Changing diets over time: knock-on effects of marine megafauna overexploitation on their competitors in the South-Western Atlantic Ocean

This study compares the δ15N values and the trophic position of two seabird species throughout the Late Holocene in three regions in the South-Western Atlantic Ocean to assess the hypothesis that the decimation of megafauna lead to changes in the trophic position of mesopredators. Modern and ancient mollusc shells were also analysed to account for changes in the isotopic baseline through time. Results revealed that modern Magellanic penguins have higher δ15N values than their ancient conspecifics in the three regions, after controlling for changes in the isotopic baseline. This was also true for modern Imperial shags compared to ancient unidentified cormorants/shags from the two areas where ancient specimens were recovered (Southern Patagonia and the Beagle Channel). Such temporal variability might be caused by three non-mutually exclusive processes: decreased availability of pelagic squat lobster resulting from decreasing primary productivity through the Late Holocene, increased availability of small fishes resulting from the sequential depletion of other piscivores (South American fur seal and sea lion and Argentine hake) since the late 18th century and modification of the migratory patterns of Magellanic penguins. Although disentangling the relative contribution of all those processes is impossible at this time, the results reported here demonstrate that the ecology of Magellanic penguins and Imperial shags has undergone major changes since the Late Holocene.

本研究对比了西南大西洋（South-Western Atlantic Ocean）三个区域内两种海鸟物种在全新世晚期（Late Holocene）的δ¹⁵N（δ15N）值与营养级（trophic position），以验证“巨型动物群（megafauna）灭绝导致中型捕食者（mesopredators）营养级发生改变”这一假说。研究同时对现代与古代软体动物壳开展分析，以校正同位素基线（isotopic baseline）随时间的演变。结果表明，在校正同位素基线变化的影响后，现代麦哲伦企鹅（Magellanic penguins）在三个研究区域的δ¹⁵N值均高于其古代同种个体（conspecifics）。对于皇鸬鹚（Imperial shags）而言，在有古代标本出土的两个区域——巴塔哥尼亚南部（Southern Patagonia）与比格尔海峡（Beagle Channel），现代皇鸬鹚的δ¹⁵N值同样高于古代未鉴定的鸬鹚/皇鸬鹚标本。这种时间变异性可能由三种非互斥的过程所导致：一是全新世晚期初级生产力下降，使得远洋铠甲虾（pelagic squat lobster）的可获得性降低；二是自18世纪晚期以来，南美海狗（South American fur seal）、海狮（sea lion）与阿根廷无须鳕（Argentine hake）等食鱼动物被持续捕捞，导致小型鱼类的可获得性提升；三是麦哲伦企鹅的迁徙模式发生改变。尽管目前尚无法厘清所有这些过程的相对贡献，但本研究结果表明，自全新世晚期以来，麦哲伦企鹅与皇鸬鹚的生态状况已发生重大改变。