物种谱系不确定性对群落谱系格局指标的影响

物种谱系关系常被用于衡量群落谱系格局及推断格局背后的生态过程,但多数研究往往忽视谱系关系的不确定性及其可能对群落谱系格局造成的影响.为此,本文以浙江天童20 hm^2样地内150个树种为研究对象,采用这些物种叶绿体DNA的rbcL和matK碱基序列构建1棵一致系统发育树和反映谱系不确定性的999棵系统发育树,然后结合样地物种分布数据计算标准化净亲缘指数(NRI)和最近亲缘指数(NTI),最后运用独立置换零模型衡量样地群落谱系格局.结果表明:物种系统发育树在拓扑结构和物种谱系分支节点年龄上均存在较大的不确定性,谱系不确定性随着谱系分支节点年龄的减小而增大,也随物种间平均谱系距离的增加而增加;在样方尺度上,物种谱系的不确定性增加了标准化NRI和NTI指数的变异,但对两个指数的影响几乎独立;其对两指数的空间分布影响不同,且程度不一,其中标准化NRI受到的影响相对更大;在群落尺度上,物种谱系的不确定性增加了标准化NRI和NTI的变异,平均变异系数分别为0.37和0.077,表明群落水平的标准化NRI更易受到谱系不确定性的影响.这说明物种谱系不确定性会传递到常用的群落谱系格局指标中,且不同指标受影响的程度不同,进而影响对群落谱系格局的衡量及相关生态过程的推断.该结论也暗示以往不考虑谱系不确定性的研究中,非随机的群落谱系格局比例可能被高估.

The impact of phylogenetic uncertainty on the metrics of community phylogenetic structure.

Phylogeny has been widely used to quantify community phylogenetic structure and to infer the underlying mechanism. Many studies, however, neglected phylogeny uncertainty and its potential impact on community phylogenetic structure. In this study, we explored the potential impact of phylogenetic uncertainty among 150 species in a 20 hm² plot in Tiantong, Zhejiang. One consensus tree and 999 phylogenetic trees representing the phylogenetic uncertainty were estimated based on two cpDNA fragments (rbcL and matK). Combined with the species distribution data, community phylogenetic structure was quantified by two common indices (NRI and NTI) and their significances were tested by the independent swap null model. Our results showed that tree topology and node age showed a large uncertainty. The uncertainty was larger for young species and significantly increased with mean phylogenetic distance. Phylogenetic uncertainty increased the variation of both standardized NRI and NTI in each quadrat. These impacts were independent between both indices in either spatial pattern or the degree of impact. NRI was more sensitive than NTI to the uncertainty. At community scale, phylogenetic uncertainty also affected the variation of the mean standardized NRI and NTI of all quadrats, with mean standard deviation of 0.37 and 0.077, respectively. Such a result suggests that mean standardized NRI at community level was more vulnerable to the phylogenetic uncertainty, which is consistent with the result at the sample level. Our findings showed that phylogenetic uncertainty could add different variation into the NRI and NTI series indices and might increase biases in the quantification of community phylogenetic structure and its underlying ecological processes. Our results implied that non-random community phylogenetic structure was probably overestimated in the previous studies which ignored phylogenetic uncertainty.