Data from: Berry production drives bottom-up effects on body mass and reproductive success in an omnivore

Obligate herbivores dominate studies of the effects of climate change on mammals, however there is limited empirical evidence for how changes in the abundance or quality of plant food affect mammalian omnivores. Omnivores can exploit a range of different food resources over the course of a year, but they often rely on seasonally restricted highly nutritious fruiting bodies during critical life stages. Brown bears Ursus arctos in Sweden are dependent on berries for fattening before entering hibernation. We used a ten-year time series to evaluate the effect of temperature and snow on annual variation in berry abundance and how this variation affected bears. We found marked interannual variation in berry production of bilberry Vaccinium myrtillus and lingonberry V. vitis-idaea, that we could attribute in part to temperature during plant dormancy and flowering and precipitation during fruit ripening. Both, autumn weights of female bears and spring weights of yearling bears increased linearly with bilberry abundance. When bilberry abundance was low, lightweight female bears had a lower reproductive success than females in better condition. This effect vanished when food abundance was above average, indicating that lightweight females could compensate for their initial weight during good bilberry years. Our study highlights the importance of considering individuals’ dynamic responses to variation in food availability, which leave some more vulnerable to food shortage than others. Individual life-history heterogeneity in response to resource variation likely affects long-term population recruitment. Our findings emphasize that Scandinavian bears can be dependent on a single food resource during a critical period of the year and are therefore less resilient to environmental change than expected for an omnivore. Future climate scenarios predict ambiguous trends for weather covariates that affected crucial stages of berry phenology, preventing a clear prognosis of how climate change may affect long-term bilberry production.

当前有关气候变化对哺乳动物影响的研究，多以专性植食动物（obligate herbivores）为核心研究对象，但针对植物食物资源丰度与品质变化如何影响哺乳类杂食动物（omnivores）的实证证据仍较为匮乏。杂食动物全年可利用多种不同的食物资源，但在关键生命阶段，它们通常依赖于受季节限制的高营养结实体。瑞典境内的棕熊（Ursus arctos）在进入冬眠前，需依赖浆果积累脂肪。本研究依托十年时间序列数据，评估了温度与积雪对浆果丰度年际变化的影响，以及该变化如何作用于棕熊种群。研究发现，欧洲越橘（Vaccinium myrtillus）与红豆越橘（Vaccinium vitis-idaea）的浆果产量存在显著年际波动，该波动部分可归因于植物休眠与开花期的温度条件，以及果实成熟期的降水状况。雌性棕熊的秋季体重与一岁龄幼熊的春季体重，均随欧洲越橘丰度呈线性增长趋势。当欧洲越橘丰度较低时，体况较差的雌性棕熊的繁殖成功率（reproductive success）显著低于体况良好的雌性个体。当食物丰度高于平均水平时，该效应消失，表明在欧洲越橘丰收年份，体况较差的雌性个体可弥补初始体况的不足。本研究强调，需关注个体对食物资源可获得性变化的动态响应——此类响应使得部分个体相较于其他个体更易遭受食物短缺的威胁。个体在响应资源变化时展现出的生活史异质性（life-history heterogeneity），或会对长期种群补充（population recruitment）过程产生影响。本研究结果进一步表明，斯堪的纳维亚棕熊在年度关键阶段可依赖单一食物资源，因此作为杂食动物，其对环境变化的恢复力低于此前预期。未来气候情景（climate scenarios）对影响浆果物候（berry phenology）关键阶段的气象协变量（weather covariates）的预测趋势尚不明确，这使得我们无法清晰预判气候变化将如何长期影响欧洲越橘的产量。