Wildlife Institute of India, Dehradun, 248001, India.
Biomathematics Program, North Carolina State University, Raleigh, North Carolina, 27695, USA.
Ecology. 2019 Jan;100(1):e02548. doi: 10.1002/ecy.2548.
Along elevational gradients, species richness often peaks at intermediate elevations and not the base. Here we refine and test eight hypotheses to evaluate causes of a richness peak in trees of the eastern Himalaya. In the field, we enumerated trees in 50 plots of size 0.1 ha each at eight zones along an elevational gradient and compared richness patterns with interpolation of elevational ranges of species from a thorough review of literature, including floras from the plains of India. The maximum number of species peaks at similar elevations in the two data sets (at 500 m in the field sampling and between 500 m and 1,000 m in range interpolation); concordance between the methods implies that statistical artefacts are unlikely to explain the peak in the data. We reject most hypotheses (e.g., area, speciation rate, mixing of distinct floras). We find support for a model in which climate (actual evapotranspiration [AET] or its correlates) sets both the number of species and each species optimum, coupled with a geometric constraint. We consider that AET declines with elevation, but an abrupt change in the association of AET with geographical distance into the plains means that the location of highest AET, at the base of the mountain, receives range overlaps from fewer species than the location just above the base. We formalize this explanation with a mathematical model to show how this can generate the observed low-elevation richness peak.
在海拔梯度上，物种丰富度通常在中间海拔处达到峰值，而不是在底部。在这里，我们改进并测试了八个假设，以评估喜马拉雅山东部树木丰富度峰值的原因。在实地，我们在海拔梯度的八个区域的每个 0.1 公顷大小的 50 个样方中对树木进行了计数，并将丰富度模式与对文献中物种海拔范围的插值进行了比较，包括来自印度平原的植物志。两个数据集（实地采样的 500 米和范围插值的 500 米至 1000 米之间）中的物种数量峰值出现在相似的海拔高度；两种方法的一致性表明，统计数据不太可能解释数据中的峰值。我们拒绝了大多数假设（例如，面积、物种形成率、不同植物区系的混合）。我们支持这样一种模型，即气候（实际蒸散量[AET]或其相关因素）决定了物种的数量和每个物种的最佳条件，并与几何约束相结合。我们认为 AET 随海拔升高而下降，但 AET 与平原地区地理距离的关联的突然变化意味着，在山脚下 AET 最高的位置，来自较少物种的范围重叠比刚好在山脚以上的位置要多。我们用一个数学模型来正式解释这个问题，说明这如何产生观察到的低海拔丰富度峰值。
