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24 25 The combined effects of isolation within an unfavourable matrix and vegetation 26 development were studied in patches of scrub vegetation on a tropical inselberg (Nouragues, 27 French Guiana). Three thickets ~10 m 2 area were sampled, located at ~30 m distance from 28 each other, in which the same vegetation types were present at less than 3 m distance: 29 canopies of Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrtaceae), two stages of thicket 30 development, and zones of destruction by fire. The shallow organic soil was sampled over ~2 31 dm 2 and down to 10 cm. The matrix was comprised of granite without any soil development. 32 Arthropods were classified into morphospecies and their food diets were characterized by 33 optical methods. The effect of isolation (between-thicket variation) on morphospecies 34 composition was negligible, while that of vegetation type and fire (within-thicket variation) 35 was prominent, as ascertained by principal component analysis and tested by partial 36 redundance analysis. The importance of keystone structures (stages of vegetation 37 development) was stressed, and interpreted to the light of taxonomic and foodweb knowledge. 38 39