Data from: Rapoport effect and the climatic variability hypothesis in Early Jurassic ammonites

The increase of species range size towards high latitudes, known as the Rapoport’s rule, remains one of the most debated and poorly understood macroecological patterns. Numerous studies have challenged both its universality and the main mechanism originally proposed to explain it, i.e. the climatic variability hypothesis. Here we study this pattern on a group of fossil marine organisms: the early Pliensbachian ammonites of the western Tethys. We further take into account the influence of the marked provincialism prevailing at that time, with a Mediterranean province (MED) and a Northwest European province (NWE) located on each side of a latitudinally-oriented palaeobiogeographic barrier. We find that only species from the NWE province display a Rapoport effect, whereas species from the more tropical MED province show a boundary effect and have larger range sizes in average. This dual pattern can be explained by an alternative climatic variability hypothesis that better captures latitudinal seasonal variations and outlines the influence of the intertropical zone, characterized by stable and homogeneous climate that allows species to disperse over very large areas, regardless of their thermal tolerance. Accordingly, the NWE province probably displayed a gradient of seasonal climatic variations which caused the emergence of a Rapoport effect, whereas the MED province was probably located in the intertropical zone where no gradient in species range size is expected. Our multi-scale approach further shows that the Rapoport effect is scale dependent and may be labile through time. This probably explains the conflicting results of previous studies carried out at various spatio-temporal scales.

物种分布范围大小随纬度升高而增大的规律被称为拉波波特法则（Rapoport’s rule），迄今仍是宏观生态学领域最具争议且最难以阐释的格局之一。诸多研究已对其普适性以及最初用以解释该规律的核心机制——气候变异性假说（climatic variability hypothesis）——提出了质疑。本研究以一类化石海洋生物为研究对象，即西特提斯洋域早普林斯巴期的菊石，对该宏观生态格局展开探究。此外，本研究还纳入了当时盛行的显著生物地理分区效应的影响：当时存在以纬向古生物地理屏障为界的两个生物地理区系——地中海生物区（MED）与西北欧生物区（NWE）。研究结果显示，仅西北欧生物区的物种呈现拉波波特效应，而地处更热带环境的地中海生物区物种则表现出边界效应，且其平均分布范围更大。这种二元分布格局可通过修正后的气候变异性假说来解释：该假说更精准地捕捉了纬度梯度上的季节气候变异特征，并阐明了热带间区域的影响——热带间区域气候稳定且均一，无论物种的热耐受能力如何，均能支持其在超大范围内扩散。据此推测，西北欧生物区可能存在季节气候变异梯度，进而催生了拉波波特效应；而地中海生物区大概率处于热带间区域，该区域本不应出现物种分布范围大小的纬度梯度。本研究采用的多尺度分析方法进一步表明，拉波波特效应具有尺度依赖性，且可能随时间发生变化。这一结论或可解释此前不同时空尺度下开展的相关研究所得出的矛盾结果。