Ecosystems are interconnected by energy fluxes that provide resources for the inhabiting organisms along the transition zone. Especially where in situ, i.e. autochthonous, production is low, whole ecosystems built up on allochthonous input. It is unclear how relevant allochthonous input is for systems with high autochthonous production. Therefore, we investigated the role of marine input for small tropical islands with high autochthonous production. Similar to studies on desert islands, a dependency of the insular food web on allochthonous resources was expected. Using stable isotope analysis, we demonstrated, however, that only the beach fauna depends on allochthonous input, while the inland fauna primarily builds up on autochthonous resources. This shows that small island ecosystems can support two independent food webs. Therefore, our findings suggest that disturbances do not cascade throughout the whole ecosystem but only affect single food webs.