Detrimental impacts of climate change may be exacerbated by density dependent population regulation in blue mussels

1. The climate on our planet is changing and the range distributions of organisms are shifting in response. In aquatic environments, species might not be able to redistribute poleward or into deeper water when temperatures rise because of barriers, reduced light availability, altered water chemistry, or any combination of these. How species respond to climate change may depend on physiological adaptability, but also on the population dynamics of the species. 2. Density dependence is a ubiquitous force that governs population dynamics and regulates population growth, yet its connections to the impacts of climate change remain little known, especially in marine studies. Reductions in density below an environmental carrying capacity may cause compensatory increases in demographic parameters and population growth rate, hence masking the impacts of climate change on populations. On the other hand, climate-driven deterioration of conditions may reduce environmental carrying capacities, making compensation less likely and populations more susceptible to the effects of stochastic processes. 3. Here we investigate the effects of climate change on Baltic blue mussels using a 17-year data set on population density. Using a Bayesian modelling framework, we investigate the impacts of climate change, assess the magnitude and effects of density dependence, and project the likelihood of population decline by the year 2030. 4. Our findings show negative impacts of warmer and less saline waters, both outcomes of climate change. We also show that density-dependence increases the likelihood of population decline by subjecting the population to the detrimental effects of stochastic processes (i.e., low densities where random bad years can cause local extinction, negating the possibility for random good years to offset bad years). 5. We highlight the importance of understanding, and accounting for both density dependence and climate variation when predicting the impact of climate change on keystone species, such as the Baltic blue mussel. 08-Oct-2020

1. 地球气候正发生变化，生物的分布范围也随之发生偏移。在水生环境中，当温度升高时，物种可能因阻隔物、光照可利用性降低、水化学性质改变，或上述因素共同作用，而无法向极地迁移或向更深水域扩散。物种对气候变化的响应，不仅取决于其生理适应性，还与其种群动态（population dynamics）密切相关。
2. 密度依赖（density dependence）是一种普遍存在的作用力，它调控种群动态并调节种群增长，但目前其与气候变化影响之间的关联仍鲜为人知，在海洋研究领域尤为如此。当种群密度降至环境容纳量（environmental carrying capacity）以下时，可能会引发种群统计参数与种群增长率的补偿性升高，从而掩盖气候变化对种群的影响。另一方面，气候变化驱动的环境条件恶化，可能会降低环境容纳量，使得补偿效应更难发生，进而让种群更易受到随机过程（stochastic processes）的影响。
3. 本研究基于一项长达17年的种群密度数据集，探究气候变化对波罗的海贻贝（Baltic blue mussels）的影响。本研究采用贝叶斯建模框架（Bayesian modelling framework），评估气候变化的影响、量化密度依赖的强度与效应，并预测至2030年的种群下降可能性。
4. 研究结果显示，气候变暖与水体盐度降低这两种气候变化的产物，均会产生负面影响。同时我们发现，密度依赖会通过使种群承受随机过程的不利影响，进而提升种群下降的可能性：具体而言，当种群密度较低时，随机发生的灾年可能引发局域灭绝，从而消除随机丰年抵消灾年负面影响的可能性。
5. 本研究强调，在预测气候变化对关键物种（keystone species，如波罗的海贻贝）的影响时，理解并同时考虑密度依赖与气候变异的重要性。
2020年10月8日