For many decades ecologists have studied mechanisms influencing species coexistence, and many theories has been proposed. Niche theory, that suggests diversity is mainly influenced by environmental heterogeneity, and neutral theory, that suggests diversity is maintained by stochastic immigration, are the most popular theories and still generating much interest. Due to the limitations of two main contrasting theories (i.e. niche and neutrality) to completely explain species coexistence, this project will attempt to find a link between them through the inclusion of the spatial scale as a key concept in explaining diversity. Specifically, I aim to explore how spatial scale influences the environmental heterogeneity - diversity relationship by the influence of scale on seed dispersal and plant size. The project is divided in three sub-projects. 1) the relationship between large-scale environmental heterogeneity and small-scale species richness, because I expect that regional species pool can influence small-scale plant species richness; 2) the effect of seed and vegetative dispersal on the ability of species to colonize isolated habitats within a plant community; and 3) the relationship between plant size (trees, shrubs and grasses) and the spatial scale at which these different growth-forms are affected by environmental heterogeneity. The project will be carried out in the Macroecology workgroup of the Tartu University, where Professor Pärtel and his research work have a long experience in studying plant species coexistence. We will use a combination of experiments and meta-analyses to completely address the proposed questions. I expect that the results obtained will be useful to understand how scale influences seed dispersal, and thus species coexistence. This will provide a further step to understanding the factors that determine species diversity, and potentially create a novel consensus theory.