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.

专性植食动物（obligate herbivores）在气候变化对哺乳动物影响的相关研究中占据主导地位，但目前关于植物食物的丰度或质量变化如何影响哺乳动物杂食动物的实证证据十分有限。杂食动物全年可利用多种不同食物资源，但在关键生命阶段，它们往往依赖具有季节性限制特征且营养高度富集的果实。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，评估了气温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，越橘（bilberry, Vaccinium myrtillus）与红豆越橘（lingonberry, V. vitis-idaea）的浆果产量存在显著年际波动，该波动在一定程度上可归因于植物休眠期与开花期的气温，以及果实成熟期的降水量。雌性棕熊的秋季体重与一岁幼熊的春季体重，均随越橘丰度呈线性增长趋势。当越橘丰度较低时，体况较差的雌性棕熊的繁殖成功率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，该效应便会消失，这表明在越橘丰度较高的年份，体况较差的雌性个体可弥补初始体重的不足。本研究强调，需关注个体对食物可获得性变化的动态响应——这种响应会使部分个体比其他个体更易遭受食物短缺的影响。个体在响应资源变化时的生活史异质性，可能会对种群长期补充量产生影响。研究结果表明，斯堪的纳维亚棕熊在一年中的关键时期可能仅依赖单一食物资源，因此其应对环境变化的恢复力低于杂食动物的预期水平。未来气候情景下，影响浆果物候关键阶段的气象协变量呈现出模糊的变化趋势，这使得我们无法明确预测气候变化将如何长期影响越橘的产量。