Interrelations among three groups of ideas are considered. (1) The place where a plant is rooted, or a sessile animal is attached, may be termed a microsite. The microsites for a community form a mosaic that is differentiated by physical environment or biological effects or both. Populations function and regulation, and community self-maintenance and response to environmental fluctuation, can be approached in terms of the flow of reproducing populations through the mosaic. (2) Most communities are subject to disturbance followed by succession. Communities are diverse in the kinds and frequencies of disturbances and in kinds of successions and climaxes; and the species of a biota diversify in their relationships to successional time and patterns of successional and climax communities. (3) Although mosaic phenomena are general, two broad groupings may be recognized: intracommunity patterns that relate to microsite differentiation and species response to this, and intercommunity successional mosaics and climax complexes for which community disturbance is a major determining force. Relationships between elements of a mosaic can often be formulated in terms of a chain or network or replacement rates, but formulations should allow for the influence of both terms and occurrence of semipermanent plateau stages in some successions.