Data from: Shifting niches of marine predators due to human exploitation: the diet of the South American sea lion (Otaria flavescens) since the late Holocene as a case study

Stable isotope ratios of carbon and nitrogen in archaeological and modern bone samples have been used to reconstruct the dietary changes of the South American sea lion Otaria flavescens from the late Holocene to the present in the southwestern Atlantic. We sampled bones from archaeological sites in northern-central and southern Patagonia, Argentina, and bones housed in modern scientific collections. Additionally, we analyzed the stable isotope ratios in ancient and modern shells of intertidal molluscs to explore changes in the isotope baseline and allow comparison between bone samples from different periods after correction for baseline shifts. Results confirmed the trophic plasticity of the South American sea lion, demonstrated the much larger impact of modern exploitation of marine resources as compared with that of hunter-gatherers, and underscored the dissimilarity between the past and modern niches of exploited species. These conclusions are supported by the rather stable diet of South American sea lions during several millennia of aboriginal exploitation, in both northern-central and southern Patagonia, and the dramatic increase in trophic level observed during the twentieth century. The recent increase in trophic level might be related to the smaller population size resulting from modern sealing and the resulting reduced intraspecific competition. These results demonstrate how much can be learned about the ecology of modern species thanks to retrospective studies beyond the current, anthropogenically modified setting where ecosystem structure is totally different from that in the pristine environments where current species evolved.本研究通过分析考古及现代骨骼样本中的碳、氮稳定同位素比值（stable isotope ratios），重建了西南大西洋海域全新世晚期至今南美海狮（Otaria flavescens）的食性变化历程。我们采集了阿根廷中北部巴塔哥尼亚与南部巴塔哥尼亚考古遗址中的骨骼，以及现代科研馆藏的骨骼样本。此外，我们还分析了古、现代潮间带软体动物壳体的稳定同位素比值，以探究同位素基线（isotope baseline）的变化情况，从而在校正基线偏移（baseline shifts）后，实现不同时期骨骼样本的比对分析。研究结果证实了南美海狮的营养可塑性（trophic plasticity），证明现代海洋资源开发对其的影响远大于史前狩猎采集者（hunter-gatherers），并凸显了开发利用物种的历史与现代生态位之间的显著差异。在巴塔哥尼亚中北部与南部地区，历经数千年土著开发活动期间，南美海狮的食性始终保持相对稳定，而20世纪其营养级（trophic level）出现了显著上升，这一现象为上述结论提供了支撑。近期营养级的上升，可能与现代海狮捕猎导致的种群规模缩小、进而种内竞争（intraspecific competition）减弱有关。本研究结果表明，相较于当前受人为改造、生态系统结构与物种演化原始环境截然不同的生境，通过回溯性研究可极大拓展我们对现代物种生态学的认知。