Data from: Isotopic niche partitioning between two apex predators over time

1. Stable isotope analyses have become an important tool in reconstructing diets, analyzing resource use patterns, elucidating trophic relations among predators and understanding the structure of food webs. 2. Here, we use stable carbon and nitrogen isotope ratios in bone collagen to reconstruct and compare the isotopic niches of adult South American fur seals (Arctocephalus australis); n = 86) and sea lions (Otaria flavescens); n = 49) –two otariid species with marked morphological differences– in the Río de la Plata estuary (Argentina - Uruguay) and the adjacent Atlantic Ocean during the second half of the 20th century and the beginning of the 21st century. Samples from the middle Holocene (n = 7 fur seals and n = 5 sea lions) are also included in order to provide a reference point for characterizing resource partitioning before major anthropogenic modifications of the environment. 3. We found that the South American fur seals and South American sea lions had distinct isotopic niches during the middle Holocene. Isotopic niche segregation was similar at the beginning of the second half of the 20th century, but has diminished over time. 4. The progressive convergence of the isotopic niches of these two otariids during the second half of the 20th century and the beginning of the 21st century is most likely due to the increased reliance of South American fur seals on demersal prey. 5. This recent dietary change of South American fur seals can be explained by at least two non-mutually exclusive mechanisms: (i) the decrease in the abundance of sympatric South American sea lions as a consequence of small colony size and high pup mortality resulting from commercial sealing; and (ii) the decrease in the average size of demersal fishes due to intense fishing of the larger class sizes, which may have increased their accessibility to those eared seals with a smaller mouth gape, i.e., South American fur seals of both sexes and female South American sea lions.

1. 稳定同位素分析（Stable isotope analyses）已成为重建物种饮食组成、解析资源利用模式、阐明捕食者间营养级关系以及理解食物网结构的重要研究工具。
2. 本研究利用骨胶原（bone collagen）中的稳定碳、氮同位素比值，重建并比较了20世纪下半叶至21世纪初，拉普拉塔河口（阿根廷-乌拉圭交界）及邻近大西洋海域内，成年南美海狗（Arctocephalus australis，样本量n=86）与南美海狮（Otaria flavescens，样本量n=49）——两种形态特征差异显著的海狮科物种——的同位素生态位（isotopic niche）。同时纳入中全新世样本（南美海狗7例、南美海狮5例），以提供人类活动大规模改造环境前的资源分化参照基准。
3. 研究发现，中全新世时期南美海狗与南美海狮拥有截然不同的同位素生态位；20世纪下半叶初期，二者的同位素生态位分化程度相似，但随时间推移逐步减弱。
4. 这两种海狮科物种的同位素生态位在20世纪下半叶至21世纪初逐步趋同，其核心成因大概率为南美海狗对底栖猎物的依赖程度显著升高。
5. 南美海狗近期的这种食性转变，可通过至少两种非互斥机制解释：（i）商业海豹捕猎活动导致同域分布的南美海狮种群规模偏小、幼崽死亡率偏高，进而造成其种群数量下降；（ii）高强度捕捞移除了体型较大的底栖鱼类个体，使得底栖鱼类平均体型降低，这提升了口裂更小的海狮类群的可获取性，即两性南美海狗以及雌性南美海狮。