Abstract: This research selected eight grassland sites located in Shenmu City,Shaanxi Province. The field investigation method of “binary occurrence times” was adopted to obtain the data of the occurrence frequencies of each plant species in the site. We calculated the species diversity index values of the whole community and the species diversity index values assuming one specific plant species in the community did not exist. The difference between these two indices was used to investigate the role of each plant species in the overall species diversity of the community. The results are as follows:(1) In the eight grassland sites,the plant species with the greatest effect on species diversity was not the first dominant species in the community,but the plant species with the dominant position from the second to seventh. (2) In three of these sites,the first dominant species was absolutely dominant with a consequence of that their presence in the community decreased the overall diversity of the community. (3) The contribution of different species in diversity showed a unimodal trend. With the decrease of the dominant position of species,the diversity index first showed a logarithmic increase to the maximum value. Then showed an exponential or linear decline trend,and finally approached 0 value.In conclusion,in different grassland communities,the plant species that contributes most significantly to the species diversity is not the first dominant species,the plant species in the absolute dominant position reduced the species diversity of the community.

Key words: Grassland community, Plants species, Dominant species, Companion species, Species diversity index

摘要： 本研究选取位于陕西省神木市境内的8个草地样地，采用“二值出现次数”野外调查法，得到样方中各植物种的出现次数数据，以此计算群落整体的物种多样性指数值，以及组成群落的各植物种不存在时的物种多样性指数值，利用这两个多样性指数值的差值，探明各植物种在群落整体物种多样性中的作用。研究结果如下：(1)8个草地样地中，对物种多样性作用最大的植物种并非该群落的第1优势种，而是优势地位处于第2~7位的植物种；(2)3个样地中第1位优势种占据了绝对优势，它们降低了群落整体的多样性；(3)物种在多样性中的作用大小整体上呈现单峰型变化趋势，即随着物种优势地位的降低，对数式增加达到最大值，然后指数或线性下降，最终趋近于0。综合显示，不同的草地群落中，在物种多样性中作用最大的不是第1优势种，占绝对优势地位的优势种降低了该群落的物种多样性。

关键词: 草地群落, 植物种, 优势种, 伴生种, 物种多样性指数