Bottom-up effects of sodium regulate an Amazonian brown (detrital) food web through increases in detritivores

Essential nutrients, such as Na, are physiologically important and limit individual metabolic activity. Nutrient availability affects not only individual behavior but can shape whole ecological communities through bottom-up processes. Because predators consume high-Na diets whereas herbivores and detritivores obtain very little Na from their diets (plants or detritus), we tested the hypothesis that Na addition to the Na-poor Peruvian Amazon (far from oceanic aerosol input) would increase Na-limited herbivores and detritivores but not predators in a brown (or detritial) food web. We added 250 ml 0.5% NaCl solution or just river water to paired 0.25 m2 plots every other day. After 18 days, Winkler extractions revealed increases in detritivores such as collembola and oribatids and shredders such as isopods, diplopods and isopterans (often 2 fold more) on Na-addition plots but not predators such as araneids, chilopods, and staphylinids. Noted exceptions were the specialist predator Strumigenys spp. (Formicidae) and predatory mites which only increased on Na addition plots after increases in prey abundance. In the same experiment, we also found decomposition rates to be Na limited, mainly driven by changes in the brown food web. We suggest that Na is a limiting nutrient in Amazonian brown food webs that regulates community structure through bottom-up processes with a diminished effect in higher trophic levels.