Environmental And Biological Determinants Of Species Richness Gradients In Plant Communities

The determinants of species richness gradient ha ve been one of the key contents in ecological research. It is important to understand the general determinants of species richness. The factors that affect species richness generally include two aspects: environmental factors and biological factors. Many hypotheses about the environmental determinants of species richness have been raised, but only a small fraction of these hypotheses are feasible and have the potential to predict species richness. The environmental heterogeneity hypothesis and the productivity hypothesis in energy hypothesis are all frequently discussed, but there are still no satisfactory results. In addition to the positive correlation between species richness and environmental heterogeneity which has been widely supported by evidences, negative correlation and peak type correlation have been also reported. Similarly, productivity has a strong effect on species ric hness, but there is no a unified pattern. How to understand these various relationships between species richness and environmental heterogeneity, and whether these patterns will be related to the level of productivity? In addition, the effect of environmental factors on species richness is not necessarily direct, and it may also affect species richness indirectly through other biological factors, especially in small scale. These biological factors not only include the biological interactions, but also the composition of the regional species pool and the total abundance of the community. What is the relationship between species richness and these biological factors? Species richness-environmental heterogeneity relationship and species richness-biological factors relationship, which is more general? In this paper, we used the computer simulation, field experiments and statistical inference to explore the determinants of species richness gradient amongs plant communities.Through an individual-based spatially explicit model incorporating a long-recognized tradeoff between competitive ability and stress tolerance of plant species, we found that the shape of species richness-environmental heterogeneity relationship depends on the community position along the productivity gradient: at either end of the gradient of productivity, a positive species richness-environmental heterogeneity relationship occurred, whereas at the intermediate levels of the gradient, a unimodal species richness-environmental heterogeneity relationship emerged. It indicates that the level of productivity should be considered as a potential factor influencing the shape of the species richness-environmental heterogeneity relationship.All plant communities are composed of individuals, we construct a new formula: S=C*(1-exp（-（pa*E+pb/J+pc） *J） by further improving the individual-based species cumulative curve(S=C(1-e^-ZJ)) and tested the formula with a trade-off model. The results showed for spatial uniform communities this formula could effectively explain species richness gradient（P < 0.01, R² = 0.9464）, namely the variation in species richness can be jointly explained by abundance（J）, the evenness of species in species pool（E） and the size of species pool（C）; but for spatial hetergenous communities the effectiveness of the formula is not so good : its ability to explain species richness is very low, and even ineffective. Furthermore, the new formula can effectively explain the variation in species richness among aspects of slope from a field experiment in an alpine meadow（P < 0.01, R² = 0.7667）: the increase in species richness is due to the increase in eveness and total individual number along the aspect of slope gradient.For spatial uniform communitis, the relationship between species richness and biological factors is more general, namely the influence of environmental factors on species richness in plant communities is mainly indirectly achieved through abundance（J）, the evenness（E） or the size of species pool（C）. However, for spatial hetergenous communities the relationship between species richness and environmental factors is more general, that is, environmental heterogeneity has a direct effect on species richness. Thus, the explanation ability of biological factors on species richness depends on whether the spatial environment is uniform or not. Our study provides a new perspective for understanding the variation in species richness amongs plant communities.