Community structure and positive interactions in constraining environments

Spatial patterns in plant communities are thought to be controlled by the interplay of species interactions and environmental constraints. To evaluate the role of plant-plant interactions in shaping these communities we quantified species co-occurrence and interaction in seven environmentally distinct communities. These included four different semiarid habitats in southeast Spain, one alpine system in the Sierra Nevada range (Spain), and two sites in Venezuela, a secondary savanna near Caracas (Altos de Pipe), and a sclerophyllous shrubland in the Gran Sabana plateau. We expected that facilitation would be stronger at sites with more spatial associations. The four semiarid sites in Spain and the shrubland in Gran Sabana showed a high degree of positive species associations. Of the other two communities, one showed both positive and negative associations while negative ones predominated in Altos de Pipe. The direct experimental measure of neighbors' effect showed that positive interactions among species prevailed in communities where positive species associations dominated. The appearance of benefactor species in patches increased species richness compared with the surrounding inter-shrub spaces. Our results provide a link between spatial patterns and species interactions, where aggregation points to positive interactions and segregation to competitive or interference effects. Facilitation appears as a relevant process shaping communities under environmental constraints.