The nature of ecological communities is a key question in ecology. After nearly a century of debate there is evidence for communities functioning as either relatively deterministic structures shaped by predictable local interactions, or highly dynamic entities determined by the environmentally-driven spatial dynamics of species. We do know that both interactions and environmental requirements determine the geographic distribution of species and their coexistence in local communties. But we lack a comprehensive theoretical framework that allows assessing how the influence of both types of factors determines species’ occurrences at different scales. In UNITED we will seek to develop such framework using the two aspects of the niche concept (i.e. abiotic conditions and biotic interactions) as the pivotal element that connects already existing models at different scales. To do that, we will first seek to fill in several gaps in the knowledge on how interactions and environmental conditions interplay to determine the spatial responses of the speciesm through three work packages. In the first work package we will use experimental bryophyte communities –both placed in the field along a climatic gradient, and under the controlled conditions of a growth chamber– to study how neutral competitive interactions and responses to enviromental conditions determine species occurrence and local success. In a second work package we will conduct detailed field work on two different types of coastal dune xerophytic plant communties, in order to assess the effects of positive and negative interactions (i.e. facilitation and competition) on the distribution and local co-occurrence of species. In the third, we will use existing field data on similar systems (epyphytic bryophytes and inland dune xerophytic shrubs) and environmental niche models to study how climatic suitability and coexistence with other species determine the local success of each species. The same analytical framework will be applied to the data generated by the first two work packages. Finally, a fourth work package will develop theoretical models at three scales (local, landscape and regional) based on existing models on the dynamics of local communities, metacommunities and species’ geographic ranges. These models will be adapted to ensure the interoperability between them, and we will integrate them into a single analytical framework through a novel approach based on developing biologically meaningful model summaries at each scale, that can be used as inputs for the other models. If UNITED is fully successful, it may constitute a cornerstone in the development of ecological theory, particularly in what refers to the scaling of different processes.