An ice-obligate seabird responds to a multi-decadal decline in Arctic sea ice - population matrix model

The Arctic has experienced greatly decreased sea ice and increased ocean temperatures in recent decades but there is a paucity of biological time-series data allowing assessment of resulting temporal variation in the region’s marine ecosystems. Seabirds, as highly mobile and highly visible, upper-trophic level predators, can be valuable monitors of modifications in marine ecosystems, especially for regions lacking commercial fisheries or regular oceanographic sampling. Since 1975, we have studied annually an Arctic Alaskan colony of Mandt’s black guillemot (Cepphus grylle mandtii), an ice-obligate diving seabird, specializing on Arctic cod (Boreogadus saida), the primary forage fish of the ice-associated cryopelagic ecosystem. Using multistate capture–mark–recapture models, matrix population models and perturbation analysis we quantified the environmental and demographic drivers of population change from 1980-2019 for the individually-marked population. The colony increased rapidly, from <20 to >200 breeding pairs from 1975-1990 in response to increased availability of nesting cavities, before experiencing intermittent declines to <50 pairs in 2021. Immigration and apparent survival were the primary demographic parameters affecting population growth with sea ice extent in late summer and fall the primary environmental driver. The initial growth occurred during a period of primarily negative winter Arctic Oscillations (WAO) and extensive summer sea ice. The decline began when an extremely positive WAO in 1989/90 initiated changes in atmospheric and oceanographic circulation causing major reductions in summer sea ice throughout the region. The three-decade decline in the population saw plateaus or minor growth with increasing frequency of negative WAOs and increasing declines following two previously identified “tipping points” in sea ice loss. Breeding success at the study colony declined with decreased availability of Arctic cod due to sea ice loss and increasing SST and is presumed to have occurred at the source colonies for immigrants where similar oceanographic changes were occurring. Quasi-extinction of the colony (reduction to <25 pairs) is predicted within the next two decades. The sensitivity of Mandt’s black guillemot to multi-decadal changes in the Arctic’s cryopelagic ecosystem makes it an excellent sentinel species for the region with its recent collapse having dire implications for the Arctic Ocean’s constituent species.

近几十年来，北极地区海冰大幅减少、海洋温度持续升高，但目前生物时序数据（biological time-series data）的匮乏，制约了对该区域海洋生态系统由此引发的时间序列变化的评估。海鸟作为移动性强、辨识度高的高营养级捕食者，是海洋生态系统变化的优质监测类群，尤其适用于缺乏商业渔业或常规海洋学采样的区域。自1975年起，我们每年对阿拉斯加北极地区的芒氏黑海鸠（Cepphus grylle mandtii）繁殖种群开展监测研究：该物种为依赖海冰的潜水海鸟，主要捕食北极鳕鱼（Boreogadus saida）——后者是海冰关联冰下浮游生态系统的关键饵料鱼种。我们借助多状态标记-重捕模型（multistate capture–mark–recapture models）、矩阵种群模型（matrix population models）与扰动分析（perturbation analysis），针对该个体标记种群，量化了1980年至2019年间驱动种群变化的环境与种群统计因子。该繁殖种群在1975年至1990年间快速扩张，从不足20对繁殖个体增至200对以上，这一增长得益于巢穴可利用性的提升；随后种群出现间歇性下降，至2021年时繁殖对数量不足50对。影响种群增长的核心种群统计参数为迁入率与表观存活率，而主要环境驱动因子为夏末与秋季的海冰覆盖范围。种群初始增长阶段对应以负相位冬季北极涛动（Winter Arctic Oscillations, WAO）为主、夏季海冰覆盖广泛的时期。种群下降始于1989/90年的极端正相位WAO，该事件引发了大气与海洋环流的改变，导致整个区域夏季海冰大幅减少。在长达三十年的种群下降过程中，随着负相位WAO出现频率的提升，种群增长平台期或小幅增长的情况愈发少见，且在两次此前已被证实的海冰流失临界点（tipping points）之后，种群下降幅度进一步加剧。本研究繁殖地的繁殖成功率随海冰流失与海表温度（Sea Surface Temperature, SST）升高导致的北极鳕鱼可利用性下降而降低，且该繁殖成功率下降被认为发生在迁入种群的来源繁殖地——彼时这些区域正经历相似的海洋学变化。研究团队预测，该繁殖种群将在未来二十年内出现准灭绝（种群规模降至25对以下）。芒氏黑海鸠对北极冰下浮游生态系统的多年代际变化极为敏感，因此是该区域的优质指示物种；其近期的种群崩溃，对北冰洋的各组成物种均具有严峻的警示意义。