [1], European silver eels [2], white sharks [3], bluetongue lizards [4] , bowhead whales [5], and brown bears [6]. We also have published a variety of techniques-based articles on ultraviolet radiation for sterilization of surgical tools [7], different types of tissueclosure sutures during tag implantation [8], tag attachment designs [9], and criteria for determining whether a tag has been released and is floating on the surface versus being attached to a whale shark [10].…”