Geographical Area and Life History Traits Influence Diet in an Arctic Marine Predator

Global changes are thought to affect most Arctic species, yet some populations are more at risk. Today, the Barents Sea ecoregion is suffering the strongest sea ice retreat ever measured; and these changes are suspected to modify food access and thus diet of several species. Biochemical diet tracers enable investigation of diet in species such as polar bears (Ursus maritimus). We examined individual diet variation of female polar bears in Svalbard, Norway, and related it to year, season (spring and autumn), sampling area and breeding status (solitary, with cubs of the year or yearlings). Sampling areas were split according to their ice cover: North-West (less sea ice cover), South-East (larger amplitude in sea ice extent) and North-East/South-West (NESW) as bears from that zone are more mobile among all regions of Svalbard. We measured fatty acid (FA) composition in adipose tissue and carbon (δ13C) and nitrogen (δ15N) stable isotopes in plasma and red blood cells. Females feeding in the North-West area had lower δ15N values than those from the NESW. In South-East females, δ13C values were lower in autumn compared to spring and females seemed less selective in their diet as depicted by large variances in stable isotope values. Considering the differences in FA composition and stable isotope values, we suggest that females from the North-West and South-East could ingest a higher proportion of avian prey. With regard to breeding status, solitary females had higher δ15N values and smaller variance in their stable isotopic values than females with cubs, suggesting that solitary females were more selective and prey on higher trophic level species (i.e. seals). Overall, our results indicate that prey availability for Svalbard polar bears varies according to geographical area and prey selectivity differs according to breeding status. Our findings suggest that complex changes in sea ice and prey availability will interact to affect Svalbard polar bear feeding patterns and associated nutrition.

学界普遍认为全球变化会影响多数北极物种，但部分种群面临的风险更高。当前，巴伦支海生态区（Barents Sea ecoregion）正经历有记录以来最为剧烈的海冰消退；这类变化被认为会改变多种物种的食物获取途径，进而影响其食性。生化食性示踪剂（biochemical diet tracers）可用于探究北极熊（Ursus maritimus）等物种的食性。本研究针对挪威斯瓦尔巴群岛的雌性北极熊个体食性差异展开分析，并将其与采样年份、季节（春季与秋季）、采样区域以及繁殖状态（独居、带当年新生幼崽或带一岁龄幼崽）进行关联。采样区域依据海冰覆盖特征划分为三类：西北海域（海冰覆盖度更低）、东南海域（海冰范围波动幅度更大）以及东北/西南海域（NESW）——该区域的北极熊在斯瓦尔巴群岛所有区域内的移动性均更强。我们对北极熊脂肪组织中的脂肪酸（FA）组成，以及血浆和红细胞中的碳稳定同位素（δ¹³C）与氮稳定同位素（δ¹⁵N）进行了检测。栖息于西北海域的雌性北极熊，其δ¹⁵N值显著低于NESW海域的个体。东南海域的雌性北极熊秋季的δ¹³C值较春季更低，且其食性选择似乎更不专一，这一点可通过稳定同位素值的较大方差得到体现。结合脂肪酸组成与稳定同位素值的差异，我们推测西北与东南海域的雌性北极熊可能摄入了更高比例的鸟类猎物。就繁殖状态而言，独居雌性北极熊的δ¹⁵N值高于带幼崽的个体，且其稳定同位素值的方差更小，这表明独居雌性的食性选择更具专一性，且捕食的猎物营养级更高（即海豹）。总体而言，本研究结果表明，斯瓦尔巴群岛北极熊的猎物可获取性随地理区域而异，且猎物选择的专一性也因繁殖状态不同而存在差异。本研究结果还显示，海冰与猎物可获取性的复杂变化将相互作用，进而影响斯瓦尔巴群岛北极熊的摄食模式与相关营养状况。