Species richness patterns at the transition from the Tropics to the Subtropics: Studying environmental and historic effects

人们早就知道从赤道到两极生物多样性显著下降的现象，自发现以来，人们对其进行了深入调查，但对其原因的讨论仍存在争议。尤其令人着迷的是，沿着较大的热带山脉，这种与温度相关的梯度的明显镜像，在那里可以找到类似的植被带和植物适应性。在过去的几十年里，不仅有文献表明，纬度梯度没有直接反映在海拔梯度上（这里的相关性不是线性的，而是呈驼峰状，在中海拔高度有明显的多样性最大值），而且很明显，只要保持地形和土壤参数的可比性，热带地区的多样性模式就不会发生显著变化。到目前为止，从纬度梯度必须变得明显的热带和亚热带之间的过渡区获得的可比数据是不可用的。一个原因是，在世界上大多数地区，热带山地雨林并不与可比的森林接壤，而是被广阔的低地和干旱带所孤立。在这里，我们建议在墨西哥东部韦拉克鲁斯州建立一个标准化的蕨类植物多样性的海拔样带，那里最北部的热带低地雨林最终融入了山地松林。这种方法的优点是该地区已知的生物多样性，以及我们工作组过去和当前项目的可比数据的可用性。我们还考虑了所研究分类群的系统发育位置，以解开物种丰富度格局的不同驱动因素的相对作用。

The long known notable decline of biodiversity from the equator to the poles has been investigated intensively since its discovery but the causes are still controversially discussed. Especially fascinating is the apparent mirroring of this temperature-related gradient along larger tropical mountains, where comparable vegetation belts and plant adaptations can be found. During the last decades, it has not only been documented that the latitudinal gradient is not directly reflected in this elevational gradient (here the correlation is not linear but shows a hump-shape, with a clear diversity maximum at mid-elevations), it became also clear that the diversity patterns do not vary significantly within the tropical belt as long as orographic and edaphic parameter are kept comparable. Until now, comparable data from the transition zone between the tropics and sub-tropics where the latitudinal gradient must become apparent is not available. One reason is that in most areas in the world, tropical mountain rainforests do not border to comparable forests but are isolated by extensive lowlands and arid belts. Here we propose to establish a standardized elevational transect of the fern diversity in eastern Mexico, Veracruz state, where the northern-most tropical lowland rainforest eventually blends into a montane pine forest. The advantages of this approach are the already well-known biodiversity of the area and the availability of compara-ble data from past and current projects of our work group. We also regard phylogenetic position of the studied taxa in order to disentangle the relative roles of different drivers of species richness patterns.