How can terrestrial animals survive in a desert with scant primary productivity? The Peruvian coastal desert is hyper-arid, but faces one of the world's most productive marine ecosystems, the Peru-Chile cold current. Given the stark difference in productivity between these adjacent ecosystems, we expected to find strong linkages connecting the terrestrial and marine food web. We investigated how marine resources are incorporated in the diet, and influence the distribution of terrestrial consumers (geckos, scorpions, solifuges and darkling beetles). Stomach contents from geckos, and δ13C and δ15N values of geckos and other terrestrial consumers suggest that marine green algae of the genus Ulva provide energy and nutrients to the terrestrial food web. Isotopic values suggest that amphipods, which feed on stranded Ulva, make marine resources available to terrestrial predators by moving between the intertidal and supratidal zones. The relative contribution of terrestrial and algal carbon sources varied among terrestrial predators, because scorpions assimilated a lower proportion of energy from Ulva than did geckos and solifuges. These δ13C patterns reflected differences in the spatial distribution of consumers. Our study supports the idea that in places where ecosystems with contrasting productivity levels are spatially juxtaposed, it is not possible to understand the structure and dynamics of food webs without taking into account the effects of energy and nutrients flowing from adjacent ecosystems. In contrast to other studied systems, especially those in Baja California, our site in Peru receives very little rainfall and the amount of precipitation is not affected by El Niño events. The near absence of rainfall promotes an extreme dependence of terrestrial consumers on marine resources, and causes permanent indirect food-web effects that are affected by temporal variability in marine productivity, rather than temporal patterns of plant growth. © Oikos.