Elevational gradient in biodiversity (or species richness) is one of the most obvious ecological patterns on our Earth. Understanding how and why diversity varies systematically with elevation has been a focus of research in spatial ecology since the days of the earliest naturalists. Dated back to two and half centuries ago, Linnaeus documented that diversity of animal and plant life is highest at the bottom and decreases as you climb, which was once taken an a wisdom. It should be noted that, nearly one thousand year ago, it was similarly documented as “It Is Lonely at the Top (高处不胜寒[Gao Chu Bu Sheng Han])” in the Chinese poem “Prelude to the Melody of Water” by ancient Chinese poet Su Shi (1037-1103).

Compared to those studies in larger plants and animals, the elevational patterns in microbial diversity have been less examined. Thus given the important roles of microorganisms in ecosystems as decomposers and primary producers, filling this gap is urgent. Two primary questions regarding microorganisms remain unanswered, namely (1) whether microorganisms show an elevational gradient in diversity, and (2) whether such a pattern, if it exists, resembles the patterns observed for macroorganisms. Further emergent question should be for the underlying causes of elevational diversity gradients.

Since 2008, a research program, carried by researchers from Nanjing Institute of Geography and Limnology, Chinese Academy of Science, was aimed to answer above primary questions focusing on aquatic environments. This is the first study on such ecosystem so far and the primary findings have been achieved as following:

(1) Bacteria exhibited a monotonically increasing pattern along the elevational gradient was unexpected. This increasing pattern is extremely rarely observed in nature, considering the large number of elevational gradients studied. This pattern is less likely to be affected by methodological issues as different methods came to the similar result, such as fingerprinting method (DGGE) and high-throughput DNA sequencing (454).

(2) Diatoms monotonically decreased in richness from the lowest elevation to the highest elevation. The variation in TN/TP was most likely to be the strongest factor in controlling the diatom richness, and it was also among the most important factors in explaining variation in diatom evenness. This indicated the effects of support the effect of climate as underlying the richness gradient in diatoms.

(3) Macroinvertebrate richness followed a clear unimodal pattern, with a steeper decline towards higher elevations. The Mid-domain Effect Hypothesis (MDE) had the best statistical fit among the factors considered in respect of the elevational richness gradient of macroinvertebrates. The MDE, based solely on geometric constraints on species richness, predicts that when hard boundaries (such as oceans and mountaintops) limit species ranges, variously sized ranges create a peak in species richness in the middle of the gradient. It is regarded as a spatial factor.

(4) The underlying mechanism for the elevational diversity in microbes is likely to be climatic hypothesis. So far, the proposed hypotheses for explaining the elevational patterns in biodiversity can be classified into four major categories: climatic, spatial, historical and biotic. However, this study only supported the first category. The alpha or beta diversity of bacteria is mainly related to aquatic physical-chemical variables. The bacterial communities (the phylum of Proteobacteria as well) consisted of closer relatives than expected and displayed increasing terminal phylogenetic clustering towards mountain top. The effects of environmental filtering (water temperature, for instance) increased towards higher elevations. Thus, different from high plants or animals, environmental filtering related to climatic variables play a dominant role in structuring bacterial biodiversity along the elevational gradient.

It seems that it is not lonely at the top for microbes. However, there are still few studies related to microbial biodiversity on mountainsides, especially aquatic microbes. This is an emergent field for exploring.

Other research groups were included into this study, for instance, University of Helsinki, Nanjing Agricultural University, Anhui Normal University, and Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Current primary results have been published by Journal of Biogeography, Global Ecology and Biogeography and Environmental Microbiology Reports. Furthermore, these results were orally presented in Ecological Society of America Annual Meeting 2011, and will be presented with a poster in the 14th International Symposium on Microbial Ecology (2012).

The Chinese version of this report can be found by following this link:
http://www.niglas.ac.cn/xwdt/kydt/201206/t20120620_3603237.html

Related references:
Wang, J., J. Soininen, Y. Zhang, B. Wang, X. Yang, and J. Shen. 2011. Contrasting patterns in elevational diversity between microorganisms and macroorganisms. Journal of Biogeography 38:595-603.
Wang, J., J. Soininen, Y. Zhang, B. Wang, X. Yang, and J. Shen. 2012. Patterns of elevational beta diversity in micro- and macroorganisms. Global Ecology and Biogeography 21:743-750.
Wang, J., J. Soininen, J. He, and J. Shen. 2012. Phylogenetic clustering increases with elevation for microbes. Environmental Microbiology Reports 4:217-226.