Department of Biology, University of Turku, Turku, Finland.
Bioeconomy Research Team, Novia University of Applied Sciences, Ekenäs, Finland.
PLoS One. 2018 Apr 10;13(4):e0195415. doi: 10.1371/journal.pone.0195415. eCollection 2018.
In contrast to theoretical predictions of even adult sex ratios, males are dominating in many bird populations. Such bias among adults may be critical to population growth and viability. Nevertheless, demographic mechanisms for biased adult sex ratios are still poorly understood. Here, we examined potential demographic mechanisms for the recent dramatic shift from a slight female bias among adult eider ducks (Somateria mollissima) to a male bias (about 65% males) in the Baltic Sea, where the species is currently declining. We analysed a nine-year dataset on offspring sex ratio at hatching based on molecularly sexed ducklings of individually known mothers. Moreover, using demographic data from long-term individual-based capture-recapture records, we investigated how sex-specific survival at different ages after fledgling can modify the adult sex ratio. More specifically, we constructed a stochastic two-sex matrix population model and simulated scenarios of different survival probabilities for males and females. We found that sex ratio at hatching was slightly female-biased (52.8%) and therefore unlikely to explain the observed male bias among adult birds. Our stochastic simulations with higher survival for males than for females revealed that despite a slight female bias at hatching, study populations shifted to a male-biased adult sex ratio (> 60% males) in a few decades. This shift was driven by prime reproductive-age individuals (≥5-year-old), with sex-specific survival of younger age classes playing a minor role. Hence, different age classes contributed disproportionally to population dynamics. We argue that an alternative explanation for the observed male dominance among adults-sex-biased dispersal-can be considered redundant and is unlikely, given the ecology of the species. The present study highlights the importance of considering population structure and age-specific vital rates when assessing population dynamics and management targets.
与理论上对成年性别比例的预测相反，许多鸟类种群中雄性占主导地位。这种成年个体之间的性别偏差可能对种群的增长和生存能力至关重要。然而，导致成年性别比例偏差的人口机制仍未得到很好的理解。在这里，我们研究了导致波罗的海海雀（Somateria mollissima）成年个体性别比例从轻微的雌性偏向到雄性偏向（约 65%为雄性）的潜在人口机制，目前该物种在该地区的数量正在减少。我们分析了基于分子性别鉴定的雏鸭孵化时的性别比例的九年数据集，这些雏鸭的母亲是已知的个体。此外，我们利用长期个体基础的捕获-再捕获记录的人口数据，研究了在幼鸟离巢后不同年龄的性别特异性存活率如何改变成年性别比例。更具体地说，我们构建了一个随机的两性别矩阵种群模型，并模拟了雄性和雌性不同存活率的情景。我们发现，孵化时的性别比例略有雌性偏向（52.8%），因此不太可能解释成年鸟类中观察到的雄性偏向。我们的随机模拟结果表明，与雌性相比，雄性的存活率更高，尽管孵化时性别比例略有雌性偏向，但在几十年内，研究种群的成年性别比例仍会偏向雄性（>60%为雄性）。这种转变是由主要生殖年龄个体（≥5 岁）驱动的，年龄较小的个体的性别特异性存活率则起次要作用。因此，不同年龄组对种群动态的贡献不成比例。我们认为，对于成年个体中观察到的雄性主导地位的另一种解释——性别偏向的扩散，可以认为是多余的，而且考虑到该物种的生态学，这种解释不太可能成立。本研究强调了在评估种群动态和管理目标时，考虑种群结构和年龄特定的关键生存率的重要性。
