Data from: Complementary food resources of carnivory and frugivory affect local abundance of an omnivorous carnivore

A major unresolved question for omnivorous carnivores, like most species of bears, is to what degree are populations influenced by bottom–up (food supply) or top–down (human-caused mortality) processes. Most previous work on bear populations has focused on factors that limit survival (top–down) assuming little effect of food resource supply. When food resources are considered, most often they consider only the availability/supply of a single resource, particularly marine-subsidized or terrestrial sources of protein (carnivory) or alternately hard or soft mast (frugivory). Little has been done to compare the importance of each of these factors for omnivorous bears or test whether complementary resources better explain individual animal and population measures such as density, vital rates, and body size. We compared landscape patterns of digestible energy (kcal) for buffaloberry (a key source of carbohydrate) and ungulate matter (a key source of protein and lipid) to local measures in grizzly bear Ursus arctos abundance at DNA hair snag sites in west-central Alberta, Canada. We tested support for bottom–up hypotheses in either single (carnivory [meat] versus frugivory [fruit]) or complementary (additive or multiplicative) food resources, while accounting for a well-known top–down limiting factor affecting bear survival (road density). We found support for both top–down and bottom–up factors with complementary resources (co-limitation) supported over single resource supplies of either meat or fruit. Our study suggests that the availability of food resources that provide complementary nutrients is more important in predicting local bear abundance than single foods or nutrients (e.g. protein) or simply energy per se. This suggests a nutritionally multidimensional bottom–up limitation for a low density interior population of grizzly bears.对于多数熊类这类杂食性食肉动物而言，一个尚未解决的核心科学问题是：种群数量在多大程度上受自下而上（bottom–up，即食物供给）或自上而下（top–down，即人为致死）过程的调控。此前针对熊类种群的多数研究均聚焦于限制种群存活的自上而下（top–down）因子，且默认食物资源供给对种群的影响微乎其微。即便考虑食物资源，多数研究也仅关注单一资源的可获得性或供给量，尤其是海洋支撑型或陆地来源的蛋白质（食肉性（carnivory）），或是硬果类（hard mast）与软果类（soft mast）（食果性（frugivory））资源。目前鲜有研究针对杂食性熊类比较各类因子的重要性，也未验证互补资源是否能更好地解释个体及种群层面的指标，如种群密度、生命率与体型大小。 我们以加拿大阿尔伯塔省中西部地区的DNA毛发采样（DNA hair snag）位点为研究场景，比较了水牛果（buffaloberry，一种关键碳水化合物来源）与有蹄类动物组织（ungulate matter，一种关键蛋白质与脂质来源）的可消化能量（千卡，kcal）空间格局，并结合当地灰熊（Ursus arctos）的种群丰度实测数据进行分析。我们分别检验了单一资源（食肉性（肉食）与食果性（果实））以及互补资源（加和或相乘效应）对应的自下而上调控假说的支持程度，同时纳入了已知的影响熊类存活的自上而下限制因子——道路密度（road density）。 研究结果同时支持自上而下与自下而上调控因子的作用，且相较于单一的肉食或果实资源供给，互补资源的协同限制（co-limitation）效应得到了更多数据支持。本研究表明，相较于单一食物或营养物质（如蛋白质），或是单纯的能量本身，能够提供互补营养的食物资源可获得性，在预测当地熊类种群丰度方面更为关键。这表明，低密度内陆灰熊种群受到营养维度多元的自下而上调控限制。