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

自发现以来，人们对从赤道到两极的生物多样性显著下降的现象进行了深入的研究，但其原因仍存在争议。尤其令人着迷的是，这种与温度相关的梯度明显反映了更大的热带山脉，在那里可以找到类似的植被带和植物适应。在过去的几十年里，不仅证明纬度梯度不直接反映在这种海拔梯度中(这里的相关性不是线性的，而是呈现驼峰形状，在中等海拔有一个明显的多样性最大值)，而且还清楚地表明，只要地形和土壤参数保持可比，热带地区的多样性格局就不会有明显的变化。到目前为止，还没有热带和亚热带之间过渡区的可比数据，在那里纬度梯度必须变得明显。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森林接壤，而是被广阔的低地和干旱地带隔绝开来。在这里，我们建议在墨西哥东部的韦拉克鲁斯州建立一个蕨类多样性的标准化海拔样带，在那里，最北部的热带低地雨林最终融入到山地松林中。这种方法的优点是该地区已经众所周知的生物多样性，以及我们工作组过去和目前项目的可比数据的可用性。我们还考虑了所研究的分类群的系统发育地位，以理清不同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.