Data from: Trait-demography relationships underlying small mammal population fluctuations

Large-scale fluctuations in abundance are a common feature of small mammal populations and have been the subject of extensive research. These demographic fluctuations are often associated with concurrent changes in the average body mass of individuals, sometimes referred to as the ‘Chitty effect’. Despite the long-standing recognition of this phenomenon, an empirical investigation of the underlying coupled dynamics of body mass and population growth has been lacking.
Using long-term life-history data combined with a trait-based demographic approach, we examined the relationship between body mass and demography in a small mammal population that exhibits non-cyclic, large-scale fluctuations in abundance. We used data from the male segment of a 25-year study of the monogamous prairie vole, Microtus ochrogaster, in Illinois, USA. Specifically, we investigated how trait–demography relationships and trait distributions changed between different phases of population fluctuations, and the consequences of these changes for both trait and population dynamics.
We observed phase-specific changes in male adult body mass distribution in this population of prairie voles. Our analyses revealed that these changes were driven by variation in ontogenetic growth, rather than selection acting on the trait. The resulting changes in body mass influenced most life-history processes, and these effects varied among phases of population fluctuation. However, these changes did not propagate to affect the population growth rate due to the small effect of body mass on vital rates, compared to the overall differences in vital rates between phases. The increase phase of the fluctuations was initiated by enhanced survival, particularly of juveniles and fecundity, whereas the decline phase was driven by an overall reduction in fecundity, survival and maturation rates.
Our study provides empirical support, as well as a potential mechanism, underlying the observed trait changes accompanying population fluctuations. Body size dynamics and population fluctuations resulted from different life-history processes. Therefore, we conclude that body size dynamics in our population do not drive the observed population dynamics. This more in-depth understanding of different components of small mammal population fluctuations will help us to better identify the mechanistic drivers of this interesting phenomenon.

种群丰度的大规模波动是小型哺乳动物种群的常见特征，亦是诸多深入研究的核心议题。这类种群动态波动往往伴随个体平均体重的同步变化，该现象有时被称为‘奇蒂效应（Chitty effect）’。尽管该现象早已被学界认知，但针对体重与种群增长的耦合动态机制所开展的实证研究仍有所欠缺。
本研究结合长期生活史数据与基于性状（trait）的种群动态分析方法，针对一个呈现非周期性大规模丰度波动的小型哺乳动物种群，探究了其体重与种群统计特征之间的关联。我们使用了美国伊利诺伊州一项为期25年的一夫一妻制草原田鼠（Microtus ochrogaster）研究中的雄性个体数据集。具体而言，我们分析了种群波动不同阶段中，性状-种群统计特征关系与性状分布的变化，以及这些变化对性状动态与种群动态的影响。
本研究观测到该草原田鼠种群的成年雄性体重分布存在阶段特异性变化。分析结果表明，这类变化由个体发育生长的差异所驱动，而非由针对该性状的选择作用所导致。体重的此类变化会影响多数生活史过程，且其影响效应随种群波动阶段的不同而存在差异。不过，由于相较于种群波动阶段间生命率（vital rates）的整体差异，体重对生命率的影响较小，这类变化并未传递至种群增长速率。种群波动的增长阶段由存活率提升所启动，尤其是幼体存活率与繁殖力，而衰退阶段则由繁殖力、存活率与成熟速率的整体下降所驱动。
本研究为伴随种群波动的性状变化提供了实证依据与潜在的作用机制。体型动态与种群波动源自不同的生活史过程，因此我们认为，本研究种群中的体型动态并未驱动观测到的种群动态变化。对小型哺乳动物种群波动各组成部分的更深入理解，将有助于我们更好地识别这一有趣现象的机制性驱动因素。