Beta Diversity And Its Causes In The Loess Plateau Grassland Community Inquiry

Loess Plateau is China’s unique ecological environment area. Because the ecosystem is seriously degraded due to the effect of long-term soil and water loss, and in recent years the influence of human grazing activity, it is imminent to accelerate the restoration and recovery of degenerative ecosystem on the Loess Plateau. Malan forest region is the most intact natural vegetation area so far, it plays a very important role in Loess Plateau central area, watering conservation, adjusting the climate, keeping the ecological balance and promoting the sustainable development of the green economy. Therefore understanding the mechanism of beta diversity maintenance is helpful to protect the biodiversity of the region, as well as to the degradation of ecosystem recovery. On the other hand, the construction mechanisms of plant communities are a hot topic in the field of ecology. However, for a long time the question of how communities are constructed has not yet been adequately explained. Whether these mechanisms are driven by niche theory, neutral theory, or a combination of the two has been confirmed in a variety of research studies. In view of this, we looked at grassland communities on China’s Loess Plateau as example, through the analysis and probe the effect of environmental factors and species dispersal on species assemblages in grassland communities, hoping to provide some grounds for answering this question in this paper. Three different grass habitats were selected:land which had been abandoned for five years, north-facing slopes and south-facing slopes.In this paper, by using Whittaker index (βw) and Routledge index (βR) as the binary data, and Bray-Curtis index (βB-C) and Morisita-Horn index (βM) as the numerical data, the β diversity between sample plots was measured. Canonical Correspondence Analysis (CCA) was used to select some environmental factors, which have a significant influence on β diversity. To analyze statistic in the method of Mantel test and multiple regression, The Mantel test analysis was applied to test the correlation between similarity in community composition, geographic distance and environment distance, multiple regression analysis was used to test for fitness:the percentage of community assembly can be explained in terms of the geographic distance and environmental distance. The empirical results are summarized as follows:(1) The results of the binary data show that curv βW and curve βR could be described as a certain family resemblance. Overall, β diversity shows a rise and falling to fluctuate, and the β diversity of abandoned land is at a relatively high level. Taken individually, the lower slope of both abandoned land and south-facing slopes has the best β diversity, while the middle slope of north-facing slopes has the best β diversity.(2) The results of the matrix of β diversity indices show that β diversity indices measured by binary data is at a relatively low level inside the three sampling sets, and β diversity indices of different belt transects is at a relatively high level, while the result of β diversity indices measured by numerical data was contrary to the former.(3) The result of CCA sort shows that the effect of soil humidity and available nitrogen was relatively obvious of the eight environmental variables examined, especially available nitrogen (r2=0.9078, P=0.010).(4) The result of Mantel test suggested that geographic distance has a significant effect on the community composition; there is a correlation between environment distance and community composition, but the function of environment distance is not as good as geographic distance. In order to further explore the influence, multiple regression analysis was used to test for fitness:the percentage of community assembly can be explained in terms of the geographic distance and environmental distance. The results indicate the distribution of plants are highly variable and more strongly related to geographical distance rather than to environmental differences. Together, geographical distance and environmental distance explained79.3%of the variation in floristic similarity. Pure geographic distance or pure environmental distance alone explained33.8%and14.2%of the variation in floristic similarity, respectively. The neutral theory plays a strong role in the species assemblages within ecological communities, but the function of the niche can’t be ignored.