The combined effects of isolation within an unfavourable matrix and vegetation development were studied in patches of scrub vegetation on a tropical inselberg (Nouragues, French Guiana). Three thickets similar to 10m2 area were sampled, located at similar to 30 m distance from each other, in which the same vegetation types were present at less than 3 m distance: canopies of Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrticeae), two stages of thicket development, and zones of destruction by fire. The shallow organic soil was sampled over similar to 2 dm2 and down to 10 cm. The matrix was comprised of granite without any soil development. Arthropods were classified into morphospecies and their food diets were characterized by optical methods. The effect of isolation (between-thicket variation) on morphospecies composition was negligible, while that of vegetation type and fire (within-thicket variation) was prominent, as ascertained by principal component analysis and tested by partial redundance analysis. The importance of keystone structures (stages of vegetation development) was stressed, and interpreted to the light of taxonomic and foodweb knowledge.