The sand dune ecosystem of the Namib Desert of southwestern Africa contains a species-rich and highly endemic biota. The ecosystem's abiotic and biotic characteristics are described and quantified, and the causal relationships between 18 of these variables are compared. Wind and dune morphology affect 16 of the 18 environmental parameters and are the major environmental factors in this ecosystem. Organic detritus cycles, fog moisture, soil temperature and sand grain size also have important direct or indirect influences on the evolution of the dune biota. Four distinct biotic communities are defined and described for the north-central portion of the southern dune field; these are the interdune, dune base, plinth and slipface communities. Their distribution along the dune elevation gradient appears to be limited mainly by differences in sand particle size, soil moisture and sand stability. It is hypothesized that the subtropical, coastal Namib desert has an inordinately heterogeneous, and probably unique, physical environment that contains several adaptative zones not found in temperate continental dune ecosystems. Under such conditions, there has existed a high potential for extensive adaptive radiations within the numerous lineages of the psammophilic ancestral biota.