Rapid morphological change in a small mammal species after habitat fragmentation over the past half-century

Study Aim: To compare the rapid shifts in body size of mainland and island populations of a native rodent and examine the mechanisms underlying these changes.

Location: Thousand Island Lake, China, which was created in 1959 when the Xin’anjiang Dam was constructed for generating hydroelectricity.

Taxon: The Chinese white-bellied rat, Niviventer confucianus.

Methods: Field surveys were conducted from 2015 to 2018 to collect data on body size of the rodents from a set of islands and nearby mainland sites. We constructed multiple linear models to examine the relationships between body size (length and mass) of rodents and biological variables (predators, interspecific and intraspecific competitors, and food availability). We also conducted structural equation modeling (SEM) by constructing models via confirmatory path analysis.

Results: All island populations of N. confucianus had significantly larger body size (both body mass and body length) than their mainland counterparts. Moreover, populations on small and more isolated islands had larger body size than their relatives on big islands. The relative absence of predators (large-bodied mammals, snakes, and raptors) on islands was most strongly associated with shifts in the body size of rodents. The documented changes occurred after only a half-century of fragmentation.

Main conclusions: The observed rapid body enlargement of rodents after habitat fragmentation is consistent with a release from predation pressure. SEM indicated that island area, rather than island isolation, had positive effects on the richness of predators, interspecific competitors and food resources, which then had an indirect impact on body size of the rodents. In this study, we report a remarkably rapid case of mammal morphological shifts in a small mammal in response to habitat fragmentation. Given the omnipresence of dams and other anthropogenic disturbances, our findings suggest that a wave of rapid phenotypic shifts in terrestrial vertebrates is taking place in the Anthropocene.

Methods
How was this dataset collected? 

We gathered these data from our field survey. We collected the data on body size by live-trapping rodents along transects from July to September in 2016 and 2017. The primary predators of N. confucianus in the region are snakes, owls, and large carnivorous mammals such as wild boars and leopard cats. We surveyed the bird community (with an emphasis on the rodent predators like Otus bakkamoena) on mainland and island sites during 2015–2018. To assess species composition and abundance of carnivore mammals that are predators of rodents, we set camera traps on study islands and mainland sites from 2015 to 2017. We conducted surveys of all vascular plant species occurring on each mainland and island site during the growing seasons (April to November) in 2017. 

All aspects of this study were approved by Chun'an Forestry Bureau, Thousand Island Lake National Forest Park and Zhejiang University.

How has it been processed?  

We constructed multiple linear models to examine the relationships between body size (length and mass) of rodents and biological variables (predators, interspecific and intraspecific competitors, and food availability). We also conducted structural equation modeling (SEM) by constructing models via confirmatory path analysis.

研究目的：比较本土啮齿类大陆种群与岛屿种群的体型快速变化，并探究此类体型变化背后的潜在机制。
研究地点：中国千岛湖，该水域于1959年因修建新安江水电站以水力发电而形成。
研究类群：中华白腹鼠（*Niviventer confucianus*）。
研究方法：2015年至2018年间开展野外调查，从多个岛屿及邻近大陆样点收集啮齿动物的体型数据。本研究构建多元线性模型，以探究啮齿动物体型（体长与体质量）与各类生物变量（捕食者、种间与种内竞争者、食物可获得性）之间的关联。同时，通过验证性路径分析构建结构方程模型（SEM）开展分析。
研究结果：所有中华白腹鼠的岛屿种群的体型（体质量与体长）均显著大于其大陆对应种群。此外，面积更小、隔离程度更高的岛屿上的种群，体型较大岛屿上的种群更大。岛屿上捕食者（大型哺乳类、蛇类与猛禽）的相对缺失，与啮齿动物的体型变化关联最为紧密。此类体型变化仅在生境碎片化发生半个世纪后便已出现。
主要结论：生境碎片化后啮齿动物出现的快速体型增大现象，与捕食压力释放的假说相符。结构方程模型（SEM）结果显示，岛屿面积而非岛屿隔离程度，对捕食者、种间竞争者及食物资源的丰富度具有正向影响，进而间接作用于啮齿动物的体型。本研究报道了小型哺乳动物在生境碎片化响应下，发生显著快速形态变化的案例。鉴于水坝及其他人为干扰无处不在，本研究结果表明，人类世（Anthropocene）中陆生脊椎动物正经历一轮快速的表型变化浪潮。

数据集采集方式：
本研究的数据来源于野外调查。2016年与2017年的7月至9月，我们通过样线活捕法收集啮齿动物的体型数据。该区域中华白腹鼠的主要捕食者包括蛇类、鸮形目鸟类，以及野猪、豹猫等大型食肉哺乳类。2015年至2018年间，我们在大陆与岛屿样点开展鸟类群落调查，重点关注以啮齿动物为食的捕食者（如领角鸮*Otus bakkamoena*）。为评估啮齿动物捕食者——食肉哺乳类的物种组成与丰度，我们于2015年至2017年在研究样地的岛屿与大陆点位布设红外相机。2017年的生长季（4月至11月），我们对每个大陆与岛屿样点的所有维管植物物种开展调查。本研究的所有环节均已通过淳安县林业局、千岛湖国家森林公园及浙江大学的伦理审批。

数据集处理方式：
我们构建多元线性模型，以探究啮齿动物体型（体长与体质量）与各类生物变量（捕食者、种间与种内竞争者、食物可获得性）之间的关联。同时，通过验证性路径分析构建结构方程模型（SEM）开展分析。