Community Assembly Rules Of The Eastern Edge Of Qinghai-Tibetan Plateau

Over the past century, a central theme in ecology is the underling mechanisms of formation and maintenance of species diversity in community, i.e., community assembly rules. Niche and neutral theory are the two distinct, yet not mutually exclusive forces of community structuring. Niche theory states that the long-term coexistence of species is possible only if their niches are sufficiently separated, and community structure was controlled mainly by environmental factors and niche differences among species. It predicts that the spatial pattern of community should be closely related to environmental heterogeneity, and local communities with similar environmental conditions should be similar in species composition, and community similarity should decrease with increasing environmental distance (i.e., environmental dissimilarity). In contrast, neutral theory assumes that all individuals on the same trophic level are functional equivalent, i.e., demographically identical on a per capita basis in terms of their vital rates, of birth, death, dispersal and speciation, and emphasizes the role of random dispersal processes in shaping local communities, and speciation-extinction events in forming large-scale metacommunities. It predicts that due to the spatial effect of dispersal limitation, community similarity should decrease with increasing geographical distance, irrespective of any environmental factor. The relative importance of niche and neutral processes is still controversial due to the species-specific dispersal ability, sensitivity towards environmental variables, organism's abundance and the effect of spatial scale.In the present paper, we explored niche versus neutral processes in shaping community composition (i.e.,β-diversity) and species turnover (i.e., change ofβ-diversity) at an alpine meadow along a slope aspect gradient on the Qinghai-Tibetan Plateau at different spatial scales of sampling (quadrats and plots), by taking account of seed dispersal mode and abundance. We examined the relative importance of niche (environmental factors) and neutral (spatial) processes using all species and four additional subsets of selected species based on their seed dispersal mode and abundance. Moreover, we attempted to explore the effect of scale (quadrat refers to scale of～0.3 m and plot of～8 m) on their counter balance. The data were analyzed both by variation partitioning of community composition and multiple regressions on distance matrices. The spatial structures of quadrats and plots were modeled using Moran's eigenvector maps.The main conclusions are as follow:Both spatial processes and environmental factors were important determinants of community composition and species turnover. The community composition in the alpine meadow was controlled by spatially structured environment (17.6%), space independent of environment (18.0%) and a negligible effect of environment independent of space (4.4%) at the scale of quadrats. These three components contributed 21.8,9.9 and 13.9%, respectively, at the scale of plots. The balance between the forces at different spatial scales drove community structures along the slope aspect gradient. The importance of environmental factors onβ-diversity increased with scale while that of spatial processes decreased or kept steady, depending on dispersal mode and abundance of species comprising the subset. However, the'pure'effect of spatial processes on species turnover increased with scale while that of environmental factors decreased. This discrepancy highlights that (3-diversity and species turnover were determined jointly by spatial processes and environmental factors. We also found that the relative roles of these processes vary with spatial scale. These results underline the importance of considering species-specific dispersal ability and abundance of species comprising the communities and the appropriate spatial scale in understanding the mechanisms of community assembly.In future, the main issues of community assembly will be to:1) define the position of communities from different ecosystems along the niche-neutrality continuum; 2) deconstruct communities into several sub-communities based-on selected plant functional traits according to the component species under consideration and phylogenetic structure of the communities, and compare the relative importance of niche and neutral processes among these sub-communities; 3) determine the effect of spatial scale (extent, sampling interval, grain size) on the relative importance of niche and neutral processes on shaping community, and compare among different communities.