This study quantifies the flow of energy and biomass from a productive marine system to a relatively unproductive terrestrial system. Biomass from marine food webs (here, the Gulf of California) enters the terrestrial webs of islands and coastal areas through two conduits: (1) shore drift of algal wrack and carrion and (2) colonies of seabirds. Both conduits support dense assemblages of consumers: arthropods are 85-560 times more abundant in the supralittoral than inland and 2.2 times more abundant on islands with seabird colonies than those without. Marine input (MI), not terrestrial primary productivity (TP) by land plants, provides most energy and biomass for terrestrial communities on 16 of 19 study islands. The ratio of perimeter to area (P/A) significantly predicts arthropod abundance on islands and is the major determinant of the relative importance of allochthonous flow; we expect P/A ratio to be important wherever transport of nutrients, detritus, and organisms among habitats occurs. Similar transport phenomena generally take place, often with significant impact, on coastal habitats and islands worldwide. Such input subsidizes a diverse array of terrestrial consumers; in many cases, subsidized consumers reach extraordinarily high densities and thus can depress their in situ resources. In general, we propose that such flow is often a key feature of the energetics, structure, and dynamics of populations, food webs, and communities whenever any two habitats, differing in productivity, are juxtaposed.