Distributional patterning of terrestrial herpetofauna on the Wessel and English Company Island groups, northeastern Arnhem Land, Northern Territory, Australia

Forty-four species of terrestrial reptiles and eight species of frogs were recorded from 60 continental islands of the Wessel and English Company grou ps off northeastern Arnhem Land, Northern Territory. Two gecko species, Oed ura rhombifer and Heteronotia binoei, were present on the most islands (34 and 31, respectively), and occurred on islands < 5 ha. Zn contrast, agamids , pygopodids and varanids were absent from islands < 18 ha, and snakes and frogs were not reported from islands < 240 ha. Island size explained 82% of the variation in species richness for terrestrial reptiles, and 84% of tha t for lizards. The relationship was less good for (i) groups with generally uncommon species (notably snakes), for which sampling effort explained mor e variation, and (ii) groups with species which had relatively specific hab itat requirements (notably frogs), for which island size and isolation fact ors were not especially relevant. For most taxonomic groups considered, iso lation factors added little to the relationship between species richness an d island size. Across all reptiles, larger species were found on fewer isla nds, and had larger island size thresholds. This relationship broke down wi th analysis restricted to the single most species-rich family, Scincidae. O nly 6 of the 20 most frequently recorded species showed significant variati on in abundance among 8 vegetation types sampled by 226 quadrats across 40 islands. The number of species (alpha-diversity) and total abundance of her petofauna within quadrats was generally unrelated to island size; however, (with analysis restricted to islands on which they occurred) six individual species were significantly more abundant on smaller islands than on larger islands, with no species showing the opposite pattern. The islands' herpet ofauna is largely a relatively depauperate subset of that of the far more c omplex sandstone massif and escarpment of western Arnhem Land, especially m issing species associated with rugged sandstone gorges, riparian areas, ope n forests, swamps and clay soils. Patterns in species richness and composit ion are explained by greater range of environments on larger islands allowi ng better retention of species since isolation and/or richer tallies at the time of isolation. The evidence suggests that there has been relatively li ttle colonization, although at least two gecko species and one varanid may have moved reasonably frequently.