Alexander Forde studied classic top-down and bottom-up forces in food webs with his experimental study in an unusual ecosystem. On mangrove cays inside the Belizean barrier reef, severe P-limitation on the peat substrates curbs red mangrove (Rhizophora mangle) growth within interior ponds. Along the pond fringes, subsidies from wind-blown flocculent microbial biomass transform leaf traits and growth form, creating fine-grained productivity gradients and heterogeneity in arthropod and bird communities. On Twin Cays, Belize, our study leveraged this steep productivity gradient and experimentally excluded insectivorous birds for more than 1 year. Although bird predation consistently disrupted arthropod numbers and reduced their herbivory, microbial detrital subsidies alone explained red mangrove growth dynamics.