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 (<em>Cepphus grylle mandtii</em>), an ice-obligate diving seabird, specializing on Arctic cod (<em>Boreogadus saida</em>), 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 &lt;20 to &gt;200 breeding pairs from 1975-1990 in response to increased availability of nesting cavities, before experiencing intermittent declines to &lt;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 &lt;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，北极地区海冰大幅缩减、海洋温度持续升高，但目前仍缺乏充足的生物时序数据，难以评估该区域海洋生态系统由此产生的时间动态变化。海鸟作为移动性强、辨识度高的顶级营养级捕食者，是监测海洋生态系统变化的优质指示生物，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曼氏海鸠（*Cepphus grylle mandtii*）繁殖种群开展长期研究。该物种为依赖海冰的潜水海鸟，主要以北极鳕鱼（*Boreogadus saida*，北极冰栖生境的关键饵料鱼类）为食。

我们利用多状态标记-重捕（capture–mark–recapture）模型、矩阵种群模型及扰动分析，量化了1980-2019年间该个体标记种群的种群动态变化的环境与种群统计驱动因子。1975年至1990年，由于巢洞可获得性提升，该繁殖种群从不足20对繁殖个体快速增长至200对以上；但随后种群出现间歇性下降，2021年时种群规模降至不足50对。

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。种群初期增长阶段对应冬季北极涛动（WAO）以负值为主、夏季海冰覆盖广泛的时期。而种群下降始于1989/90年极端正相位的北极涛动，该事件引发了大气与海洋环流的改变，导致整个北极区域夏季海冰大幅缩减。

在这三十年的种群下降过程中，随着冬季北极涛动负值出现频率升高，种群增长出现停滞或小幅回升；而在两次已被证实的海冰流失“临界点”之后，种群下降幅度进一步加剧。受海冰流失与海表温度上升影响，北极鳕鱼可获得性降低，导致研究种群的繁殖成功率持续下降；这种繁殖失败现象推测同样发生在迁入种群的来源种群中，当地也正经历相似的海洋环境变化。

模型预测，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）。曼氏海鸠对北极冰栖生态系统的多年代际变化高度敏感，因此是该区域极佳的指示物种；其种群近期的崩溃，对北极海洋各组成物种均具有严峻的警示意义。